Kinetics and Mechanism of the Reactions of Bis(biguanide)copper(II) Ion with Different Amino Acids in Aqueous Media

Abstract

Abstract The kinetics of the reaction of [Cu(bigH)2]2+ (bigH = biguanide) with an excess of different amino acids (LH) like ß-alanine, L-phenylalanine and L-valine in aqueous solution in the 7.60-9.00 pH range at different temperatures (30-40°C) have been studied by stopped-flow spectrophotometry and compared with those of different ligands reacting with the title copper(II) complex. The ligand replacement process has been suggested to pass through an intermediate formation of the ternary complex, [Cu(bigH)L]+ as the rate determining step, followed by the rapid transformation into the binary complex, [CuL2]. The overall ligand replacement process follows a two-term rate law, kobs = k0 + k1Ka[L]T/([H+] + Ka, where [L]T gives the total concentration of the entering ligand, i.e. amino acid, and Κa gives the deprotonation constant of the amino acid (LH). Thus the process experiences both the ligand independent path (i.e. k0) and ligand dependent path (i.e. k0). The solvent assisted k0 path leads to a copper(II) mono-biguanide complex followed by the rapid nucleophilic substitution; the k1 path is in agreement with an associative mechanism. The k0 path is independent of the structure and basicity of the entering amino acids while the k0 path is dependent on the basicity and steric crowding of the entering amino acid. The reactivity order is: glycine > α-alanine > ß-alanine > L-valine > L-phenylalanine. The activation parameters have been evaluated. An iso-kinetic trend for different amino acids and other nucleophiles reacting with the title copper(II) complex has been found to prevail.