Malonylcoenzyme A models. Part 1. E1cB (Keten) pathway for acyl transfers of malonic acid thiolmonoesters including S-malonylcoenzyme A

The aminolysis of a series of malonate thiolmonoesters (HO ₂CCH₂COSR) has been studied kinetically at 25°C. No indication of substrate ionisation was observed for the 5-4-chlorophenyl and S-phenyl esters up to 0.45M-sodium hydroxide. Morpholinolysis showed saturation kinetics, i.e. the observed pseudo-first-order rate constant became insensitive to morpholine concentration at higher morpholine concentrations. This behaviour was analysed in terms of the rate equation k_obs = k _max.[B]/(K + [B]), where k_max. is the limiting rate constant at higher buffer (B) concentrations and K the concentration of buffer required for k_obs = 0.5 k_max. Saturation aminolysis was also observed for S-malonylcoenzyme A. Arrhenius parameters were determined for the above kinetic parameters (k_max., K, k_max./K) for the S-benzyl ester. Trapping experiments with aniline showed that rate-determining and product-determining steps differed. The results were interpreted in terms of an intermediate keten formed from the ester dianion (-O₂CCHCO-SR). However, to explain leaving-group dependences of k_max., K, etc., a degree of leaving-group protonation in the transition-state had to be invoked, although mechanisms involving the zwitterion-anion [-O₂CCH· COS(H)R] could be excluded.