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A 47 years-old story that started with the discovery of an ordered organosilane monolayer that assembles itself on various polar surfaces has evolved into an ongoing “research thriller” craving explanations for a series of unusual experimental findings. Using interfacial electron beam lithography – a novel approach to chemical surface patterning that allows fabrication of hybrid inorganic-organic monolayer structures spanning nano-to-macroscale dimensions, we fabricate metal (Ag)-monolayer quasinanowires on silicon with micrometer-centimeter lengths and planned layouts that exhibit puzzling electrical conduction. Depending on the composition and structure of the quasinanowire and the nature of the silicon support, the room-temperature resistivities of such surface entities may vary between that of a practical insulator to some extremely low values. These findings defy rationalization in terms of conventional electrical conduction mechanisms. Interfacial systems with characteristic structural features akin to those of our quasinanowires have, however, been proposed in both the exciton model of high-temperature superconductivity (Little, Ginzburg, 1964-70) and that of superconductivity by the pairing of spatially separated electrons and holes (Lozovik & Yudson, 1976). While gathering additional clues that might shed light on the mystery of our thriller, these theoretical predictions spur us to seek the shining light at the end of the tunnel...

Proteins are highly dynamic biomolecules that can undergo ligand-induced
conformational changes, thus often playing a crucial role in biomolecular processes. For
an in-depth understanding of protein function, the conversion of one conformational state
into another has to be resolved over space and time. Pulsed electron-electron double
resonance spectroscopy (PELDOR/DEER) in combination with site-directed spin
labelling (SDSL) is a powerful tool for obtaining distributions of interspin distances in
proteins [1, 2]. It allows for measurements with Angstrom precision, but it cannot directly
determine the time scale and the mechanism of the conformational change. However,
coupling PELDOR with rapid freeze-quench techniques adds the time axis to the
distance distribution and thus permits studying conformational changes with temporal
resolution.
Here, we show that the combination of Microsecond Freeze-Hyperquenching (MHQ) [3]
and PELDOR resolves ligand-triggered conformational changes in proteins on the
Angstrom length and microsecond time scale. It allows taking snapshots along the
trajectory of the conformational change by rapid quenching within aging times of
82-668 μs, and it is applicable at protein amounts down to 7.5 nmol (75 μM, 100 μL) per
time point. We applied MHQ/PELDOR to the cyclic nucleotide-binding domain (CNBD)
of the MloK1 channel from Mesorhizobium loti, which undergoes a conformational
change upon binding of cyclic adenosine monophosphate (cAMP). We observed a
gradual population shift from the apo to the holo state on the microsecond time scale,
but no distinct conformational intermediates (Fig. 1a, b). [4]
Figure 1: a) Interspin distance distributions obtained at different aging times and b) the corresponding
fractions of apo and holo state. c) Free-energy profile of the ligand-induced conformational change.
Corroborated by measurements of ligand-binding kinetics and molecular dynamics (MD)
simulations, we interpret the data in terms of a dwell time distribution. The transitions
across the free-energy barriers (Fig. 1c) i.e., ligand binding and the conformational
change, are on the nanosecond time scale and thus below the time resolution of the
MHQ device. However, the dwell time of the apo state in complex with the cAMP ligand
is in the microsecond range and can be monitored by MHQ/PELDOR. [4]
Literature:
[1] A.D. Milov et al., Fiz. Tverd. Tela 1981, 23, 975-982. [2] G. Jeschke, Annu. Rev. Phys. Chem.
2012, 63, 419-446. [3] A.V. Cherepanov et al., Biochim. Biophys. Acta 2004, 1656, 1-31.
[4] T. Hett et al., J. Am. Chem. Soc. 2021, 143, 6981-6989.