Modern Polymer Spect..

5.4 Polypeptides 257
nonhydrogen-bonded conformers. While NH ib contributes (at the level of 2 10)
to the higher frequency C5 and C7E modes, it is absent from the C7A modes.
Furthermore, if we ascribe amide I11 character to H" b, CN s modes with minimal
NH ib, then C7A has such modes at frequencies well over 1300 cnrl, again
reflecting the conformation-dependent contribution of H" b. (The situation is
somewhat reversed in the crystalline Ala dipeptide [74], with a calculated mode at
1325 cni-' showing traditional CN s, NH ib character and modes at 1272 and 1258
cm-' having H' b, CN s character.) Since NH ib can contribute throughout the
-1400-1200 cn1-l region, and can mix variably with CN s and Ha bend, it is not
surprising that the amide I11 modes are a complex function of conformation.
The amide V modes are dominated by the effects of hydrogen bonding. Thus, the
free amide V mode is at -580 cni-' in all three conformers, while the bonded
modes reflect the increasing hydrogen bond strengths: -690 cm-l in C5 (the splitting
probably due to an accidental resonance with another mode), 754 cm-' in C7E,
and 809 cni-' in C7A. Hydrogen bonding thus seeins to mask conforinational
differences in amide V.
These studies on NMA and the Ala dipeptide illustrate that, although it is useful
to characterize peptide spectra in terms of supposedly localized amide modes, it is
the subtle differences in the detailed nature of these vibrations with varying conformation
and hydrogen bonding that provide the spectral clues to differences in
structure. This is demonstrated by analyses of tripeptides [28, 75-77], which also
involve interactions between two peptide groups. Although done with an empirical
force field [5], these analyses show the sensitivity of vibrational spectra to specific
structural features. This emphasizes the importance of highly accurate force fields
in enabling vibrational analysis to realize its potential for being a powerful tool to
study the structures and forces in macromolecules.
5.4 Polypeptides
As will be obvious from our previous discussion, a comprehensive understanding of
the vibrational spectra of polypeptides depends on having reliable normal mode
analyses to combine with appropriate experimental studies. Such analyses in turn
rest on the quality of the force field. At the present time, an SDFF for the polypeptide
chain is not available, the most detailed normal mode analyses being based
on extensive empirical force fields [5]. These force fields have been tested on tripeptides
of known structure with excellent predictive results [28, 75-77], and have
proven to be very satisfactory for polypeptide systems. For reasons given in Section
5.2.1, however, we must be careful about a final acceptance of all of their detailed
predictions. Acknowledging these reservations, the general features of polypeptide
spectra provided by the present detailed empirical force fields can nevertheless be
accepted with confidence.
In this section, we describe results of some of the vibrational analyses that
have been done on the main secondary conformations found in polypeptides, viz.,