Pressure effect on the dynamics of an isolated alpha-helix studied by N-15-H-1 NMR relaxation

Dynamics and structure of (1-36)bacteriorhodopsin solubilized in chloroform /methanol mixture (1:1) were investigated by H-1-N-15 NMR spectroscopy unde r a hydrostatic pressure of 2000 bar. It was shown that the peptide retains its spatial structure at high pressure. N-15 transverse and longitudinal r elaxation times, N-15{H-1} nuclear Overhauser effects, chemical shifts and the translation diffusion rate of the peptide at 2000 bar were compared wit h the respective data at ambient pressure [Orekhov et al. (1999) J. Biomol. NMR, 14, 345-356]. The model free analysis of the relaxation data for the helical 9-31 fragment revealed that the high pressure decreases the overall rotation and translation diffusion, as well as apparent order parameters o f fast picosecond internal motions (S-f(2)) but has no effect on internal n anosecond motions (S-s(2) and tau(s)) of the peptide. The decrease of trans lation and overall rotation diffusion was attributed to the increase in sol vent viscosity and the decrease of apparent order parameters S-f(2) to a co mpression of hydrogen bonds. It is suggested that this compression causes a n elongation of H-N bonds and a decrease of absolute values of chemical shi ft anisotropy (CSA). In particular, the observed decrease of S-f(2) at 2000 bar can be explained by 0.001 nm increase of N-H bond lengths and 10 ppm d ecrease of N-15 CSA values.