Complex Bilding Behavior of L-Tryptophan and Related Amino Acids, a Comparative Investigation

Abstract

The acidity constants of Tryptophan 1 were determined by potentiometric pH titration. The stability constants of the 1:1 complexes formed between M 2+ : Ca 2+, Mg 2+, Mn 2+, Co 2+, Ni 2+, Cu 2+ or Zn 2+ and Trp 2 −, were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO3, 25°C). The order of the stability constants was reported. It is shown that the stability of the binary M(Trp) complexes is solely determined by the basicity of the carboxyl or amino group. All the stability constants reported in this work show the usual trend. The obtained order is Ca 2+ < Mg 2+ 2+ < Co 2+ < Ni 2+ < Cu 2+ Zn 2+. The observed stability order for Tryptophan follows the Irving-Williams sequence. It is shown that regarding to M ion–binding properties vital differences on complex bilding were considered. It is demonstrated, that in M−Trp complexes, M ion is coordinated to the carboxyl group, M ion is also able to bild macrochelate over amino group. The up mentioned results demonstrate that for M−Trp complex the stability constants is also largely determined by the affinity of Cu 2+ for amino group. It is shown that Trp can exert a direct influence on reaction mechanism through different kinds of metal ions and donor groups of Trp.