Reinterpretation of the Bronsted α for redox reactions based on the effect of substituents on hydride transfer reaction rates between NAD+ analogues

Rate and equilibrium constants have been determined spectrophotometrically for two sets of hydride transfer redox reactions between acridine and benzothiazoline derivatives and between pyridine and benzimidazoline derivatives that can be regarded as NAD(+)/NADH analogues. According to generally accepted ideas of the relation between equilibrium constants, K, and rate constants, k, these reactions would all have Bronsted alpha values close to 0.5 since the equilibrium constants, K, for these reactions range from 10(-1) to 10(2). However, when the structural variation is in the hydride acceptor, the Bronsted alpha is less than 0.5 (0.38 and 0.42, respectively), and when the structural variation is in the hydride donor, the Bronsted alpha is greater than 0.5 (0.63 and 0.61, respectively) for the present systems. The Marcus theory of atom and group transfer can explain the difference of alpha values in terms of the tightness factor in the critical configuration. When the transition state is loose and symmetrical, the deviation of the Bronsted alpha from 0.5 can be obtained by adding or subtracting a tightness factor that depends on the location of the substituents.