Modern Polymer Spect..

280 5 Vibrational Spectroscopy of Polvpeytides
essentially threefold helices [ 122, 1231 are completely intermolecularly N-H . .. O=C
hydrogen bonded (Figure 5-8), with strong spectroscopic evidence [ 1231 for additional
C"-H...O=C hydrogen bonds. The basis for this conclusion is of interest
since it demonstrates the sensitivity of the spectroscopic technique to subtle features
of the structure.
The vibrational analysis [ 1231 was based on comparing the normal modes of
a parallel-chain and an antiparallel-chain structure of PGII. In both structures all
N-H... O=C hydrogen bonds can be made. However, whereas in the parallel-chain
arrangement all CH2 groups participate equivalently in C"-H ... O=C hydrogen
bonds (implying that all C=O groups have bifurcated hydrogen bonds), in the
antiparallel-chain arrangement only every third CH2 group can be involved in such
interactions, and only between like-directed chains. The result is that the chain no
longer has strict threefold symmetry, and as a result the degeneracy of the E species
modes is broken. This leads to the expected, and observed, presence of a larger
number of bands than predicted by a strictly threefold-symmetric chain structure,
and also of bands that occur in regions where no modes are predicted for the
parallel-chain structure. Such symmetry-based conclusions are powerful constraints
on structural possibilities.
The antiparallel-chain structure (Figure 5-8) has only a screw axis of symmetry,
and its modes are distributed as follows: A-62 modes, Raman, IR; B-61 modes,
Raman, IR. As for PGI, extensive IR spectra on isotopic derivatives (NH, CD2;
ND, CH?; and ND, CD2) were available [97] as well as Raman spectra on the N-
deuterated molecule [98]. The PGI force field [96] was used as a starting point, and
refinement required small adjustments in 10 of -70 intramolecular force constants
[123]. Amide mode frequencies are given in Table 5-12; the overall rnis frequency
error is 5.4 cm-'.
The hydrogen bond in PGII is stronger than that in PGI (N...O=2.69 b, vs
2.91 A), so it is not surprising that the average amide I mode frequency is lower in
PGII (the unperturbed modes are at 1651.0 f 1.7 cm-I). The very different splittings
are due to the different TDC interactions for the two structures. The somewhat
higher amide I1 modes are also consistent with stronger hydrogen bonds. The
amide I11 modes with significant NH ib, CN s contributions are found near 1280
cm-', strongly mixed with CH2 tw, quite different from the situation in PGI. The
isolated NH ib contribution is now found in the 1400-1300 cm-' region. The aniide
V modes in PGII are divided between a higher frequency group near 750 cmP1 and
a lower frequency group near 670 cm-'. This is a much larger separation than
found for other helical structures, and may be a consequence of the interchain
hydrogen bonding.
5.5 Summary
The main message conveyed by the body of present vibrational spectroscopic
studies of peptides and polypeptides is that detailed information on their structures