Quantum Tunneling-Mediated Activation of Molecular Hydrogen by Semiquinone Radicals and Superoxide: Implications for Quantum Biology and Metal Catalyst-Free Redox Regulation

Abstract The therapeutic effects of molecular hydrogen (H₂), particularly in ischemia-reperfusion (I/R) injury and deleterious inflammation, have been increasingly attributed to its modulation of redox balance. However, the precise molecular mechanisms underlying H₂-mediated redox modulation, particularly in mitochondrial reverse electron transfer (RET)-driven superoxide (O₂•⁻) generation, remain unclear. Here we demonstrate that H₂ can be activated through quantum tunneling-assisted electron transfer involving semiquinone radicals (Q•⁻) without requiring any metal catalysts or hydrogenase enzymes. Using enzymatic (xanthine oxidase/hypoxanthine; XO/Hx) and non-enzymatic (potassium superoxide; KO₂) O₂•⁻-generation systems combined with the O₂•⁻-specific fluorescence probe, 2-methyl-6-p-methoxyphenylethyl-imidazopyrazinone (MPEC), we observed distinct bell-shaped and U-shaped O₂•⁻ kinetic profiles depending on the concentrations of ubiquinone (Q) and H₂. Unexpectedly, even in the absence of Q, O₂•⁻ directly activated H₂, resulting in a clear bell-shaped kinetic profile indicative of quantum tunneling-mediated electron transfer from H₂ to O₂•⁻. In contrast, when Q was present, distinct U-shaped kinetics were observed, suggesting a dual-mode electron transfer mechanism involving Q•⁻-mediated electron buffering and subsequent H₂ activation. Electron spin resonance (ESR) radical scavenging experiments and quantitative high-performance liquid chromatography (HPLC) analyses confirmed that H₂ participates in semiquinone-mediated redox cycling, leading to the formation of ubiquinol (QH₂). Thus, our findings shed light on an unprecedented metal catalyst-independent quantum pathway for H₂ activation, suggesting the theoretical possibility that mitochondria, despite lacking hydrogenases, could potentially utilize exogenous H₂ through Q-mediated unrecognized mechanisms. This provides critical insights into quantum biology within mitochondrial bioenergetics.