Publications

In the quest for fast throughput metal biosensors, it would be of interest to prepare fluorophoric ligands with surface-adhesive moieties. Biomimetic analogues to microbial siderophores possessing such ligands offer attractive model compounds and new opportunities to meet this challenge. The design, synthesis, and physicochemical characterization of biomimetic analogues of microbial siderophores from Paracoccus denitrificans and from the Vibrio genus are described. The (4S,5S)-2-(2-hydroxyphenyl)-5-methyl-4,5-dihydro-1,3-oxazole-4-carbonyl group (L(a)), noted here as an HPO unit, was selected for its potential dual properties, serving as a selective iron(III) binder and simultaneously as a fluorophore. Three tripodal symmetric analogues cis-L(b), cis-L(c), and trans-L(c), which mainly differ in the length of the spacers between the central carbon anchor and the ligating sites, were synthesized. These ferric-carriers were built from a tetrahedral carbon as an anchor, symmetrically extended by three converging iron-binding chains, each bearing a terminal HPO. The fourth chain could contain a surface-adhesive function (L(c)). A combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, and electrochemistry was used to fully characterize the corresponding ferric complexes and to determine their stability. The quenching mechanism is consistent with an intramolecular static process and is more efficient for the analogue with longer arms. Detection limits in the low nanogram per milliliter range, comparable with the best chemosensors based on natural peptide siderophores, have been determined. These results clearly demonstrate that these tris(phenol-oxazoline) ligands in a tripodal arrangement firmly bind iron(III). Due to their fluorescent properties, the coordination event can be easily monitored, while the fourth arm is available for surface-adhesive moieties. The tripodal system is therefore an ideal candidate for integr

Over the past decade, there has been remarkable progress in the development of molecular logic and arithmetic systems, which has brought chemists closer to the realization of a molecular scale calculator (a Moleculator). This paper describes a significant step in this direction. By integrating past and new approaches for molecular logic reconfiguration, we were able to load advanced arithmetic calculations onto a single molecular species. Exchanging chemical inputs, monitoring at several wavelengths simultaneously, as well as using negative logic for the transmittance mode significantly increase the input and output information channels of the processing molecule. Changing the initial state of the processor is an additional approach used for altering the logical output of the device. Finally, introducing degeneracy to the chemical inputs or, alternatively, controlling their interactions to form identical chemical states minimizes the complexity of realizing three-bits addition and subtraction at the molecular scale. Consequently, using a commercially available fluorescein molecule, acid and base chemical inputs, and a simple UV-vis measurement setup, integration of a full-adder and, for the first time, a full-subtractor is now possible within individual molecules.

A highly efficient and simple molecular arithmetic system based on a plain fluorescene dye, capable of performing a full scale of elementary addition and subtraction algebraic operations is reported. For achieving boolean functions through scid-base interactions and their control over multistate molecular switches are described. A half-adder, performing a+b algebraic operations, is obtained with two identical sodium hydroxide solutions as inputs and by monitoring the outputs at 447 and 501 nm. Subtraction is achieved by operating the half-subtractor first using acid and then using base as input numbers. This molecular arithmetic unit demonstrates a general applicable method for resetting of chemical computation system.

The biological evaluation and synthesis of lipophilic iron chelators as protective agents from oxidative stress were analyzed. A spontaneously transformed cell line of oligodendroglia origin (OLN 93) was chose as a model for neural cells. These cells were used to demonstrate remarkable sensitivity to geontoxic stress, culminating in cell death when both divalent iron and H ₂O₂ were added to cells. OLN 93 cells were seeded in 96-well polyethyleneimine precoated plates, and after 24 h attachment were further incubated for 3 h with the synthetic analog at various concentrations. The results show the proactive effect of analog 5-7 at a 200 μM.

Coordination self-assembly of bishydroxamate-based metal-organic multilayers on gold employing a layer-by-layer (LbL) approach was investigated. It is shown that the solution chemistry of the participating metal ion has a marked influence on the composition and properties of the multilayers. Use of Ce4+ and particularly zirconium(IV) acetylacetonate (Zr(acac)(4)) solutions in the ion-binding step of multilayer construction leads to multilayers with a near-stoichiometric metal ion-to-ligand ratio, suggesting a structure close to that predicted by a simple coordination self-assembly scheme. On the other hand use of a ZrCl4 solution as the source of metal ions in the multilayer construction leads to a multilayer with greater thickness and a large excess of Zr(IV), evenly distributed between the organic layers. In the latter case, a ratio of ca. 1:2 between the excess Zr and oxygen, as well as long-term Zr4+ binding experiments showing deposition of ZrO2, suggest the formation of a zirconia-type nanophase between the bishydroxamate organic repeat units during multilayer self-assembly. Hence, while the multilayer prepared using Zr(acaC)(4) solution appears to represent a "true" coordination-based structure, the one prepared using ZrCl4 is best described as a composite organic-ceramic multilayer. Composite multilayers prepared in this way display different properties from those of the stoichiometric ones, such as improved dielectric behavior and higher stiffness. Even greater mechanical stability is obtained with multilayers constructed using alternate binding of ZrCl4 and Ce4+. The concept of LbL formation of coordination-based composite organic-ceramic structures may be useful in obtaining nanometer-scale structures with tunable properties.

Ruthenium complexes with three bipyridyl ligands, one of which is modified by attaching one or two hydroxamic acids groups (Ru-1 and Ru-2, respectively), were synthesized. Using EPR spectroscopy, we have found that photoexcitation leads to formation of nitroxyl radicals. The nitroxyl radical concentration in Ru-2 increased dramatically in the presence of spin traps DMPO (5,5́-dimethyl-1-pyrroline-N-oxide) and PBN (N-tert-butyl-α-phenylnitrone) characterized by strong affinity to Superoxide radicals. We have attributed this behavior to the formation of a cage complex between Ru-2 and the Superoxide radical. This paper concerns the study of cages formed between ruthenium complexes and molecular oxygen and the effect of functional groups attached to modified bipyridyl ligands on cage formation. The complex between Ru-2 and O ₂ was formed in the ground state, probably with participation of the hydroxamic acid groups. The equilibrium constant of this complex was determined by EPR as K _eq ∼ 3 M ^-1. The formation of the Ru-2-O ₂ complex is supported by the temperature-dependent rate of appearance of the EPR signal in the presence of PBN. Additional evidence comes from observation of paramagnetic shifts of the peaks in the ¹H NMR spectrum of specific aromatic protons in the substituted bipyridyl ring upon exposure to O ₂. Similar shifts were observed in the spectrum of Os-2, with osmium replacing ruthenium. Model compounds with functional groups that replace the hydroxamic acid or compounds without the metal center, but with the two hydroxamic acids, were synthesized. No shifts in the ¹H NMR spectra of these derivatives were observed in the presence of O ₂. These results lead to the conclusion that both metal ions, Ru(II) or Os(II), and hydroxamic acid groups are essential components for the formation of the oxygen cage.

The multifunctional nature of proteins that have iron-heme cofactors with noncovalent histidine linkage to the protein is controlled by the heme environment. Previous studies of these active-site structures show that the primary difference is the length of the iron-proximal histidine bond, which can be controlled by the degree of H-bonding to this histidine. Great efforts to mimic these functions with synthetic analogues have been made for more than two decades. The peroxidase models resulted in several catalytic systems capable of a large range of oxidative transformations. Most of these model systems modified the porphyrin ring covalently by directly binding auxiliary elements that control and facilitate reactivity; for example, electron- donating or -withdrawing substituents. A biomimetic approach to enzyme mimicking would have taken a different route, by attempting to keep the porphyrin ring system unaltered, as close as possible to its native form, and introducing all modifications at or close to the axial coordination sites. Such a model system would be less demanding synthetically, would make it easy to study the effect of a single structural modification, and might even provide a way to probe effects resulting from porphyrin exchange. We introduce here an alternative model system based on these principles. It consists of a two component system: a bis-imidazolyl ligand and an iron-porphyrin (readily substituted by a hemin). All modifications were introduced only to the ligand that engulfs the porphyrin and binds to the iron's fifth and sixth coordination sites. We describe the design, synthesis, and characterization of nine different model compounds with increased complexity. The primary tool for characterizing the environment of each complex Fe¹¹¹ center was the Extended X-ray Absorption Fine Structure (EXAFS) measurements, supported by UV/Vis, IR, and NMR spectroscopy and by molecular modeling. Introduction of asymmetry, by attaching different imidazoles as head groups, led to the formation of two axial bonds of different length. Addition of H-bonds to one of the imidazoles in an advanced model increased this differentiation and expanded the porphyrin ring. These complexes were found to be almost identical in structure to peroxidase active sites. Similarly to the peroxidases and other synthetic models, these compounds stabilize the green, compound I-like intermediate, and catalyze the oxidation of organic substrates.

Hopobactin is a 1-hydroxy-2-oxopyridine-6-carboximide analog of enterobactin that forms isostructural complexes.

The use of evaporated ultrathin gold films on mica to obtain transmission UV/vis spectra of monomolecular overlayers containing chromophores is demonstrated. The gold substrates (thickness, 13-100 Angstrom) were studied by scanning force microscopy. UV/vis spectroscopy, and X-ray diffraction and found to exhibit an array of {111} textured islands, whose sizes and optical properties can be controlled by the evaporation conditions and subsequent annealing. The use of gold substrates of this kind enables one to clearly resolve UV/vis absorption spectra of (sub)monolayers. This methodology is applied to the study of the binding of cobalt and iron tetraphenylporphyrins (CoTPP and FeTPPCl, respectively) and cobalt phthalocyanine (CoPc) to various N-donor ligand monolayers, self-assembled on ultrathin Sold films. Enhancement of the gold surface plasmon absorption, resulting from molecular self-assembly on the gold island film, is used to semiquantitatively follow porphyrin binding to the monolayer.

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices(1). But physical implementations using discrete molecules are neither conceptually(2,3) simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a 'soft' method to avoid damaging the molecules(4). By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.

Geringe Konzentrationen von NO in physiologischen Pufferlösungen (1 ppm, ca. 3 μM) lassen sich mit dem gezeigten maßgeschneiderten molekularen Zweikomponentensystem – ein pinzettenartig von einem organischen Liganden gehaltenes Eisen(III)‐porphyrin – nachweisen, das mit einer Seite an einen Detektor auf GaAs‐Basis gekoppelt ist. Bindet NO an das Eisen(III)‐porphyrin, ändert sich die Stromstärke.

Metal-organic coordination is an attractive means for constructing supramolecular systems, providing versatility, simple synthesis, and a defined geometry. The convenience of changing 'building blocks' during multilayer assembly is exploited for the fabrication of novel ion-coordinated hybrid multilayers on gold. Two bifunctional linkers are used, a tetrahydroxamate and an organic diphosphonate, while the connection between layers is accomplished through Zr(IV) coordination, to form a well-defined hybrid multilayer. The two ion binders are compatible with respect to multilayer assembly, allowing the change of linkers during construction while maintaining the film structural integrity and organization. The different chemical reactivity of the binders enables rational structural manipulation of the multilayer, by selective dissolution of the acid-sensitive hydroxamate layers while keeping the acid-resistant phosphonates (and underlying hydroxamates) intact. The process demonstrates the multilayer structural quality, where two diphosphonate monolayers are capable of effectively blocking proton penetration to underlying hydroxamate layers. This allows nanometer-scale reshaping of the molecular film according to a scheme introduced during its construction.

Perpendicularly oriented iron porphyrins are absorbed onto a gold surface when interconnected long-chain diimidazolyl groups coordinate axially to the metal center from either side of the ring plane (see schematic representation). The stacking of the rings is simultaneously prevented. The monolayers have been characterized structurally and electrochemically.

Synchrotron radiation (SR) pulses are used to eject electrons from a gold substrate covered with organized organic thin films (OOTF) in order to investigate their transmission probability through the OOTF as a function of the electron initial kinetic energy. By variation of the SR photon energy within a few eV above the Au-4f binding energy levels we controlled the initial kinetic energy of the substrate electrons. The observed oscillations in the transmission probability for porphyrin-based films as a function of the kinetic energy is argued to be due to effects of band structure above the vacuum level in the well-ordered molecular adsorbate. We also present valence photoemission spectra (PES) of different type OOTF and demonstrate how their coverage of the substrate affects the PES.

A two-dimensional experiment, termed DONUT-HYSCORE (double nuclear coherence transfer hyperfine sublevel correlation) designed to obtain correlations between nuclear frequencies belonging to the same electron spin manifold is presented. The sequence employed is π/2-τ₁-π/2-t₁-π-τ₂- π-t₂-π/2τ₁-echo, and the echo is measured as a function of t₁ and t₂ whereas τ₁ and τ₂ are held constant. It is complementary to the standard HYSCORE experiment which generates correlations between nuclear frequencies belonging to different M(s) manifolds and is particularly useful for ¹⁴N nuclei. The experiment is first demonstrated on a single crystal of copper- doped l-histidine hydrochloride monohydrate where the modulations are induced by a single ¹⁴N nucleus, the remote nitrogen in the imidazole group. HYSCORE and DONUTHY-SCORE experiments were carried out on two crystal orientations. In the first, one Cu²⁺ site contributes to the echo and all six nuclear frequencies together with the expected correlation were observed. In the second, 12 frequencies corresponding to two Cu²⁺ ions at different crystallographic sites appeared and all expected correlations were detected as well. This rather trivial example demonstrates that the DONUT-HYSCORE pulse sequence indeed generates correlations within the M(s) manifolds. The value of the DONUT-HYSCORE experiment is demonstrated on a frozen solution of a vanadyl complex with a bis-hydroxamate ion binder (VO-RL515). The modulations in this complex arise from the two InN nuclei in the hydroxamate groups, and orientation-selective three-pulse ESEEM (electron spin-echo envelope modulation) spectra showed a number of well-resolved peaks. An unambiguous assignment of all peaks and their orientation dependences could not be achieved through HYSCORE alone because at certain orientations frequencies of one of the M(s) manifolds were absent or overlapped with those of the other manifold. The application of the DONUT-HYSCORE experiment provided new correlations that led to the complete assignment of the ESEEM frequencies, thus paving the way for future systematic spectral simulations for the determination of the best-fit Hamiltonian parameters. This example shows that, in the case that the HYSCORE experiment cannot distinguish between two sets of frequencies belonging to the same M(s) manifold in different centers (or orientations) because signals from the other manifold are missing or overlapping, the DONUT-HYSCORE becomes most valuable.

In this study, we monitored and compared the uptake of iron in the fungus Ustilago maydis by using biomimetic siderophore analogs of ferrichrome, the fungal native siderophore, and ferrioxamine B (FOB), a xenosiderophore. Ferrichrome-iron was taken up at a higher rate than FOB- iron. Unlike ferrichrome-mediated uptake, FOB-mediated iron transport involved an extracellular reduction mechanism. By using fluorescently labeled siderophore analogs, we monitored the time course, as well as the localization, of iron uptake processes within the fungal cells. A fluorescently labeled ferrichrome analog, B9-lissamine rhodamine B, which does not exhibit fluorescence quenching upon iron binding, was used to monitor the entry of the compounds into the fungal cells. The fluorescence was found intracellularly 4 h after the application and later was found concentrated in two to three vesicles within each cell. The fluorescence of the fluorescently labeled FOB analog CAT18, which is quenched by iron, was visualized around the cell membrane after 4 h of incubation with the ferrated (nonfluorescent) compounds. This fluorescence intensity increased with time, demonstrating fungal iron uptake from the siderophores, which remained extracellular. We here introduce the use of fluorescent biomimetic siderophores as tools to directly track and discriminate between different pathways of iron uptake in cells.

A novel type of bilayer on a gold surface, based upon metal-ion coordination to hydroxamate moieties, is described, Tailor-made bifunctional ligands containing hydroxamate groups (for metal coordination) and a cyclic disulfide residue (for surface attachment) have been prepared. The bishydroxamate binding site forms 2:1 ligand/metal complexes with octacoordinating metal ions such as Zr-IV Ce-IV, and Ti-IV:the cyclic disulfide moiety anchors the complex to the gold surface, Two routes to bilayer formation are demonstrated: i) a one-step process from preformed 2:1 complexes, and ii) a stepwise process including formation of the ligand monolayers followed by binding of a guest ion and a second layer of ligand molecules. The former approach allows full characterization of the complexes before bilayer assembly, whereas the latter enables construction of either symmetric (identical) or asymmetric (nonidentical) bilayers. Both types of bilayers were characterized by ellipsometry, contact angle, and XPS measurements. Symmetric bilayers obtained by the two processes have similar properties.

The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time- and wavelength-dependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductor-based sensors is indicated.

The manner of hydrogen-bonding to peptide selectors in enantiomer separation is examined with the help of a structural model. This model relies on a C₃-symmetric trispeptide selector, which is stabilized by a network of distinct intramolecular hydrogen bonds. A combination of experimental and theoreticaI tools enables us to identify the lowest-energy conformation of the trispeptide selector and the sites of selector-substrate interactions. Experimental tools include temperature dependent ¹HNMR studies, 1D-NOE- measurements, and titration experiments, with the theoretical tools being EFF and CFF91 molecular mechanics calculations. The structural information deduced from these investigations is shown to bear on the enantioseparation of the corresponding chiral stationary phase towards derivatized amino acids. These observations, taken together, help to rationalize the mode of enantiomer-separation by amide phases as involving predominantly C₇-hydrogen bonding sites.

Iron chelators belonging to three distinct chemical families were assessed in terms of their physicochemical properties and the kinetics of iron chelation in solution and in two biological systems. Several hydroxypyridinones, reversed siderophores, and desferrioxamine derivatives were selected to cover agents with different iron-binding stoichiometry and geometry and a wide range of lipophilicity, as determined by the octanol- water partition coefficients. The selection also included highly lipophilic chelators with potentially cell-clearable ester groups that can serve as precursors of hydrophilic and membrane-impermeant chelators. Iron binding was determined by the chelator capacity for restoring the fluorescence of iron- quenched calcein (CA), a dynamic fluorescent metallosensor. The iron- scavenging properties of the chelators were assessed under three different conditions: (a) in solution, by mixing iron salts with free CA; (b) in resealed red cell ghosts, by encapsulation of CA followed by loading with iron; and (c) in human erythroleukemia K562 cells, by loading with the permeant CA-acetomethoxy ester, in situ formation of free CA, and binding of cytosolic labile iron. The time-dependent recovery of fluorescence in the presence of a given chelator provided a continuous measure for the capacity of the chelator to access the iron/CA-containing compartment. The resulting rate constants of fluorescence recovery indicated that chelation in solution was comparable for the members of each family of chelators, whereas chelation in either biological system was largely dictated by the lipophilicity of the free chelator. For example, desferrioxamine was among the fastest and most efficient iron scavengers in solution but was essentially ineffective in either biological system when used at ≤200 μM over a 2-hr period at 37°. On the other hand, the highly lipophilic and potentially cell-cleavable hydroxypyridinones and reversed siderophores were highly efficient in all biological systems tested. It is implied that in K562 cells, hydrolysis of these chelators is relatively slower than their ingress and binding of intracellular iron. The chelator-mediated translocation of iron from cells to medium was assessed in ⁵⁵Fe-transferrin-loaded K562 cells. The speed of iron mobilization by members of the three families of chelators correlated with the lipophilicity of the free ligand or the iron-complexed chelator. The acquired information is of relevance for the design of chelators with improved biological performance.

Wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy was used to investigate the surface state properties of CdTe crystals before and after the adsorption of specially designed organic molecules. One photon was used to modify the population of the surface states and a second photon to eject electrons from the substrate. We measured the dependence of the photoemission signal on the energy of the first photon and on the delay between the two light pulses. The energy of surface states, relative to the bands, was found to correlate with the relaxation time of the semiconductor surface after being photoexcited. This is explained in terms of a simple kinetic model for electron transfer. These findings demonstrate that the properties of surface states of semiconductors can be manipulated by adsorbing suitable organic molecules on the semiconductor surface and that the WD-TPPE method is a useful tool for optoelectronic characterization of semiconductor surfaces, with sensitivity exceeding that of most commonly used techniques.

Reversed siderophores (RSFs) are artificial hydroxamate-based iron chelators designed after the natural siderophore ferrichrome. The modular molecular design of RSF derivatives allowed the synthesis of various congeners with controlled iron-binding capacities and partition coefficients. These two physicochemical properties were assessed by a novel fluorescent method and were found to be the major determinants of RSF permeation across erythrocyte membranes and scavenging of compartmentalized iron. The partition coefficient apparently conferred upon RSFs two major features: (i) the ability to rapidly access iron pools of in vitro-grown Plasmodium falciparum at all developmental stages and to mobilize intracellular iron and transfer it to the medium and (ii) the ability to suppress parasite growth at all developmental stages. These features of RSFs were assessed by quantitative determination of the structure-activity relationships of the biological activities and partition coefficients spanning a wide range of values. The most effective RSF containing the aromatic group of phenylalanine (RSFm2phe) showed 50% inhibitory concentration of 0.60 ± 0.03 nmol/ml in a 48-h test and a 2-h onset of inhibition of ring development at 5 nmol/ml. The lipophilic compound RSFm2phe and the lipophilic and esterase-cleavable compound RSFm2pee inhibited parasite growth at all developmental stages whether inhibition was assessed in a continuous mode or after discontinuing drug administration. The antimalarial effects of RSFm2phe and clearable RSFm2pee were potentiated in the presence of desferrioxamine (DFO) at concentrations at which DFO alone bad no effect on parasite growth. These studies provide experimental evidence indicating that the effective and persistent antimalarial actions of RSFs are associated with drug access to infected cells and scavenging of iron from intracellular parasites. Moreover, the optimal antimalarial actions of RSFs are apparently also determined by improved accessibility to critical iron pools or by specific interactions with critical parasite targets.

In order to guarantee sufficient iron(III) uptake, microorganisms produce iron(III) ligands, termed siderophores. They scavenge the scarcely available iron(III) from the environment and deliver it to the cell via a highly specific, active transport process. In this article we describe analogs of hydroxamate siderophores with emphasis on Ferrioxamine derivatives. We present the principles of their design, and synthesis and describe their coordination properties and biological activity. These data enable us to trace the relationship between structural and functional parameters, and apply the knowledge gained for the design of supramolecular arrangements.

Molecular recognition and signaling are at the heart of biological processes as they govern intercellular communication and information transfer. Here we probe the recognition phenomena of siderophore-mediated microbial iron(III) uptake, which involves two recognition processes: recognition of iron(III) by microbial siderophores (iron(III) carriers), and recognition and binding of siderophore-iron(III) complexes by microbial membrane receptors. In addition, we introduce fluorescent siderophore analogs that respond to iron binding with changes of their fluorescence and thereby provide artificial binders that combine iron recognition with signaling.

MAGNETIC storage of information requires the ability to manipulate the magnetization of thin films with high sensitivity and spatial resolution. Non-magnetic overlayers are known to affect the characteristics of magnetic films: for example, the direction of magnetization of cobalt and iron films can be altered by deposition of a monolayer of copper and gold, respectively^1–3. The magnetic properties of cobalt films seem to be particularly sensitive to copper overlayers—deposition of only sub-monolayer amounts of copper will decrease the coercive field required to invert the magnetization direction⁴. Here we show that copper coverages as small as three-hundredths of a monolayer are sufficient to rotate by 90° the magnetization of Co films up to 20 atomic layers thick. This implies that the spins of about 500 cobalt atoms switch direction for each copper atom added. Adding more copper eventually switches the magnetization back to its original direction. This fine tuning of thin-film magnetism might be useful for developing sensitive magnetic-field sensors, as well as for magnetic recording.

Hydroxamate-based chelators of iron are potent inhibitors of in vitro growth of Plasmodium falciparum. Two types of such chelators, the natural desferrioxamine and the synthetic reversed siderophore RSF(ileum 2), are prototypes of antimalarial agents whose action spectra differ in the speed of action, stage dependence, and degree of reversibility of effects. This work explores the possibility of improving the antimalarial efficacy of these agents by using them in various combinations on in vitro cultures of P. falciparum. Growth assessment was based both on total nucleic acid synthesis and on parasitemia. The results indicate that the synthetic reversed siderophore more than complements the antimalarial action of desferrioxamine when applied during either ring, trophozoite, or mixed stages. The combined drug effects were significantly higher than the additive effect of the individual drugs. Qualitatively similar results were obtained for both reversible effects and irreversible (i.e., sustained) effects. Following an 8-h window of exposure the combined drug treatment caused parasite growth arrest and prevented its recovery, even 3 days after the treatment. The fact that such a combination of iron chelators displays a wider action spectrum than either drug alone has implications for the design of chemotherapy regimens.

Fourier transform infrared spectroscopy shows that benzoic acid and its derivatives can bind chemically to the surfaces of CdSe. The dipole moment of the adsorbed benzoate correlates linearly with changes in the semiconductor work function, as determined by vibrating capacitor (Kelvin probe) measurements. Because ligand adsorption does not induce significant changes in surface photovoltage, we conclude that changes in work function are due to changes in electron affinity rather than in band bending. This means that ligand adsorption does not induce charge transfer between the surface and the semiconductor but rather adds a surface dipole. Changes in band bending do occur, however, upon conventional etching.

Controlled surface modification of CdTe single crystals and CdTe and CuInSe₂ solar cell quality thin films was achieved by chemisorption of a series of organic ligands with varying dipole moments. Contact potential difference measurements in air showed that adsorption of benzoic or hydroxamic acid derivatives on the thin films or crystals changes the semiconductors' electron affinity without significantly affecting band bending. The magnitude and direction of surface potential changes, which reach 670 mV between extreme modifications, correlate with the ligands' dipole moments. Ligand dipole moments were controlled by varying the substituents of the ligand. Quantitative Fourier transform infrared (FTIR) spectroscopy showed that benzoic acid surface coverage is about one monolayer. Finally, FTIR spectral analysis showed that the benzoic acid derivatives adsorb via coordination to Cd on CdTe and that hydroxamic acids bind to Cd on CdTe and to In on CuInSe₂. These phenomena occur in several systems (two semiconductor compounds, two types of binding groups, and two types of surface morphologies were examined) and may prove useful in band edge engineering.

Electrospray Mass Spectrometry (ESMS) has been used to analyse protein/metal ion complexes directly in solution. A synthetic siderophore analogue and two sulphur-iron proteins have been used as models for the study of protein/iron interactions. These experiments were successfully extended to a protein/cofactor complex interaction model, myohemoglobin. Our results open the door to the characterization of weak interactions between large molecules by ESMS.

Molecular recognition by microbial receptors for siderophores [natural iron(III) carriers] is examined with synthetic iron(III) carriers as structural probes. The iron(III) carriers have been designed to reproduce the two essential features of the natural siderophores: the capability to form octahedral iron(III) binding cavities and to fit specific membrane receptors. Specifically, analogs of tripodal ferrichrome and linear ferrioxamines have been prepared and examined. The ferrichrome analogs rely on C₃‐symmetric binders that are assembled from triscarboxylates as anchors, amino acids as bridges, and terminal hydroxamate groups as binding sites. The ferrioxamine analogs are based on linear assemblies of three identical monomers, each derived from a chiral amino acid. The deliberate use of animo acid residues as variable building blocks enables us to systematically modify the molecules' envelopes and the preferred absolute configuration of the iron(III) complexes until optimal performance is reached. Examination of the synthetic analogs in Pseudomonas putida demonstrates that the domains around the iron(III) center and their chiral sense dictate the extent of recognition by the membrane receptors. It is also shown that the synthetic siderophore analogs may be designed to either exert a broader, or a more narrow range of microbial activity than the natural siderophores. The implications of these findings are discussed in relation to the possible design of species‐specific antimicrobial agents. © 1993 Wiley‐Liss, Inc.

Two novel families of chiral trishydroxamate binders are described. These compounds have been modeled after the natural siderophore ferrichrome in an attempt to mimic its biological properties as iron(III) carrier and growth promoter. In these analogs the hydroxamate binding sites and tripodal topology of ferrichrome are retained, but the chiral hexapeptide anchor of the natural siderophore is replaced by C₃-symmetric tricarboxylates of two homologous types (m = 1, type 1; m = 2, type 2). The resulting loss of chirality is compensated by symmetric extension of these anchors with natural amino acids (Figure 1). In this design two elements are of particular importance: (i) the amino acid bridges that induce chirality, stabilize specific conformations of the free ligands, and allow systematic modifications and (ii) the use of two homologous anchors that dictate the conformations of the metal complexes. Type 2 binders proved to either simulate the performance of the natural ferrichrome and act as growth promoters or to inhibit the action of ferrichrome and function as growth inhibitors. On the other hand, none of the type 1 binders proved active. In an attempt to establish the origin of these differences, the structures of the two types of compounds and of their octahedral metal complexes are examined by a combination of IR, NMR, UV/vis, CD, and NMR spectroscopy. Little differences were observed in the conformations of the free ligands as both types adopt propeller-like arrangements that are stabilized by intramolecular H-bonds. Both types of complexes also predominantly assume the Λ-cis configuration when L-amino acids are used. However, pronounced differences were observed in the conformations of the metal complexes: while in type 1 complexes the amides are positioned tangentially to the molecules cross section, in type 2 complexes they are oriented radially with the amide-NH pointing inward. In the latter arrangement the amide-NH groups become fit to form intramolecular H-bonds. These findings allowed us to interpret the in vivo performance of these compounds and to identify the structural requirements for obtaining growth promoters or growth inhibitors, respectively. The systematically modified ligands allowed us to examine the effect of intramolecular interactions, specifically H-bond networks and van der Waals forces, on the complexes' stoichiometry and isomeric and optical purity. The presence of either of the two intramolecular forces is sufficient to obtain monomeric complexes of 1:1 stoichiometry, while the absence of both like in the glycine derivative of type 1 ligand, 1G, causes the formation of polymeric complexes.

We present here the physicochemical and biochemical properties of NBD-DFO, the 7-nitrobenz-2-oxa-1,3-diazole (NBD) derivative of the siderophore, desferrioxamine B (DFO) (Lytton et al., Mol. Pharmacol. 40, 584, 1991). Modification of DFO at its terminal amine renders it more lipophilic, imparts to it fluorescent properties, and is conservative of the high-affinity iron(III) binding capacity. NBD-DFO partitions readily from aqueous solution into n-octanol (P_coeff = 5) and displays solvent-induced shifts in absorption and fluorescence spectra. The relative quantum yield of the probe's fluorescence increases over a 10-fold range with decreasing dielectric constant of the solvent. Fluorescence is quenched upon binding of iron(III) to the probe. We demonstrate here the application of NBD-DFO for the specific detection and monitoring of iron (III) in solutions and iron(III) mobilization from cells. Interactions between fluorescent siderophore and the ferriproteins ferritin and transferrin were monitored under physiological conditions. Iron removal from ferritin was evident by the demonstrable quenching of NBD-DFO fluorescence by scavenged iron(III). Quantitation of iron sequestered from cells by NBD-DFO or from other siderophore-iron(III) complexes was accomplished by dissociation of NBD-DFO-Fe complex by acidification and addition of excess ethylenediaminetetraacetic acid. The sensitivity of the method and the iron specificity indicate its potential for monitoring chelatable iron under conditions of iron-mediated cell damage, iron overload, and diseases of iron imbalance such as malaria.

C₃ symmetric trispeptides are described that form chiral conformations and are therefore eminently suited to provide a new family of chiral receptor molecules when extended by appropriate binding sites. These trispeptides are composed of C₃ symmetric trisamines as anchors and three symmetrically extending chiral amino acid residues. Their conformations in apolar solvents fall into two main classes. One is comprised of propeller-like conformations of preferred chiral sense that are stabilized by a belt of intramolecular H-bonds (hydrogen bonds) between adjacent strands. The other class has two of its strands connected by two H-bonds to form a 10-membered ring, while the third strand may hydrogen-bond to one of the other two. The effect of the anchors and amino acids on the relative stability of the H-bonded, chiral conformations has been established by a combination of spectroscopic and theoretical means. Trispeptides derived from more lipophilic α-amino acids show a higher population of the chiral conformations. Moreover, trispeptides that are based on tris(2-aminoethyl)amine (TREN) as anchor form stronger H-bonds than those relying on 1,3,5-tris(aminomethyl)benzene (TRAM).

The performance of a novel tetradentate hydroxamate as selective metal ion carrier across liquid membranes has been studied. Two types of carrier impregnated membranes have been examined, bulk liquid membranes and supported liquid membranes. The dihydroxamate was found to act as a selective carrier for Cu²⁺ ions in both types of membranes with antiport of H⁺ ions. Considering the small carrier concentration in the membranes (0.3 mM), the membrane separation factors and ion transfer rates are feasible and encourage the development of selective membranes for metal ions other than Cu²⁺.

The synthetically produced fluorescent siderophore NBD-desferrioxamine B (NBD-DFO), an analog of the natural siderophore ferrioxamine B, was used to study iron uptake by plants. Shortterm (10-hour) ⁵⁵Fe uptake rates by cotton (Gossypium spp.) and maize (Zea mays L.) plants from the modified siderophore were similar to those of the natural one. In longer-term uptake experiments (3 weeks), both siderophore treatments resulted in similar leaf chlorophyll concentration and dry matter yield. These results suggest that the synthetic derivative acts similarly to the natural siderophore. The NBD-DFO is fluorescent only when unferrated and can thus be used as a probe to follow iron removal from the siderophore. Monitoring of the fluorescence increase in a nutrient solution containing Fe³⁺-NBD-DFO showed that iron uptake by plants occurs at the cell membrane. The rate of iron uptake was significantly lower in both plant species in the presence of antibiotic agent, thus providing evidence for iron uptake by rhizosphere microbes that otherwise could have been attributed to plant uptake. Confocal fluorescence microscopy revealed that iron was taken up from the complex by cotton plants, and to a much lesser extent by maize plants. The active cotton root sites were located at the main and lateral root tips. Significant variations in the location and the intensity of the uptake were noticed under nonaxenic conditions, which suggested that rhizosphere microorganisms play an important role in NBD-DFO-mediated iron uptake.

Natural ion binding molecules, specifically iron(III) carriers, are used as guiding models for the preparation of synthetic binders. This biomimetic approach takes advantage of the evolutionary edge of natural molecules and aims at reproducing the essential characteristics of the most potent natural compounds with the simplest possible synthetic structures. The potent Enterobactin siderophore is examined as a guiding model. Its H-bonding (hydrogen-bonding) network and elements of preorganization are reproduced with all-synthetic, C₃-symmetric, chiral Triscatecholates to provide efficient and selective iron(III) binders of high optical purity. The structural principles operating in Enterobactin are then applied to the synthesis of Ferrichrome analogs, but with a shift of emphasis to conformational characteristics and biological activity. Comparison between two homologous families of Trishydroxamates demonstrates the role of H-bonding networks and van-der-Waals interactions in dictating the complexes' isomeric and optical purity, and highlights the conformational consequences of H-bonds. Some analogs of Ferrichrome reproduce the function of the natural compound as microbial iron(III) carriers by binding to both membrane receptors and transport proteins. Other derivatives inhibit the performance of the natural carriers by blocking the membrane receptors. The three strands of the C₃-symmetric ligands are extended, each carrying two hydroxamates, to become ditopic ion binders. These ligands form triple-stranded, helical metal complexes which are stabilized by H-bond networks and allow systematic extension to polynuclear complexes. Our design was governed by a few basic principles. We separated functional elements from structural elements, and used chiral elements as structural probes. To allow systematic modifications we adopted a modular synthetic strategy, and made use of variable modules as building blocks. As a common theme we applied multiple weak forces, particularly H-bond networks, to shape the individual molecules, stabilize their complexes and possibly favor their assembly to supramolecular architectures. Throughout all stages of this work physicochemical and biological testing of the compounds was joined by theoretical calculations in order to put experimental observations into a coherent structural framework.

Linear hydroxamate derivatives, possessing chiral α-amino acid moieties, were synthesized and their iron transport activities were studied in bacteria and fungi. No growth-promoting activity could be detected in the Gram-positive hydroxamate-auxotroph Aureobacterium flavescens JG9. However, Gram-negative enterobacteria, such as Escherichia coli, Pantoea agglomerans and Hafnia alvei were able to utilize iron from these analogues. Uptake of⁵⁵Fe-labeled analogues was inhibited by sodium azide, suggesting an active transport process. The receptors involved during uptake in enterobacteria were identified by using appropriate indicator organisms which are defective in the transport of either ferrioxamines (P. agglomerans FM13), coprogens (H. alvei), or both of these siderophore classes (E. coli fhuE). Our data suggest that the chiral hydroxamates are recognized by the ferrioxamine receptor (FoxA) and the coprogen receptor (FhuE) at a ratio which depends on the optical ν/δ isomer fraction and the nature of side chains. Transport was also observed in the fungus Neurospora crassa, known to take up coprogen rather than ferrioxamines, suggesting that in this fungus the synthetic analogues behave like coprogen.

In an attempt to provide models for copper binding in proteins, symmetric and nonsymmetric chiral ligands designed to bind copper in a controlled geometry were synthesized. These ligands are assembled from trifunctional anchors extended by donors containing amino acids such as histidine and methionine. The copper coordination of these complexes was studied by orientation selective electron spin echo envelope modulation (ESEEM) experiments. The three-pulse FT-ESEEM spectra consist of peaks at 0.7, 1.4, 2.1, and 3.0 MHz, which positions are practically independent on the resonant magnetic field, and a peak at about 4 MHz, which shows significant field dependence. The relative intensities of all lines vary with the resonant field. These lines are typical for the remote nitrogen in the imidazole ring. By using computer simulations of the FT-ESEEM spectra recorded at the various resonant magnetic fields along the powder pattern and taking into account the selected excited orientations, the ¹⁴N isotropic and anisotropic hyperfine interactions were determined. The simulations also gave the relative orientation of the ¹⁴N hyperfine and quadrupole tensor principal axes with respect to the g tensor principal axis. From these parameters it was concluded that in the complexes consisting of three histidyl residues all three imidazoles are coordinated to the copper, not in a coplanar structure but in a propeller-like arrangement. No other nitrogens, such as the pivotal nitrogen, were found to be coordinated to the copper.

The neutral noncyclic imide and ether containing ionophore (AS701), a selective carrier for Li⁺ among alkali cations, was found to be capable of mediating the transport of NH₄⁺ and of biogenic amines (catechols and indoles) across lipid bilayer membranes also. Ionophore-mediated electrical properties of planar lipid bilayers were studied under experimental conditions where the positively-charged amine species was dominant. The ionophore was found to act as a selective carrier of the biogenic amines, mediating their electrogenic transport across the membrane, forming 2:1 carrier-amine permeant complexes, carrying a net-charge of +1. Selectively among the amines corresponding to the following sequence: tryptamine (35) > Li⁺ (1) > serotonin (0.60) > dopamine (0.19) > norepinephrine (0.13) > epinephrine (0.05) > NH₄⁺ (0.05). The molecular factors involved in determining these selectivities are assessed.

An algorithm is described for generating atomic Cartesian coordinates of conformations of macrocyclic molecules possessing exact rotational or rotation‐reflection symmetries. A fragment representing the asymmetric unit of the molecule is suitably oriented in space, and then a symmetry operator is applied to generate the initial coordinates of the molecule. An empirical force field of interatomic interactions is used to generate equilibrium conformations. Results of calculations performed on two cyclic polylactones and one crown ether using this approach are given. They reveal that symmetric conformations of these molecules are often preferred. Since the latter conformations are probably responsible for the specialized properties of these molecules, this method should facilitate doing theoretical studies on these kinds of compounds.

The design, preparation, and properties of a new series of lipophilic lithium ion carriers are described. The structure of these carriers is based on an acyclic system in which a hexafunctional lipophilic envelope is formed around the metal ion in an octahedral arrangement. The synthesis involves a series of condensation reactions using a cyclic tin-oxygen compound as an activated diol precursor. The carrier properties for lithium ions were demonstrated by in vitro experiments on liposomes using a fluorescence assay. The potential pharmacological applications of these ionophores are discussed.

A series of protected β-hydroxy α-amino acids have been converted stereospecifically to their dehydro derivatives by treatment with (diethylamino)sulfur trifluoride and pyridine, three isomers giving rise to the Z derivatives and erythro isomers to the E derivatives.

A novel synthesis of the natural iron carrier enterobactin, based on a single step conversion of the tritylated serine β-lactone (1) into the enterobactin skeleton (3) via the use of a cyclic organotin compound as a template, is described.

The synthesis and properties of a new family of diastereomeric macrocyclic compounds are described which span two different types of internal cavities. The compounds are characterized by a polylactone ring and functionalized side chains arranged in either a “syn” or an “anti” configuration. The method of synthesis involves the use of cyclic tin-oxygen compounds as covalent templates and provides either of the diastereomers with high regio- and stereospecificity. Extensive spectroscopic data are given (NMR and X-ray analyses) and evidence for the different binding properties of the diastereomers for europium ions. The relationships of these compounds with naturally occurring ionophores is discussed and their possible application for the design of ion carriers or catalysts indicated.

The neutral, noncyclic, imide and ether containing ionophore AS701, has been developed as Li⁺-selective molecule, to be used potentially as an aid in the Li⁺-therapy of manic-depressive illness. The present report is a characterization of this molecule in neutral lipid bilayer membranes. This ionophore was found to the bilayers Li⁺-selective, acting as a selective carrier of monovalent cations. In addition, this molecule was found to be capable of acting as a selective carrier of monovalent anions. For both types of ions, the rate-limitting step in the process of permeation was found to be the diffusion of the carrier-ion complex through the membrane. The membrane-permeating species were found to be 2 : 1 carrier-ion complexes, carrying either a monovalent cation or a monovalent anion. The selectivity sequences among the ions studied being: Li⁺(1) > ClO₄^-(0.7) > Na⁺(0.07) > K⁺(0.016) > Rb⁺(0.0095) > Cs⁺(0.0083) > Cl^-(0.001). Mg²⁺ and SO₄^2- were found to be impermeant (under present experimental conditions). This sequence shows that the AS701 molecule has low selectivity for ions present in biological media, among those studied (i.e. Na⁺, K⁺, Mg²⁺, Cl^2- and SO₄^2-). This indicates that these ions will not interfere in the Li⁺ permeability induced by this carrier in vivo, and that the carrier will not interfere in the normal transport processes of these ions.

The utility of covalently bonded metalloid templates is demonstrated by syntheses with (a) Bu₂SnO and (b) Me₂Si(NCO) (or Me₂Si(NCS)₂). Approach (a) leads to macrocycles from diols and diisocyanates, and approach (b) gives unsymmetrically substituted heterocycles from symmetrical reactants (Im  imidazolyl). (Figure Presented.) (Figure Presented.)

The efficient preparation of macrocyclic amino acid derivatives via the use of tin as covalent template is described.

In this paper we discuss applications of cyclic intermediates for selective derivatization of difunctional substrates and controlled condensation to macrocyclic systems. Ring compounds that incorporate metalloid elements like silicon and tin have been chosen for this purpose and we demonstrate how the combination of their properties can be applied for the solution of specific problems.