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 α‐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.