Measuring Solvent Hydrogen Exchange Rates by Multifrequency Excitation ¹⁵N CEST: Application to Protein Phase Separation

Solvent exchange rates provide important information about the structural and dynamical properties of biomolecules. A large number of NMR experiments have been developed to measure such rates in proteins, the great majority of which quantify the buildup of signals from backbone amides after initial perturbation of water magnetization. Here we present a different approach that circumvents the main limitations that result from these classical hydrogen exchange NMR experiments. Building on recent developments that enable rapid recording of chemical exchange saturation transfer (CEST) pseudo-3D data sets, we describe a ¹⁵N-based CEST scheme for measurement of solvent exchange in proteins that exploits the one-bond ¹⁵N deuterium isotope shift. The utility of the approach is verified with an application to a 236 residue intrinsically disordered protein domain under conditions where it phase separates and a second application involving a mutated form of the domain that does not phase separate, establishing very similar hydrogen exchange rates for both samples. The methodology is well suited for studies of hydrogen exchange in any ¹⁵N-labeled biomolecule. A discussion of the merits of the CEST experiment in relation to the popular CLEANEX-PM scheme is presented.