Vibrational analysis of peptides, polypeptides, and proteins. XV. Crystalline polyglycine II

A force field has been refined for the 3 1 ‐helix structure of polyglycine II, using the polyglycine I force field plus previous C α H α …︁O force constants as a starting point. Besides force constants associated with the hydrogen bonds, which must change since the hydrogen‐bond characteristics are different in the two structures, we have had to modify only 10 force constants from the polyglycine I force field to make it suitable for reproducing the polyglycine II frequencies. Most involve the NC α bond, which is the torsion angle that changes from the I to the II structure. Calculations were done for parallel chain and antiparallel chain crystal structures of polyglycine II, the observed spectra being found to agree best with the latter structure. Since this provides strong evidence for the loss of strict threefold symmetry in the chain, our analysis strengthens the support for the existence of C α H α …︁O hydrogen bonds in the structure of polyglycine II.