2H NMR determination of the global correlation time of the gramicidin channel in a lipid bilayer.

A detailed experimental description of molecular dynamics requires an accurate description of the global correlation time. For the gramicidin cation-selective channel the local dynamics of the polypeptide backbone are thought to play a very significant role in the functional process of this channel which occurs on the 10-100-ns time scale. By solid-state NMR spectroscopy an experimental description of the local dynamics including a description of the axis about which the motions occur, the amplitude of the motions, whether they are diffusional or discontinuous and the frequency of the motions is possible. Initial interpretations of the NMR data for this polypeptide backbone in fully hydrated lipid bilayers suggests that the time scale for the local motions is the same as the kinetic time scale (North and Cross, 1993). Here the global correlation time is found from d4-Ala3 and d4-Ala5-gramicidin A powder pattern spectra to be 36 microseconds at 309 K. This surprisingly long correlation time may reflect the high viscosity of the peptide environment or possibly the higher molecular weight of a peptide-lipid aggregate. Furthermore, this reassessment of the global correlation time supports the initial interpretation of relaxation data for the local motions.