Modern Polymer Spect..

272 5 ViDrutiomil Spectroscopy oj Polvpepticles
Figure 5-8. Antiparallel-chain structure of polyglycine 11. (From [ 1231)
vibrational analysis of the two structures 11231 showed that the antiparallel-chain
structure (Figure 5-8) was indeed preferred. In these studies, the suggestion that
C"-H ... O=C hydrogen bonds were present [ 1241, which was supported by spectroscopic
studies [ 125, 1261, was convincingly confirmed by normal-mode analysis
[123]. The same chain symmetry, in a required left-handed helix, is found in polyproline
II, which has trans peptide groups 1127, 1281, a short contact again suggesting
the possibility of a C"-H...O=C hydrogen bond [129]. It is interesting that,
from circular dichroisrn studies [130], a polyproline I1 type of structure, with 2.5-
3.0 residues/turn [131], was indicated to be a preferred local conformation in many
'unordered' polypeptides in solution. This proposal has since received strong support
from Rainan and normal-mode studies [ 1321 as well as from a more detailed
analysis of the circular dichroism results [133]. A triplet of left-handed PGII or
polyproline 11 helices can be thought of as the topological parent of the triplestranded
right-handed coiled-coil structure of collagen [ 134, 1351. When the peptide
groups in polyproline take a cis configuration, i.e., polyproline I, the chain adopts a
right-handed 10 residue/3 turn helical structure [ 1361.
The variety of helical as well as extended-chain and turn [5] structures emphasizes
the flexibility of the polypeptide chain, and thereby the importance of achieving a
full understanding of its vibrational dynamics as a function of conformation.