Modern Polymer Spect..

Table 5-5. Structural parameters of ,&sheet StrUCtUreS".
Paraiiieterh APPS' APRSd PPSC PRS'
4.73
3.445
138.4
135.7
2.73
1.75
9.8
164.6
4.77
3.522
149.9
146.5
2.91
2.12
31.4
134.4
4.85
3.25
119.0
114.0
2.74
1.82
5.5
172.0
4.80
3.25
- 119.0
114.0
2.82
1.75
4.9
172.0
" From [82].
Lengths in A. angles in degrees. a (or a/2): lateral separation between adjacent chains. 17: axial
separation between adjacent units in one chain.
Antiparallel-chain pleated sheet of poly(L-alanine). From [80, 81 1.
Antiparallel-chain rippled sheet of polyglycine. From [82].
Parallel-chain pleated sheet of polyil-alanine). From [83].
Parallel-chain rippled sheet of polyglycine. From [83].
of major importance to understand the vibrational dynamics of this type of polypeptide
chain conformation.
5.4.1.2 Antiparallel-Chain Structures
Antiparallel-Chain Pleated Sheet
The APPS structure is the predominant one in synthetic polypeptides, and is a
prevalent motif in proteins [2]. (Polyglycine is an exception, which xe treat below.)
An X-ray diffraction study of the simplest representative polypeptide, /I-PLA [811,
has provided the geometric parameters of the APPS. In addition to those given
in Table 5-5, we note that the axial shift between adjacent chains, measured from
the position where the hydrogen bond is linear, is 0.27A (as obtained from a TDC
analysis of the amide I mode [80]). The vibrational analysis of /I-PLA thus provides
the basic spectral characteristics of the APPS.
A PLA sample can be easily oriented, and dichroic IR spectra are therefore
readily obtainable. Such spectra [85], together with far-IR spectra [86], are shown in
Figure 5-4. Together with spectra of the ND inolccule [87, 881, as well as Rainan
spectra [89] (Figure 5-5), a large amount of spectral information is available, both
to optimize an empirical force field and to provide a reasonable description of the
normal modes.
The symmetry of the p-PLA structure determines that the normal modes will
be distributed among four symmetry species with optical activity and IR dichroism
as follows [SO]: A-30 modes, Raman; B1-29 modes, Raman, IR (11); B2-29 modes,
Raman, IR (I); B3-29 modes, Raman, IR (I). The results of the most recent nor-