X-ray structural studies of some poly(α-amino acid)s with hydrophobic side chains: Poly(l-valine), poly(l-isoleucine), and poly(l-phenylalanine)

Abstract

X-Ray structure studies of poly(L-valine), poly(L-isoleucine), and poly(L-phenylalanine) have been carried out. Poly(L-valine) has two structural forms. One is an α-helical conformation packed in the hexagonal lattice with a dimension of a=11.43 Å, though this conformation is unfavorable for poly(L-valine). The other is a β pleated sheet structure with the crystal data∧∧#x0023;x003A; P222, a=4.80 Å, b=19.14 Å, c=6.59 Å (fiber axis). The β poly(L-valine) gave an X-ray fiber photograph of high quality and a detailed structural study was carried out. Poly(L-isoleucine) also crystallizes in a β pleated sheet structure with a=4.8 Å, b=23 Å, c=6.6 Å, orthorhombic. Contraction of fiber axes in both β structures are observed. The X-ray fiber photograph of poly(L-phenylalanine) suggests the coexistence of ω- and α-helix. This specimen may be the third example of an ω-helix. The significance of the hydrophobic interaction of the side chains in the structures is discussed. © 1979 The Society of Polymer Science, Japan.