Abstract
The kinetics and stability constants of l-tyrosine complexation with copper(II), cobalt(II) and nickel(II) have been studied in aqueous solution at 25° and ionic strength 0.1 M. The reactions are of the type M(HL)(3-n)+ n-1 + HL- ⇆ M(HL)(2-n)+n(kn, forward rate constant; k-n, reverse rate constant); where M=Cu, Co or Ni, HL− refers to the anionic form of the ligand in which the hydroxyl group is protonated, and n=1 or 2. The stability constants (Kn=kn/k-n) of the mono and bis complexes of Cu2+, Co2+ and Ni2+ with l-tyrosine, determined by potentiometric pH titration are: Cu2+, log K1=7.90 ± 0.02, log K2=7.27 ± 0.03; Co2+, log K1=4.05 ± 0.02, log K2=3.78 ± 0.04; Ni2+, log K1=5.14 ± 0.02, log K2=4.41 ± 0.01. Kinetic measurements were made using the temperature-jump relaxation technique. The rate constants are: Cu2+, k1=(1.1 ± 0.1) × 109 M −1 sec−1, k-1=(14 ± 3) sec−1, k2=(3.1 ± 0.6) × 108 M −1 sec−1, k−2=(16 ± 4) sec−1; Co2+, k1=(1.3 ± 0.2) × 106 M −1 sec−1, k-1=(1.1 ± 0.2) × 102 sec−1, k2=(1.5 ± 0.2) × 106 M −1 sec−1, k-2=(2.5 ± 0.6) × 102 sec−1; Ni2+, k1=(1.4 ± 0.2) × 104 M −1 sec−1, k-1=(0.10 ± 0.02) sec−1, k2=(2.4 ± 0.3) × 104 M −1 sec−1, k-2=(0.94 ± 0.17) sec−1. It is concluded that l-tyrosine substitution reactions are normal. The presence of the phenyl hydroxyl group in l-tyrosine has no primary detectable influence on the forward rate constant, while its influence on the reverse rate constant is partially attributed to substituent effects on the basicity of the amine terminus.