A model that stands out is the one that involves redox chemistry between a tightly bound and an exchangeable Q molecule . One of the central ideas of the mechanism is that the redox potential of Q bound at site 1 is rather low (< -300 mV) . As a result, there is no major energy gain upon electron transfer to bound Q from NADH, which has a similar redox potential (~-320 mV). This means the reactions that take place beyond the reduction of Q at the site near N2 cluster are responsible for driving the proton pump of complex I. Such reactions may include diffusion of the reduced Q species to a second site, its oxidation to an exchangeable Q and/or its diffusion back to site 1 . Structural data and molecular simulations have provided support to this model in that how electron transfer between two Q molecules may accomplish. See refs. . However, further experimental work is needed to verify the involvement of two Q molecules in the coupling mechanism. Since electron transfer takes place between two Q molecules, it may be of interest to obtain EPR signatures that would reveal spin-spin coupling between the two SQ molecules.