Studies of Poly-β-benzyl-l-aspartate Helix. II. The Circular Dichroism and Optical Rotatory Dispersion of Copolymers of β-p-Methyl, Chloro, or Cyanobenzyl-l-aspartate with β-Benzyl-l-aspartate

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Abstract
The observation of the circular dichroism (240–290 mμ) due to the aromatic side chain indicates that, in a chloroform solution, even the side-chain aromatic rings of the copolyaspartate chain in the right-handed helix are in a regular arrangement to form an outer helix. On the contrary, for the copolyaspartate chain in the left-handed helix, the aromatic rings are much less regular. The ORD measurements indicate that, in a dimethylformamide solution, the poly-β-benzyl-l-aspartate chain is essentially in a random coil form, whereas the para-substituted derivatives are essentially in a right-handed helix. Accordingly, for each series of copolymers, the transition from a random coil form to a right-handed helix is observed as the molar content of the para-substituent is increased. The copolypeptide chain of β-benzyl-l-aspartate and β(p-methylbenzyl)-l-aspartate (molar ratio, 1:1) takes either the left-handed helix in a chloroform solution or the right-handed helix in a dimethylformamide solution. Accordingly, for this copolypeptide, the transition from the left to right-handed conformation is observed at the mixing volume ratio, 90 : 10, of the chloroform-dimethylformamide solution.