Specific cation effects on the conformation of α‐poly‐L‐glutamic acid

Abstract

AbstractThe effect of the binding of the alkaline earth cations and of cupric and lanthanum ions on the dissociation of the sidechain carboxyl groups in α‐poly‐L‐glutamic acid has been measured by a modified titration technique. The α‐poly‐L‐glutamic acid was pretitrated with the strong base of a weakly bound counterion followed by addition of aliquots of the neutral salt, solution of the more strongly bound counterions. The changes in the ionization of the sidechain carboxyl groups were measured as a function of degree of poly acid pretitrated and the concentration of added salt. These changes were related to the helix‐coil transition in α‐poly‐L‐glutamic acid as a function of degree of ionization. pH titration data were used to delineate the helix, helix‐coil, and pure coil regions in α‐poly‐L‐glutamic acid.For the alkaline earth cations, the binding strength in the helical region is calcium > strontium > magnesium > barium. The more strongly bound cations produced much larger poly acid dissociation with the binding strength alkaline earths ≪ cupric < lanthanum at equivalent ionic strength. All results indicated that the helix conformation promoted ion binding. The binding of the alkaline earth cations in the random coil form was similar to that reported with other coiled polyelectrolytes. The cupric and lanthanum ions appeared to stabilize the helical structure against changes due to increased poly‐acid ionization.