Modern Polymer Spect..

I I I I I I I I I I I I I
1600 1400 I200 loo0 800 Mxl 400 (crn-'l
Figure 5-10. Ratnan spectra of a-poly (L-alanine) (-)
381)
Undeuterated. ( ..' ) N-deuterated. (From
The observed and calculated amide I mode splittings agree quite well, and are
much smaller than for p-PLA. (The specific A-Ez splitting, due to TDC, remains to
be verified; this will depend on having good polarized Raman spectra.) The mode
now contains less CN s and more C"CN d than P-PLA, and a change in the small
(< 10) NH ib contribution is largely responsible for the observed 8 cni-' downshift
(calculated 10 cm-' ) on N-deuteration [ 1061. The calculated unperturbed amide I1
modes at 1529 (A), 1532 (El), and 1536 (Ez) cn1-l again show how important TDC
is in explaining the observed 29 cn1-l A-El splitting. The NH ib contribution to
amide I11 now dominates over Ha b as compared with p-PLA, and CN s does not
appear. The increase in these frequencies, from the 1240-1220 cm-I region in p-
PLA to the 1280-1260 cm-' region in a-PLA, must be due to the difference in
conformation (and the resulting change in interaction between H" b and NH ib),
since the relevant force constants are in fact smaller in the a-helix than in the p-sheet
[106]. The lower amide V frequencies in a-PLA, 658 and 618 cm-l, than in /I-PLA,
706 cin-', are undoubtedly a result of the weaker hydrogen bond in the helical
structure (N...O = 2.86 A in a-PLA versus 2.73 A in P-PLA).
Other synthetic polypeptides adopt the al-helix structure, and it is of interest to
see how their normal modes are influenced by a change in the side chain. A polypeptide
that has been analyzed with the same main chain force field as a-PLA is
a-poly(L-glutamic acid), a-PLGH [139]. Some of its modes are compared with those
of q-PLA in Table 5-10. The amide I mode is hardly affected. The observed amide
I1 mode A-El splitting increases somewhat, from 29 cm-l in a-PLA to 40 cn-I in
a-PLGH; since the eigenvectors are identical [139], this may reflect a very subtle
change in structure that alters the TDC interactions. The amide I11 modes are
somewhat more affected, and this is a result of the side chain: CitH? tw now contributes
to the E2 and El modes. Analogous changes influence skeletal modes in the
1200-900 cm-' region, in the case of the CN s, CNC" d modes even altering the
frequency order of the species. The aniide V modes are also shifted by such changes
in eigenvectors, and these have an even greater impact on the low-frequency skeletal
modes. The constellation of such changes may well be characteristic of the side
chain.