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

The backbone dynamics of the bacteriorhodopsin fragment (1-36)BR solubilized in a 1:1 chloroform/methanol mixture were investigated by heteronuclear ¹H-¹⁵N NMR spectroscopy. The heteronuclear ¹⁵N longitudinal and transverse relaxation rates and ¹⁵N{¹H} steady-state NOEs were measured at three magnetic fields (11.7, 14.1, and 17.6 T). Careful statistical analysis resulted in the selection of the extended model-free form of the spectral density 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 α-helical part of the peptide (residues 9-31) are characterised by nanosecond and picosecond motions with mean order parameters S(s)² = 0.60 and S(f)² = 0.84, respectively. The nanosecond motions were attributed to the peptide's helix-coil transitions in equilibrium. Residues 3-7 and 30-35 also exhibit motions on the pico- and nanosecond time scales, but with lower order parameters. Residue 10 at the beginning of the α-helix and residues 30-35 at the C-terminus are involved in conformational exchange processes on the microsecond time scale. The implications of the obtained results for the studies of helix-coil transitions and the dynamics of membrane proteins are discussed.