Effect of ionic strength on the protonation of various amino acids analysed by the mean spherical approximation

Abstract

The dependence of the ionic strength of stoichiometric protonation constants, obtained potentiometrically, of several amino acids has been analysed by the mean spherical approximation (MSA), a mathematically simple statistical mechanical treatment based on integral equations. Unlike the Debye–Hückel equation, which overestimates the effects of ion–ion interactions as the sizes of all ions are neglected, the MSA represents the electrolytes as a mixture of hard spheres immersed in a dielectric continuum which represents the solvent. This approach fits experimental data using concentration, electrical charge and diameter concepts and is an alternative to semiempirical models based on different ionic strengths with coefficients that defy theoretical interpretation. Analysis of data using both semiempirical models and a quasi-lattice model, not containing Debye–Hückel terms, is also included.

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