Sequence-specific resonance assignment and secondary structure of (1-71) bacterioopsin

The conformation of chymotryptic fragment C2 of bacteriohodopsin (residues 1-71) was studied by 2D 1H NMR. The fragment was solubilized in a mixture of chloroform/methanol (1:1), 0.1 M LiClO4. Most of the resonances in 1H NMR spectra of fragment C2 were assigned using phase-sensitive DQF-COSY, TOCSY, and NOESY techniques. To simplify the assignment procedure for overlapping regions of NMR spectra, an analog of fragment C2 with leucines deuterated in beta-positions was used. Deuterium exchange rates for amide protons were measured in a series of TOCSY spectra. Two right-handed alpha-helical regions Pro8-Lys30 and Lys41-Leu62 were identified on the basis of NOE connectivities and deuterium exchange rates. The N-terminal part of the fragment (Ala2-Gly6) adopts the helical conformation stabilized by 3 hydrogen bonds.