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.
