The backbone 15N chemical shift tensor of the gramicidin channel. A molecular dynamics and density functional study

The short-time-scale fluctuations in 15N chemical shift tensors are examined for an ensemble of N-methylacetamide complexes constructed from a molecular dynamics (MD) trajectory of the gramicidin A channel in a fully hydrated phospholipid bilayer. Sum-over-states density functional perturbation theory (SOS-DFPT) calculations reveal fluctuations in the magnitudes and orientations of the individual tensor components on 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 experimentally. More extensive sampling should provide first-principles predictions of the spectra under experimental conditions.