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Nuclear magnetic resonance (NMR) spectroscopy is a powerful analytical tool for the characterization of protein structure and intermolecular interactions. However, NMR is not readily applicable to determine fast structural changes and weak interactions between molecules because of low signal sensitivity and time requirements to record multi-dimensional NMR spectra. To overcome these limits, the hyperpolarization technique of dissolution dynamic nuclear polarization (D-DNP) is combined with NMR. Not all molecules can be directly hyperpolarized. Instead, polarization transfer from hyperpolarized small molecules to a target of interest can be utilized as a means of obtaining polarization, as well as for detecting intermolecular interactions between these molecules Here, hyperpolarized water-assisted NMR spectroscopy was developed to measure intermolecular interactions with water. Firstly, the use of DNP hyperpolarization was demonstrated for the accurate determination of intermolecular cross-relaxation rates between hyperpolarized water and fluorinated target molecules.[1]
Because hyperpolarized water acts as a source spin with a large deviation of the population from the equilibrium, the 19F signal on the target molecules is enhanced through NOE, allowing obtain an entire NOE buildup curve in a single, rapid measurement. When the hyperpolarized water-assisted NMR experiment is applied to a protein, water hyperpolarization can be transferred to amide protons on the protein through proton exchange. Further, this polarization spreads within the protein through intramolecular NOE to nearby protons including aliphatic groups.[2] By utilizing this polarization transfer, this method extends to measure enhanced 2D NMR spectra of the protein under folded and refolding conditions.[3] With the ability to rapidly measure protein signals that were enhanced through transferred polarization from hyperpolarized water, NMR spectra can be acquired within the timescale of the protein folding. Compared to the folded protein experiment, signals attributed to exchange-relayed NOEs are not observable in the refolding experiment (Figure 1b). These differences are explained by the absence of long-range contacts with nearby exchangeable protons such as OH protons