Effect of a helix‐coil transformation on magnesium binding to α‐poly‐L‐glutamic acid

Abstract

AbstractThe effect of magnesium ion binding on the dissociation of side chain carboxyl group in α‐poly‐L‐glutamic acid has been measured by a modified titration technique involving pretitration of the polyacid with a strong base of a weakly bound counterion followed by addition of aliquots of the neutral salt of the more strongly bound counterion, magnesium chloride. The changes in the ionization of the carboxylic acid groups in the poly‐L‐glutamic acid side chains have been measured as a function of both the degree of preionization and of the concentration of magnesium chloride added. These changes have been related to changes in the polymer configuration (helix‐coil transition) as a function of degree of pre‐ionization. The effects of ionic strength and of ion binding have been determined by a comparison of tetramethyl ammonium chloride and of magnesium chloride at the same ionic strength. The plot proposed by Wada has been used to determine the pure helical region, the region of helix‐coil transition, and the pure coil region with both magnesium chloride and tetramethyl ammonium chloride as added salts. At the same ionic strength the effects produced by magnesium chloride addition are far greater than for tetramethyl ammonium chloride. Magnesium chloride at an ionic strength of 0.25 × 10−4 produces an effect equivalent to that of tetramethyl ammonium at an ionic strength of 15.0 × 10−4. This effect is also apparent when the concentration of carboxyl groups ionized by magnesium chloride addition is determined. The much larger effects produced by the magnesium ion are attributed to the enhanced binding of this ion. The results reported indicate binding is favored in the helical configuration. The binding in the random coil configuration is similar to that reported previously for other randomly coiled polyelectrolytes.