Sequence-specific 1H-NMR assignment and conformation of proteolytic fragment 163-231 of bacterioopsin

Proteolytic fragment 163-231 of bacterioopsin was isolated from Halobacterium halobium purple membrane treated with NaBH4 and papain under nondenaturing conditions. Two-dimensional 1H-NMR spectra of (163-231)-bacterioopsin solubilized in chloroform/methanol (1:1), 0.1 M LiClO4 indicated the existence of one predominant conformation. Most of the resonances in the 1H-NMR spectra of (163-231)-bacterioopsin were assigned by two-dimensional techniques. Two extended right-handed alpha-helical regions Ala168-Ile191 and Asn202-Arg227 were identified on the basis of NOE connectivities and deuterium exchange rates. The N-terminal part of the peptide is flexible and the region of Gly192-Leu201 adopts a specific conformation. The protons of OH groups of Thr178, Ser183 and Ser214 slowly exchange with solvent, and side-chain conformations of these residues, as evaluated by NOE connectivities of OH protons, are optimal for the formation of hydrogen bonds between OH and backbone carbonyl groups.