H-1-N-15 NMR dynamic study of an isolated alpha-helical peptide (1-36)- bacteriorhodopsin reveals the equilibrium helix-coil transitions

The backbone dynamics of the bacteriorhodopsin fragment (1-36)BR solubilize d in a 1:1 chloroform/methanol mixture were investigated by heteronuclear H -1-N-15 NMR spectroscopy. The heteronuclear N-15 longitudinal and transvers e relaxation rates and N-15{H-1} steady-state NOEs were measured at three m agnetic fields (11.7, 14.1, and 17.6 T). Careful statistical analysis resul ted in the selection of the extended model-free form of the spectral densit y function [Clore et al. (1990) J. Am. Chem. Soc., 112, 4989-4991] for all the backbone amides of (1-36)BR. The peptide exhibits motions on the micro- , nano-, and picosecond time scales. The dynamics of the alpha-helical part of the peptide (residues 9-31) are characterised by nanosecond and picosec ond motions with mean order parameters S-s(2) = 0.60 and S-f(2) = 0.84, res pectively. The nanosecond motions were attributed to the peptide's helix-co il transitions in equilibrium. Residues 3-7 and 30-35 also exhibit motions on the pico- and nanosecond time scales, but with lower order parameters. R esidue 10 at the beginning of the alpha-helix and residues 30-35 at the C-t erminus are involved in conformational exchange processes on the microsecon d time scale. The implications of the obtained results for the studies of h elix-coil transitions and the dynamics of membrane proteins are discussed.