Redox interactions between Fe and cysteine: Spectroscopic studies and multiplet calculations

The biogeochemical cycle of Fe is intricately linked with that of organic matter. Cysteine represents an organic molecule with functionalities (O, S, N functional groups) and a C backbone that may mimic the functional groups present in organic matter from terrestrial and aquatic environments. In the present study we explore the redox speciation and coordination environment of Fe and the roles of the various ligand atoms of cysteine (C, N, S) in iron-organic redox coupling and transformations. The changes in oxidation state of Fe, C, N, and S in laboratory-synthesized Fe(II)–cysteine (synthesized from ferrous sulfate) and Fe(III)–cysteine (synthesized from ferric nitrate) complexes are monitored as a function of time using synchrotron X-ray absorption spectroscopy (Fe L₂,₃-edge XANES; C, N and S K-edge XANES; Fe K-edge EXAFS) and theoretical multiplet calculations using the program CTM4XAS (Charge Transfer Multiplet for X-ray Absorption Spectroscopy). CTM4XAS calculations show that 80% of the total Fe in both the Fe(II)–cysteine and the Fe(III)–cysteine complexes is present as Fe²⁺ initially (t=0), thus indicating preservation of Fe(II) in Fe(II)–cysteine and reduction of Fe(III) in Fe(III)–cysteine at initial conditions, the latter caused by an internal electron transfer reaction from S of –SH on the cysteine molecule. After 12months, however, ∼60% of the total Fe is present as Fe³⁺ in the Fe(II)–cysteine complex whereas ∼67% of the total Fe is present as Fe²⁺ in the Fe(III)–cysteine complex. The fact that a larger proportion of the Fe in the Fe(III)–cysteine complex remained reduced after 12months than that in the Fe(II)–cysteine complex suggests that the reduced Fe in Fe(III)–cysteine after 12months is further stabilized via preferential binding with the donor atoms of cysteine. Stabilization via preferential binding is supported by a coordination environment that changed from tetrahedral Fe²⁺ binding to S at a distance of 2.3Å at t=0 for both Fe(II,III)–cysteine complexes, to Fe³⁺ in an octahedral coordination with O/N atoms at a distance of 2.05Å (most prevalent in Fe(II)–cysteine) and Fe²⁺ in tetrahedral coordination with S/O/N atoms at an average distance of 2.15Å (most prevalent in Fe(III)–cysteine) at t=12months. Redox changes in the –NH₂ and –SH groups of cysteine accompanied the Fe redox changes thus reflecting the true potential of cysteine as a redox ligand. Our studies of the Fe(II,III)–cysteine complexes add valuable information to the existing literature on the redox chemistry of Fe–cysteine systems by shedding light on the electron exchange pathways that may occur within the complexes and by providing a detailed depiction of the iron-ligand structure and coordination. The presence and persistence of Fe(II) or Fe(III) in complexes with soluble organics have implications for Fe biological availability and Fe mobility and transport in terrestrial as well as in aquatic environments.