The effect of deuteration on the geometry of the α-helix

Abstract

Experiments were carried out to determine the effect of complete deuteration on the screw parameters of the α-helix. Poly-γ-benzyl-L-glutamate and poly-β-benzyl-L-aspartate were drawn into fibers in both normal and deuterated forms, and an X-ray diffraction study was carried out on the change in the meridional 1·5 Å reflexion, which is a measure of the rise per residue along the helix. Deuteration increased this spacing by 0·007 Å and 0·008 Å respectively. Accompanying this was an increase in the helical pitch spacing of the 5·4 Å layer line which indicated that the deuterated α-helix was being extended, without a change in the rotation per residue. The hydrogen bond lengths of 2·83 Å are increased by 0·025 Å and 0·029 Å upon deuteration. This increase is in agreement with changes which have been observed in hydrogen bond length in other substances upon deuteration. The internal contacts of the α-helix are discussed, together with the effect of deuteration upon the overall stability of the structure. Infrared data are presented which confirm the helical form of the polymers in both normal and deuterated forms.