Modern Polymer Spect..

5.4 Polypeptides 267
glycine residue is achiral, it can adopt the D or L configuration required of alternate
chains in the APRS.) This was aided by extensive IR spectra of isotopic derivatives
(NH, CD2; and ND, CD2) [97] and by Raman spectra [9S], which were important
because macroscopic samples of oriented PGI could not be prepared.
The conclusion from vibrational analysis that PGI has an APRS structure was
based on a detailed TDC study of the aniide I mode splittings for the APPS as
compared with the APRS as a function of the axial shift between adjacent chain
[19]. No reasonable shift values for the APPS were in agreement with experiment,
whereas the APRS gave excellent agreement for a shift of 0 A, exactly that indicated
by the electron diffraction analysis [94]. This structure (Figure 5-3) has a
center of inversion and therefore the mutual exclusion rule applies, giving the
following symmetry species and activity [ 191: As-21 modes, Raman; A,-20 modes,
IR( 11); Bg-21 modes, Rainan; Bu-19 modes, IR(I). The results of the normal mode
analysis [96] give an overall rnis error of 6.1 cm-', comparable with that for P-PLA.
(The force field for p-PLA was in fact refined from that for PGI [80].) A description
of the amide modes of PGI is given in Table 5-7.
The splitting of the amide I modes, somewhat smaller than for the APPS structure,
is again due to TDC: in its absence the calculated frequencies are 1684 (Ag),
1677 (Au), 1676 (Bg), and 1674 (B,) cm-I. Compared with APPS PLA, CTN d
now makes a noticeable contribution to the eigenvector.
The amide I1 modes, which are probably at a lower frequency because of the
weaker hydrogen bonds, are also split by the TDC interactions: in their absence
the calculated modes are at 1534 (Ag), 1535 (A"), 1559 (Bg), and 1559 (B,]) cni-'.
The absence of clearly observed high-frequency modes, particularly an IR band
near the expected frequency of 1572 cm-I, raises a very important point: it is necessary
to be able to predict intensities as well as frequencies. Intensities require
knowledge of dipole derivatives (Eq. (5-4)), but since they also depend on correct
eigenvectors (Eq. (5-5)) they provide an independent test of the force field. Such an
1R intensity calculation has been done on PGI [20], and the results show that the B,
amide I1 mode is indeed predicted to be very weak, in agreement with observation.
The amide I11 mode region is quite complex, with mainly NH ib, CN s contributing
near 1300 and 1200 ciii-', where only weak bands are observed. In the
'expected' region (1300-1200 cm-'), the strong bands are due to CH? twist (tw)-
dominated vibrations. The NH ib contributions now extend to bands about 1400
and below 1200 cm-' .
The amide V mode is at a similar frequency to that in ,!I-PLA, even though the
PGI hydrogen bond is weaker. This may be because of the large CN t contribution
and/or the somewhat different nature of the modes.
5.4.1.3 Parallel-Chain Structures
Parallel-Chain Pleated Sheet
Although there are no known PPS polypeptides, this general structure is found
in proteins [2] and in some tripeptides. in two of which it has been the subject of
detailed vibrational analyses [75, 771. The excellent prediction of observed I R and