Circular dichroism (CU) measurements were carried out on various copolymers of L-tryptophan and r-ethyl L-glutamate in ethylene glycol monomethyl ether as the solvent. On increasing the L-tryptophan content of the copolymers a gradual change in the C D spectra was observed. The typical spectrum of the right-handed a-helix becomes more and more evident as the L-tryptophan content decreases. On the basis of these results we assumed that no conformational transition occurs on proceeding from pure poly (7-ethyl L-glutamate) to pure poly-Ltryptophan in ethylene glycol monomethyl ether: therefore the conformation of poly-L-tryptophan should be that of a right-handed a-helix. Moreover we observed that the change in the CD spectra of the copolymers is gradual but not linear on increasing the tryptophan content. The deviations from linearity were attributed to interactions among side-chain chromophores whose contributions to the optical activity are not simply additive. An x-ray analysis carried out on oriented films of poly-L-tryptophan casted from solutions of the polymer in dimethylformamide shows conclusively that the solid-state conformation of the polymer is that of an a-helix.
* For example, Hashimoto"9 determined the bo values of copolymers of 0-benzyl L-aspartate and P-p-methylbenayl baspartate. In such a case the side-chain chromophores do not exhibit appreciable optical activity, and the bo values are uniquely determined by the peptide transitions.