Force-Triggered Thermodynamically Uphill Disulfide Reduction through Sulfur Oxidation State Control

In addition to thermal energy, current, and light, mechanical forces activate chemical reactions, often steering reaction pathways that result in products different from those obtained under thermodynamic control. Single-molecule mechanochemistry experiments have probed how the forced activation of a single covalent bond results in accelerated scission of both homolytic and heterolytic bonds, and the ring-opening of strained mechanophores in long polymers. Due to its mechanistic simplicity, the concerted S_N2 thiol-disulfide nucleophilic substitution has been successfully used as a model system to interrogate how the nucleophilicity of an attacking organic, low-oxidation state thiol determines the force dependency of the thiol/disulfide exchange rate. Inorganic sulfur-oxyanions are comparatively much less reactive. Whether mechanical forces can activate the rupture of a protein disulfide by sulfur-oxyanions featuring higher oxidation states remains unknown. Here we employ single-molecule force-clamp spectroscopy, complemented by density functional theory (DFT) calculations and colorimetric assay measurements, to show that the thermodynamically nonfavored reduction of a disulfide bond by inorganic oxyanions can be activated by mechanical force. Occurring within the core of a protein with a physiological mechanical role, the force-unlocked reactivity has a direct impact on protein elasticity.