Conformational aspects of polypeptides. XXV. Solvent and temperature effects on the conformations of copolymers of benzyl and methyl <scp>L</scp>‐aspartate with nitrobenzyl <scp>L</scp>‐aspartate

Abstract

Abstract A series of copolymers of β‐ p ‐nitrobenzyl L ‐aspartate with β‐benzyl L ‐aspartate and with β‐mcthyl L ‐aspartatc in helix‐supporting and helix‐breaking conditions have been reexamined by using ultraviolet isotropic, absorption, optical rotatory dispersion, and circular dichroism techniques. Many different conformations are apparent, depending on solvent and temperature. Chloroform, trifluoroethanol, and methylene dichloride support the left‐handed helical conformation of the copolymers containing less than about 20 mole‐% nitroaromatic residues and the right‐handed helical conformation of the copolymers containing more than approximately 30 mole‐% nitroaromatic residues. In trifluoroacetic acid all the copolymers are in a random‐coil conformation. In hexa‐fluoroacetone trihydrate and in trimethyl phosphate, the copolypeptides with low nitroaromatic residues content are predominantly in a disordered conformation, while those with high nitroaromatic residues content show a right‐handed helical array. Reversible helix‐ramlom‐coil transitions are observed with increasing temperature in trimethyl phosphate. An example of right‐handed‐left‐handed helix reversible transition with temperature is reported in a chloroform‐trimethyl phosphate (2:1) mixture. Nitrobenzyl‐nilrobenzyl side‐chain interactions in chloroform, but not in trifluoroacetic acid or in trimethyl phosphate, have been confirmed. For the first time we report the circular dichroism spectra in which the n ‐π* peptide band of a left‐handed helical conformation is almost completely evident.