The methanolysis of nitrocefin (1) was investigated at 25 degreesC in anhydrous methanol as a function of [Zn2+] and (s)(s)pH as a model for the chemistry believed to occur in the active site of Zn2+- beta-lactamases. In the absence of metal ion, the (s)(s)pH vs rate constant profile shows a long plateau region having k(obs), = 9 x 10(-5) s(-1) between (s)(s)pH 7.5 and 12 generated after ionization of the COOH of 1 (kinetic (s)(s)pK(a) of 7.34) followed by attack of CH30- with kCH,O 1.18 M-1 s(-1). Strong catalysis of the methanolysis is observed at all (s)(s)pH values between 7.95 and 11.34 in the presence of as little as 0.05-3 mM Zn2+. Plots of the pseudo-first-order rate constant for methanolysis (k(obs)) as a function of (s)(s)pH reveal a saturation phenomenon indicative of formation of a 1:Zn2+ Complex, followed by a linear section indicative of the intervention of a second Zn2+ ion in promoting the methanolysis of the complex. The two processes can be separated since the slope of the linear part of the plots gives the second-order rate constant (k(2)(obs)) for the second Zn process, while the intercept gives the spontaneous rate constant for methanolysis of 1:Zn2+ (k(cat,)(obs)). A plot of log k(cat.)(obs) VS (s)(s)pH is bell-shaped, maximizing at similar to(s)(s)pH 10, with ascending and descending domains both first order in [CH3O-]. A plot of log k(2) vs (s)(s)pH gives an ascending domain second order in [CH3O-], followed by a plateau above (s)(s)pH 9.5. The data are analyzed in terms of a one-Zn and two-Zn model in which the former involves rate-limiting attack of methoxide on a 1:Zn2+ complex up to the (s)(s)pH maximum and a spontaneous reaction of 1:Zn2+: (CH3O-)(2), while the latter involves bimolecular attack of Zn2+(CH3O-)(2) on both 1:Zn2+ and 1:Zn2+: (CH3O-)(2). The relevance of these observations is discussed in terms of the currently accepted mechanism for hydrolyses of beta-lactams promoted by the Zn(2+-)beta-lactamases.