Conformational studies of polymers and copolymers of L ‐aspartate esters. II. Infrared studies and the factors involved in the formation of the ω‐helix

It has already been show that the helix senses of poly(β‐benzyl L ‐aspartate) and poly(β‐methyl L ‐aspartate) are left‐handed, while the poly esters of n ‐propyl, isopropyl, n ‐butyl, and phenethyl L ‐asparate are all right‐handed. The effect of changes in helix sense from the left‐handed to the right‐handed α‐helical form on the infrared spectra of copolymers of benzyl L ‐aspartate with ethyl, n ‐butyl, isopropyl, n ‐propyl, and phenethyl L ‐aspartate have been studied. Those show that for the right‐handed helical form the amide band frequencies fall within the range given by Elliott, 7 while for the left‐handed form the frequencies are higher. The frequency ranges for the two helix senses are given and have been used to show that poly (β‐ n ‐propyl L ‐aspartate) in chloroform solution undergoes a transition from the right‐handed to the left‐handed helix form on heating. Polarized infrared studies of the different copolymers show that the disposition of the side chain ester groups is different for the two forms. Although methyl L ‐aspartate forms a left‐handed α‐helix similar to benzyl L ‐aspartate, the introduction of methyl L ‐aspartate residues into poly (β‐benzyl L ‐aspartate) prevents the formation of the ω‐helix. The factors involved in the formation of this helix form are discussed.