NOTES ON THE LOCAL FLEXIBILITY OF THE POLYPEPTIDE BACKBONE .1. MEAN AMPLITUDES OF THERMAL MOTIONS IN THE DIPEPTIDE MODEL

Mean amplitudes of thermal motions were calculated in the 0 K-300 K interval at 0, 80, 160 and 300 K using an SQM ab initio HF/4-21 complete harmonic force field for N-methyl-N'-methylalaninamide in the C7eq conformation. It is shown that no specific vibrational mode gives rise to motions which would be both of large amplitude and delocalised in nature. Rather, the two peptide units are rather rigid, allowing only small amplitudes, even if they are delocalised, whereas large amplitude motions appear to be localised. It is suggested that collective modes consist of spatially subsequent, uncoupled motions.