Modern Polymer Spect..

254 5 Vibrational Spectvoscopj? of Polypeptides
Keeping in mind that the peptide groups are not identical, because of the different
chemical surroundings, we find that aniide I frequencies, because of their highly
localized nature, generally follow the changes in CO s force constants with conformation
[33]. These in turn are determined by the changes in CO bond length (the
relationship is essentially linear [33]), which reflect the changing bonded and nonbonded
interactions in the molecule with conformation. The larger variation in CO
s2 (41 cm-') than in CO sl (16 cni-'1 is probably a result of its more variable conformational
environinent adjacent to C", and therefore the varying (even if small)
contributions of other coordinates. It is interesting that in a' the frequency order is
reversed compared with the other conformers.
The aiiiide I1 modes are less sensitive to conformation: the variation in either
mode is of the order of 10 cm-', although the splittings do vary. As with amide I,
the different frequencies of the two groups are mainly a reflection of their nonidentical
chemical environments.
The latter fact is demonstrated dramatically by the aniide I11 mode: the eigenvectors
for the two peptide groups are significantly different, that of the higher fre-
quency mode containing no PED contribution (210) of H" bend (b) although this
coordinate is a major contributor to the lower frequency mode. Nor is NH ib a
major contributor, as it was in NMA (in fact, it does not appear in the p2 modes).
The large variations in frequency separation between the two amide I11 modes
reflect the effect of conformation through H" b, since if H" is replaced by D" this
variation disappears (the bands are now at 1294 & 1 and 1266 k 6 cin-'j [33]. This
agrees with previous observations [5] that the amide I11 mode in polypeptides contains
a significant conformation-dependent contribution from H" b. The interaction
between H" b and NH ib is illustrated by the fact that in an (ND)? Ala dipeptide
H" b occurs at 1300 and 1296 (p2), 1320 and 1272 ( a~),
1305 and 1283 ( a~), and
1324 and 1284 (a') cm-' [33]. The H" b coordinate also mixes with CN s to predict
significant IR bands at 1277 cm-' in p2 and 1291 cm-' in a' [33]. Thus, with its
added sensitivity to side-chain composition [5], amide I11 in polypeptides is no
longer a simple characteristic NH ib, CN s mode.
The amide V mode, which appears as a single band near 400 cn-' in NMA (see
Table 5-l ), is split in the nonhydrogen-bonded conformers into at least two bands
with predominant NH ob, CN t contributions. (These paired coordinates can make
significant contributions to other modes, and each contributes individually to still
others.) It might be thought that this splitting (and higher average frequency) is
a result of the arbitrary scale factors that were used [33]. However, 4-31G" calculations
with scale factors transferred from NMA [59] give similar results [73]. Thus,
we must conclude that kinematic as well as potential coupling between the two
peptide groups is responsible for the nonlocalized contribution of these coupled
coordinates.
The presence of hydrogen bonding obviously introduces a new element into the
modes of the Ala dipeptide. This can be examined from the results on the three
stable hydrogen-bonded conformers, C5, C7E, and C7A (Figure 5-1) [34]. In Table
5-3 we show their calculated amide I, 11, 111, and V frequencies and PEDs.
The relative amide I mode frequencies again generally follow the relative values
of the CO s force constants [34], although mixing of the two CO s displacements can