From cysteine to longer chain thiols: Thermodynamic analysis of cadmium binding by phytochelatins and their fragments

Abstract

Isothermal Titration Calorimetry (ITC) was used to study the binding of Cd 2+ by phytochelatins ((γGlu-Cys) n-Gly, PC n; n = 1-5) and their selected fragments (Cys, Cys-Gly and γGlu-Cys) in order to understand the influence of the chain length on the complex stabilities and the origin of the enhanced affinities in Tris buffer at pH 7.5 and 8.5 and at 25 °C. Different complexes are formed with glutathione (GSH) and its fragments, Cys, Cys-Gly and γGlu-Cys, and their stabilities depend on the corresponding pK a value of the thiol group in the ligands. The stability of Cd-PC n complexes increases moving towards higher PC 2-5, as well as the complexing capacity expressed as the number of metal ions that can be bound by one ligand molecule. The affinity of Cd 2+ for the PC n can be described by the following GSH < PC 2