THE BACKBONE N-15 CHEMICAL-SHIFT TENSOR OF THE GRAMICIDIN CHANNEL - AMOLECULAR-DYNAMICS AND DENSITY-FUNCTIONAL STUDY

The short-time-scale fluctuations in N-15 chemical shift tensors are e xamined for an ensemble of N-methylacetamide complexes constructed fro m a molecular dynamics (MD) trajectory of the gramicidin A channel in a fully hydrated phospholipid bilayer. Sum-over-states density functio nal perturbation theory (SOS-DFPT) calculations reveal fluctuations in the magnitudes and orientations of the individual tenser components o n the picosecond to femtosecond time scale. The MD/SOS-DFPT technique emerges as a valuable tool for gaining insight into the nature of the terms that enter the motionally averaged spectra observed experimental ly. More extensive sampling should provide first-principles prediction s of the spectra under experimental conditions.