The Iron-Thiol-Oxygen Nexus: Thiol Molecular Architecture and Protein Encapsulation Dictates Iron Flux and Controls Oxidant Activity

Abstract

The redox interplay between iron-sulfur-oxygen underpins both biological homeostasis and geochemical cycling of iron. Prior to the Great Oxygenation Event (GOE), sulfur fostered a reducing environment that preserved Fe2+ bioavailability. The subsequent rise of atmospheric oxygen post-GOE posed oxidative environment to Fe2+ further depleting its bioavailability which likely drove the evolution of ferritin, a protein nanocage that detoxifies and cages iron as ferrihydrite (Fh) mineral while enabling regulated Fe2+ mobilization. Mobilizing this stored iron for biological use requires dissolution of mineralized Fe³⁺ and its reduction back to Fe2+, for this process thiols can act as critical electron donors. Herein, we investigated the ability of cellular and synthetic thiols to mediate Fe3+/ Fe2+ redox cycling, O2 consumption, and mineral dissolution from bare/ferritin encapsulated ferrihydrite. Antioxidative properties were further assessed through DNA protection and radical scavenging assays to establish correlations with molecular architecture. Thiol size, branching, and functional groups (–SH, –NH3+, –COO‒) exerted pronounced effects on electron transfer efficiency. Thiol-driven Fe3+ reduction led to significant O2 consumption, generating a hypoxic microenvironment analogous pre-GOE conditions. Kinetic analysis revealed that protein encapsulation restricts thiol-mediated iron release (Bare Fh > encapsulated Fh). Notably, Na2S/HS‒ emerged as a potent reductant exhibited superior performance across all assays.