Stability of the copper(I)-cysteine complex assessed using reverse titrations: implications for estuarine and seawater copper biogeochemistry

The biogeochemical cycle of copper (Cu) is mediated by its complexation with organic ligands. An emerging strategy for Cu uptake by marine and freshwater phytoplankton involves organic molecules of biological origin, such as cysteine (Cys), indicating an important role for Cys in controlling the redox state and uptake of Cu in surface waters. In this study, Cys-like compounds were detected in the surface layer of the Krka River estuary by cathodic stripping voltammetry (CSV), while high fractions of Cu(I) were simultaneously determined using an adapted solid phase extraction method. The affinity of Cys for associated redox reactions and for Cu(I) stabilisation depends on its complexation affinity towards Cu(I), which remains controversial in the literature, as values of reported stability constants for Cu(I)-Cys are inconsistent. Spectrophotometric and electrochemical approaches were used to determine the conditional stability constant of the Cu(I)-Cys complex (K″CuL), which refers to the equilibrium constant conditional with respect to both Cu and the ligand. Spectrophotometric reverse titration against a known Cu(I) probe, bathocuproine disulfonate (BCS), under seawater conditions (0.55 mol/L NaCl) revealed a previously unrecognised effect of chloride (Cl−) on the stability of the [CuBCS2]3− complex due to the possible formation of ternary complexes involving Cu(I), BCS, and Cl−, which calls for caution when determining the stability of Cu(I) complexes this way. The results of the electrochemical reverse titration of the Cu(I)-Cys complex with BCS in Cl−-containing medium are consistent with the spectrophotometric results. The logarithm of the derived K″CuL for the Cu(I)-Cys complex is 15.35 ± 0.11, corresponding to the conditional stability constants of the strong Cu-binding ligand class (L1) in seawater, which further supports the importance of Cys for Cu uptake and redox cycling in seawater and estuarine water. By elucidating the stability of the Cu-Cys complex, this study provides insight into how biologically produced and naturally occurring thiol-like ligands influence Cu redox speciation in marine and estuarine waters, which can affect Cu transport and uptake and is therefore essential for accurately representing the Cu biogeochemical cycle.