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 Cd²⁺ 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 Cd²⁺ for the PC_n can be described by the following GSH < PC₂ < PC₃ ≤ PC₄ ≤ PC₅ sequence. On the basis of these thermodynamic data it is possible to explain the abundance of certain Cd–PC_n complexes found in nature. The comprehension of the thermodynamic rules that govern the interactions of Cd²⁺ with PC_n and their constituents is of great service in the research with real plant samples subjected to metal stress and in the development of new strategies of bio/phytoremediation.