Assignment of the proton-association constants for 3-(3,4-dihydroxyphenyl)alanine (L-dopa)

Abstract

A re-examination based on linear free-energy relations and on kinetic evidence has been made of the assignment of the order of protonation of the L-dopa anion. This protonation order is reaffirmed as follows: first, one of the phenolato-groups; secondly, the second phenolato-group; this is followed closely, and overlapped, by the NH₂ group; and finally the carboxylato-group. The use of the following values of the microconstants is recommended for solutions of 0.100 mol dm^–3 ionic strength (K[NO₃]): logK₁^H(OH) 13.55, logK₂^H(OH) 9.76, logK₃^H(NH₃⁺)8.93, and logK₄^H(CO₂H) 2.31.