IONIZATION OF INDIVIDUAL GROUPS IN DIBASIC ACIDS, WITH APPLICATION TO THE AMINO AND HYDROXYL GROUPS OF TYROSINE

Abstract

A method is formulated for calculating the 4 "microscopic" ionization constants required to describe the ionization of the individual groups in a dibasic acid, from spectroscopic data which measure the fractional ionization of one of the groups as a function of pH. This method is applied to the ionization of the amino and hydroxyl groups of tyrosine. By a positive charge on the amino group the pK value of the hydroxyl group by 0.42 pH units at ionic strength 0.16, and probably by about 0.60 pH units at zero ionic strength. There is a corresponding reciprocal effect of the ionization of the hydroxyl group on that of the ammonium group. These interaction effects are compatible with a Kirkwood-Westheimer ellipsoidal model, with a distance of 7 A between the amino and hydroxyl groups[long dash]a value which is physically reasonable. Other methods of evaluating the microscopic constants, by alkyl substitution of some of the ionizable protons, are considered and applied to tyrosine derivatives. A brief discussion of the data of Benesch and Benesch, on the microscopic constants for the amino and sulfhydryl groups of cysteine, is also given.