Detecting water-protein chemical exchange in membranebound proteins/peptides by solid-state NMR spectroscopy

Water plays an important role in many essential biological processes of membrane proteins in hydrated lipid environments. In general, the1H polarization transfers between water molecules and site-specific protons in proteins can be classified as coherent（via dipolar spin diffusion） and incoherent（via chemical exchange and nuclear Overhauser effect） transfers.Solid-state NMR is the technique of choice for studying such water-protein interactions in membrane-bound proteins/peptides through the detection of1H polarization transfers from water to the proteins. These polarization transfer mechanisms often exist simultaneously and are difficult to quantify individually. Here, we review water-protein polarization transfer techniques in solid-state NMR, with a focus on the recent progress for the direct detection of site-specific kinetic water-protein chemical exchange processes on the sub-millisecond time scale in membrane-bound proteins. The measurements of the pure chemical exchange kinetics provide a unique opportunity to understand the role that water plays in the structurefunction relationships of membrane-bound species at the water-bilayer interface. In addition, the perspective of chemical exchange saturation transfer（CEST） experiments in membrane-bound proteins/peptides is further discussed.