Kinetic gating by an iron sulfur chain modulates flavin radical-pair spin chemistry in mitochondrial Complex I

Mitochondrial Complex I generates reactive oxygen species at its flavin cofactor, where spin dependent reaction pathways have been proposed. Models of this chemistry treat the flavin oxygen radical pair in isolation, although the flavin is wired to a chain of eight iron sulfur clusters. Here we model the pair with this chain using the human Complex I structure and open quantum system models of spin coupling and electron transfer. In the simulated network, spin coherence decays rapidly along the chain, whereas proximal clusters alter the calculated radical-pair singlet yield. Their influence is kinetically gated: a downstream cluster affects the modelled yield when its electron-transfer rate competes with radical pair recombination, a threshold robust to driving force and to the transfer model. Modelling does set the sign: one directional spin hops predict suppression and inflate coherence transport, whereas charge resolved transfer drives the singlet yield towards its spin-randomized limit unless triplet recombination dominates. Detectable spin dependent effects in this framework require superoxide spin relaxation to be substantially slower than in free solution, a condition not yet established for Complex I.