Substituent Effects on the Pre-Steady-State Kinetics of Oxidation of Benzyl Alcohols by Liver Alcohol Dehydrogenase.

1. The oxidation of benzyl alcohols by liver alcohol dehydrogenase under conditions [S]_o » [E]_o is a biphasic process with exponential rise to a steady state. The rate constants for the transient phase were determined by a curve-fitting procedure. 2. A large isotope effect was obtained for p-methoxybenzyl alcohol, indicating that hydrogen transfer is rate-determining for the transient phase. Rate constants for the hydrogen transfer step, k₃, were obtained for the various alcohols and correlated with the Hammett σ constant by plotting log k₃ against σ. The slope of this line (q = -0.76) is consistent with rate-limiting hydride transfer; a mechanism is proposed to account for the low magnitude of q. 3. The pre-steady-state rate constants, k_b, for ali benzyl alcohols (<20 s ^-1) are lower than that for ethanol. 2-Methylpropan-l-ol and cyclohexylmethanol have pre-steady-state rate constants of about 150 s-^-1, indicating that the lower k_b values for benzyl alcohols are due to the electronwithdrawing effect of the benzene ring, rather than to steric effects. [ABSTRACT FROM AUTHOR]