Acid dissociation of tyrosine and its related compounds.

Abstract

The acid dissociation equilibria in aqueous solution of tyrosine and its related compounds were determined by potentiomeric titration and absorption spectra at 25° and μ=0.1 (NaClO4). Microscopic equilibrium constants were calculated by three different methods. In method A, the absorbance at 295 nm is a measure of the total concentration of the dissociated phenol. Method B is based on the assumption that k2 is the same as the dissociation constants of dimethoxy-phenyl or phenyl derivatives. Method C is a modification of the Edsall method. Tyrosine, m-tyrosine, and octopamine were found to be present in maximum amount as the amino-phenol form at pH 9.5, whereas tyramine was found to be present in about 30% as a zwitter-ionic form at pH 10. From the values obtained, pK2 values seem to contribute to the dissociation of phenol group in tyrosine, m-tyrosine, and octopamine, while the dissociation of phenol group may contribute to the value of pK1 in tyramine.