Modern Polymer Spect..

966 5 Vibr-atioiial Slm-troscopj, of' Pol.vpeptides
empirical force field is capable of giving very acceptable frequency agreement:
the rms error between observed and calculated frequencies is 5.9 cm-'. Second, the
character of the symmetry species is well predicted: A species modes exhibit only
Rainan activity, B1 and B? species modes exhibit 1) and I IR dichroism. respectively.
Third, while the agreement is somewhat poorer, band shifts on N-deuteration
are reasonably well accounted for [88] (for amide I the observed downshift of
4 cn-' is reproduced exactly by the calculation). Finally, TDC is crucial to
explaining the large splittings in the amide I and amide I1 modes: in its absence the
ainide I modes are calculated at 1670 (A), 1673 (B,), 1665 (B2), and 1670 (B3j cin-.',
and the amide I1 modes are predicted at 1545 (A), 1549 (BI), 1550 (B?), and 1554
(B3) c111-l [5].
With respect to the amide modes, a comparison with the results of the ah initio
calculation on the hydrogen-bonded Ala dipeptide is of interest, even though the
force fields are different. Although localized primarily in CO s, the amide I modes
of /I-PLA are constrained by symmetry to be coupled modes of the four peptide
groups in the unit cell. Interestingly, CN s is now the second major component
compared with NH ib foi- the Ala dipeptide. The coupled amide I1 modes contain,
in addition to NH ib and CN s, C"C s and CO ib contributions not present (at the
level of 2 10) in the Ala dipeptide. With respect to the strong amide 111 modes, at
1243 and 1224 cni-', H" b dominates and NC" s also contributes in addition to NH
ib and CN s. As noted above, NH ib is a contributor to many other modes in
the 1400-1200 cm-l region. The amide V modes are well accounted for, and in
particular the observed absence of any significant dichroism in this band 1911 is
explained by the near coincidence in frequency of the predicted B1 and B3 species
modes. Besides the aniide modes, local as well as other skeletal modes are very well
accounted for.
The excellent agreement obtained for /I-PLA would indicate that this force field
should be transferable to other APPS polypeptides, of course taking due account of
side-chain differences. This has been done for Ca-poly( L-glutamate), Ca-PLG (921,
which X-ray and electron diffraction data had indicated to have an APPS structure
but which were not extensive enough to provide a definitive conclusion. A proposed
model [93] was used as the basis for the vibrational analysis, and the good prediction
of the observed bands [92] supports both the model as well as the viability
of the force field. The subtle differences between the P-PLA and /I-Ca-PLG spectra
are accounted for by the normal mode calculation, and emphasize the influence
of the side chain on the main-chain modes, particularly on amide 111. A similar
successful vibrational analysis has been done on an alternating copolymer, APPS
poly( L-alanylglycine) [ 191.
Antiparallel-Chain Rippled Sheet
Although early studies assumed that PG also adopts an APPS structure, electron
diffraction studies of 'single crystals' and oriented thin films [94] suggested that
extended-chain PG, PGI, has an APRS structure. This was supported by conforniational
energy calculations [95], and was the basis for a detailed vibrational
analysis of the PGI spectra [19, 961, strongly confirming this proposal. (Since the