Magnesium binding to poly(L‐aspartic acid)

The optical rotatory dispersion of poly( L ‐aspartic acid) in magnesium chloride and tetramethylammonium chloride solutions has been measured to determine helix content. At pH 4, helix content estimated by using the Moffitt b o coefficient is about 20%. Optical rotation has been used to determine changes in conformation with polymer ionization. The changes in rotation are very sensitive to the relative concentration of added salt. At very low salt concentrations no ordered structure appears to be present. The effect of magnesium ion binding on the dissociation of side‐chain carboxyl groups in poly( L ‐aspartic acid) has been measured by a modified titration technique. The changes in ionization of the carboxylic acid side chains were measured as a function of both the degree of polymer preionization and of added magnesium chloride. These changes have been related to possible changes in the polymer conformation as a function of degree of polymer ionization. The effect of ionization on magnesium ion binding to poly( L ‐aspartic acid) and to poly( L ‐glutamic acid) have been compared with essentially equivalent polymer concentrations at equivalent salt additions. pH titrations have been used to compare ionization in the two polyacids. The pH titration method proposed by Wada has been used to determine the extent of the helix‐‐coil transition region in this polymer concentration range. Poly( L ‐aspartic acid) appears to be more easily ionized than poly( L ‐glutamic acid). For both polyacids, a helical conformation appears to favor ion binding. However, in this region, binding to poly( L ‐aspartic acid) is approximately double that observed with poly( L ‐glutamic acid) at a corresponding degree of ionization. In the coil region the trends observed with magnesium binding are similar to those observed with other extended polyelectrolytes.