Energetics of Multistep versus One-step Hydride Transfer Reactions of Reduced Nicotinamide Adenine Dinucleotide (NADH) Models with Organic Cations and <i>p</i>-Quinones

Abstract

Free energy changes of each elementary step involved in the formal hydride transfer (H(-)(T)) reactions (including the so-called "one-step" H(-)(T) and "multistep" H(-)(T) mechanisms) of the reduced nicotinamide adenine dinucleotide (NADH) models with various cations and quinones (17) were investigated either by direct thermodynamic measurements or by calculations from thermochemical cycles. Based on the energetic data thus derived, combined with kinetic observations (particularly kinetic isotope effects), the mechanistic characteristics of the NADH-mediated reductions were systematically analyzed. Practical guidelines that resulted from these analyses suggest that a mutistep mechanism (e(-)-H(*) or e(-)-H(+)-e(-)) would be followed if the energy gap of the initial electron transfer [DeltaG(e(-)(T))] between the NADH model compound and the reducing substrate is considerably smaller than the empirical critical limit of 1.0 V for an endothermic e(-)(T). In contrast, if DeltaG(e(-)(T)) is much greater than 1.0 V, a concerted one-step H(-)(T) may take place. The guideline also suggests that a "hybrid" mechanism is possible if the DeltaG(e(-)(T)) is in an intermdiate situation.