Molecular conformation of poly‐S‐carboxytmethyl‐ L ‐cysteine in aqueous solutions

Abstract Poly‐S‐carboxymethyl‐L‐cysteine has been prepared by debenzylation of poly‐S‐carbobenzoxymethyl‐L‐cysteine with hydrogen bromide in acetic acid. By the infrared spectroscopic method the polymer is found to be in the extended β‐conformation with an antiparallel arrangement of polypeptide chains in solid film, if it has been regenerated from dimethyl sulfoxide solution. Aqueous solutions of the polymer have been investigated by measurements of optical rotatory dispersion and viscosity. Various properties sharply change around pH 5 at different ionic strengths. By combining these with infrared studies in D2O solutions, it has been shown that the polymer exists in the random coil conformation at higher ionization but associates into the intermolecular β‐conformation at lower ionization. At the lowest pH attainable in solution, the β‐form is partly converted into the random coil as the temperature is raised. The rotatory dispersion of the polymer is described by the Moffitt equation. While the random coil form has a large negative a0 value and a zero b0 value, the β‐form is characterized by a positive a0 value and a negative b0 value, −130°.