Modern Polymer Spect..

5.4 Polypeptides 271
Figure 5-7. Structure of 3lo-helix of poly(a-amiiioisobutyric acid).
Methyl groups are represented by large circles. (From [S])
group in a11 is inclined to the helix axis whereas it is almost parallel to the axis in MI.
The a-helix conformation is not only coniinoii in polypeptides, and is a basic component
of proteins [2], but its topology forins the basis for two-stranded coiled-coil
structures such as tropomyosin [108]. The 3lo-helix is similar to the a-helix, but has
a 4 i 1 hydrogen-bonding pattern. Although segments are found in proteins [2],
the only polypeptide in which it has been found, as suggested by electron diffraction
[ 1091 and supported by vibrational analysis [ 1101, is poly(a-aminoisobutyric acid),
PAIB (Figure 5-7). Its structure, as indicated by conformational energy calculations
[111], has a nonplanar peptide group. The o-helix is a left-handed 5 - 1
helix, -4.013, proposed from X-ray analysis for the structure of poly(j?-benzyl-Laspartate)
[ 1121. Coiiformational energy calculations have indicated somewhat different
parameters [ 1131, with other calculatioiis suggesting a further variant [114].
The n-helix, said to be present in poly(j?-phenethyl-L-aspartate [115], is also a lefthanded
helix, -4.2516, although a right-handed form with a 4" distorted NC"C
angle has been suggested [116]. The 0- and n-helices also have nonplanar peptide
groups. Other helical conformations have been proposed, a y-helix [lo?, 1031 and a
2.27-helix [117], but neither of these has as yet been observed in polypeptides. When
L and D residues alternate along a chain, which occurs in some transinembrane
peptides, a new class of helices results, the so-called /?-helices[I 18, 1191. Such chains
can also form double-stranded structures with interchain hydrogen bonds.
The simplest polypeptide structure that has intermolecular hydrogen bonds
throughout its lattice is that of PGII, which basically has threefold helical symmetry
[120]. (Because of its achiral C", PGII can be a right- or left-handed helix.) Although
the originally proposed structure was modeled with parallel chains, the ease
of mechanical conversion of PGII to PGI and the evidence for antiparallel chains
in single crystals of PGI led to the suggestion [ 1211 that an antiparallel-chain structure
should exist. Such a model was subsequently proposed 11221, and a detailed