A statistical study of optical rotation for synthetic d,l-copolypeptide solutions

The right-handed helical fraction H(L) in a D,L-copolypeptide chain is evaluated by a simple probability theory for each L-fraction, p. The curves of optical rotation at a single wavelength against p for poly(gamma-benzyl-D,L-glutamate) and D,L-copolyleucine of a series of different D:L ratios in several positive helix-solvents are calculated with a modified semi-empirical Moffitt equation. From the comparison between the rotations calculated by us and observed by Downie et al., it is shown that to reproduce the observed rotations, the minimum number of right-handed helical nuclei or sequences of L-residues appropriate to the Moffitt equation are eight and four for degrees of polymerization 320 and 1860, respectively.