Modern Polymer Spect..

5.4 Polypeptides 269
Raman bands in these cases, using the same empirical force field as for APPS PLA
[5], led to the belief that it would be useful to obtain the frequencies of PPS PLA in
order to characterize the vibrational spectrum of this structure. The unperturbed
normal modes have been obtained, and the influence of TDC interactions on the
aniide I and ainide I1 modes, as determined by the intersbeet separation, has been
assessed [82]. While unperturbed aniide I modes occur at 1678 and 1675 cm-I, TDC
interactions within one sheet shift these to 1663 and 1642 cm-' and a multisheet
array ( 11 x 11 x 11 residues) with a separation of 5.3 A (comparable with that in
APPS PLA) leads to modes at 1669 and 1677 cm-'. A similar effect is predicted for
ainide 11: single sheet modes are at 1553 and 1586 c~ii-', with the above niultislieet
separation giving 1549 and 1583 cin-' .
Parallel-Chain Rippled Sheet
A PRS structure is only possible for PG, and is ruled out by the evidence for an
APRS structure for this polypeptide. However, the norinal, modes of PRS PG have
also been calculated (821. It is interesting that, when multisheet TDC coupling is
included, the strong Raman amide I mode is predicted to be at a much lower frequency
than the strong IR amide I mode, just the opposite of what is observed and
predicted for the APRS structure. This demonstrates the power of detailed vibrational
spectroscopic analysis in revealing fine details of polypeptide structure.
5.4.2 Helical Polypeptide Structures
5.4.2.1 General Features
The concept of a helical structure as the most general one for a long polymeric
chain was developed quite early [99, 1001, and possible polypeptide helices having
an integral number of residues per turn were described [loll. However, it was only
when this constraint was relaxed and well-defined stereochemical criteria invoked
[lo21 that a specific conforination, the or-helix, was discovered [102, 1031 that is in
fact found in polypeptides and proteins [2]. A number of other helical conforniations
have been found since then, and their structural parameters, are given in Table
5-8. Vibrational analyses of some of these are described in this section.
Polypeptides are found in both single-stranded intramolecularly hydrogenbonded
helices as well as in conformations in which hydrogen bonds are fornied
between helices. In the former case, the structure is conveniently described by
several parameters: the number of residues per turn, n (positive if right-handed,
negative if left-handed); the axial translation per residue, h; and the backbone torsion
angles, p, $, and o. Sometimes the number of atoms in the 'ring' formed by the
hydrogen bond, in, is specified [loll (and the residue connectivity via the hydrogen
bond can be given; thus, 5 - 1 iiieans the NH group on residue 5 is bonded to the
CO group on residue 1). In such a case, the helix will be designated PI,?, [loll.
Among such helices. the a-helix (3.6213) structure has been obtained froin a
careful analysis of X-ray fiber diffraction patterns of PLA [104]. As is general for
helical structures [ 1051, this helix, designated, a ~, has a counterpart, designated all,