Verification of the Ionic Constants of Proteins by Calorimetry

Abstract

Dissociation curves of proteins have been extensively used as an analytical tool to provide useful information on the nature and number of the ionic groups in a protein. Changes in the ionic behavior of these groups have been related to structural and functional features of a large number of biologically important molecules. An exact count of the number of groups in any region of a Potentiometrie titration is difficult because of the overlapping ionizations of the ionizing groups. The residues which ionize in proteins can be grouped into three major sets corresponding to their ionization constants. (In this paper, the ionization constant is referred to by its negative logarithm, i.e., pK’. The concentrations are expressed as N which is μmoles ionized/μmoles present.) These sets include the acid region which is composed of exterminai carboxyl groups and the γ and δ carboxyls of aspartic and glutamic, the mid-region which includes the imidazoyl and α-amino group ionizations and the alkaline region comprising the ionizations of the sulphy-dryl, phenolic and ε-amino groups from cysteine, tyrosine and lysine (1), respectively.