Electrostatic interactions in ionic homopolypeptides in solutions of moderate ionic strength

Abstract

AbstractAt sufficiently high ionic strength, long‐range electrostatic interactions in a polyelectrolyte such as poly(L‐glutamic acid) might be adequately approximated in matrix calculations by use of statistical weights representing second‐order interactions. The validity of this assumption has been investigated making use of experimental observations (CD spectra and titration curves) for poly(L‐glutamic acid) as a function of temperature in 0.1–0.5M sodium chloride. Theoretical analysis, using a statistical weight matrix proposed by Warashina and Ikegami, is based on the Zimm‐Rice theory. Implementation differs from that of Warashina and Ikegami in one respect. Refinement of the initial estimates is achieved using a form of the configuration partition function which does not assume diagonalization of the statistical weight matrix. This difference is of no consequence for the values of σ and s, but it does produce somewhat different values for the statistical weights used to represent the electrostatic interactions. The method used to treat electrostatic interactions in poly(L‐glutamic acid) in 0.1M sodium chloride can be viewed as successful in that it properly reproduces the helix–coil transition and titration curves in this solvent and the molecular‐weight dependence of the titration curves yields values for s in harmony with those obtained using a treatment which is independent of model, and gives a reasonable ionic‐strength dependence for the electrostatic parameters. Furthermore, the model can account for measured helix–coil transitions and titration curves in homopolypeptides in which the side chain is —(CH2)xNHCO(CH2)yCOOH. The model, however, is not exact. It does not properly account for the molecular‐weight dependence of the helical content for polymers of low degree of polymerization.