Modern Polymer Spect..

Table 5-12. Aniide mode frequencies (in cm-' ) of antiparallel-chain polyglycine 11.
Observed' Calculated Potential energy distribution
Rainan IR A B
(1656 CO s(71) CN ~(20) C"CN d(l0)
1654 CO s(74) CN s(20) C"CN d( 10)
1654VS 1655W
1653 CO s(72) CN ~(20) CTN dil0)
( 1651
CO s(73) CN ~(19) C"CN d(10)
1649
CO 473) CN s(20) CTN d(10)
1640VS 1645 CO 474) CN s(19) CTN d(l0j
1560W 156OW 1565 1565 NH ib(59) CN s(18)
1550s
1555 1552 NH ib(52) CN s(20) C"C s(10)
1548 1548 NH ib(54) CN s(21) '2°C s(12)
1383MS 1377M 1353 1380 CH? ~(58) C"C ~(15) NH ib(13)
1350 1350 CH2 w(46) CH2 b(16) NH ib(16) C"C s(14)
1334VW 1332VW 1344 1345 CH2 w(50) NH ib(16) CH2 b(14) C" C ~(13)
1303 1304 CH1 tw(31) NH ib(14) CH2 w(13) CN s(12)
1283M 1283M 1290 1290 CH? tw(29) CH2 w(23) NH ib(13) CN s(12)
752VW 751W 760 759 CN t(20) NH ob(19) NH-.O ib(14) NC"C d(13)
CO ib( 12)
742VW 740 CN t(53) NH.-.O ib(22) NH ob(l8)
673M
740M 738
678
661
CN t(49) NH...O ib(20) NH ob( 16) CO ib( 11)
CN t(77) NH ob(27j NH t(l1) CO ob( 10)
CN t(83) NH ob(28) NH t( 11) NH.-O ib( 10)
CN t(88) NH ob(30) NH t(14)
{
a Data taken from [123]. S: strong, M: medium, W: weak, V: very.
b s: stretch, b: bend, ib: in-plane bend, ob: out-of-plane bend, d: deformation, w: wag, tw: twist,
t: torsion. Contributions 2 10.
and interactions can be obtained if experimental data are interpreted by careful
normal-mode analysis. Although the systematic application of extensive empirical
force fields has revealed the validity of this approach [5], the full power of this
technique remains to be exploited. Our discussion has concentrated, because of
space limitations, on the amide modes, but of course the sensitivity to conformation
and forces resides in the entire spectrum. The key to extracting this information will
be the physical reliability of the force fields used in the normal-mode calculations.
We have come a substantial distance toward this goal with existing force fields [5],
but improvements can be made and some are already in sight. These presently involve
empirical force fields, such as the development of an ab initio-based MI-PLA
force field [145] utilizing better IR [145] and polarized Raman [146] data as well as
an understanding of helix vibrations [ 1471 that avoids the weak-coupling and perturbation
approximations 191. However, the ultimate goal will be achieved when
we have a conformation-dependent force field as represented by an SDFF for the.
polypeptide chain [51].