| Summary: | Solid-state NMR spectroscopy was used to site-specifically investigate the protein-water interface of a seven alpha-helical transmembrane protein, Anabaena sensory rhodopsin (ASR). Water-edited experiments, which employ a T2-filter to select for mobile protons, provided a means to detect residues which appear to be in close contact to water molecules, and to gain insights about the water-protein interface of ASR. First, through the application of Lee-Goldburg homonuclear decoupling, it was determined that polarization transfer across this interface is dominated by through-space interaction mechanisms, as opposed to chemical exchange. A series of two-dimensional experiments were also performed to detect polarization transfer along the backbone and to the sidechains of the protein. Residues located in solvent-accessible regions of the protein, such as the B-C loop, were found to obtain polarization quickly, as expected, and in agreement with previous H/D exchange data. Residues known to be in contact with bound crystal water molecules were also detected. In addition to these, we found new residues which appear to be in contact with water, indicating additional HN-H2O interactions, or additional contacts with bound water molecules. Most of these residues were located beside exchangeable regions of ASR. Sidechains of residues located in the cytoplasmic side of helix F were seen to be in close contact with mobile water molecules, supporting evidence of a hydrophilic chain along the cytoplasmic half of the protein, which is suggested to cause a functional outward tilt of the cytoplasmic half of helix F upon light-activation.
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