Acylation. Part XXX. Metal ion catalysed hydrolysis of acetyl phosphate

Abstract

The hydrolysis of acetyl phosphate in aqueous solution, catalysed by various uni- and bi-valent cations has been studied. The results partly confirm and partly refute conclusions drawn from previous studies. Lithium ions detectably catalyse the hydrolysis. The magnitude, and nature, of the much greater effects produced by M2+ ions depend upon the pH. At 25°, and pH 8·45, the sequence of catalytic reactivity is Mg2+ [graphic omitted] Ca2+ > Sr2+ > Ba2+, but this sequence can change at lower pH values. Previous conclusions about the mechanism of catalysis are shown to be faulty and a new reaction scheme, compatible with all the available evidence, is suggested. This scheme involves, at high pH, an important contribution from a path involving reaction between AcPO4M and MOH+.The association constant, K1, between Mg2+ and AcPO42– ions has been independently determined at two temperatures. At 4°K1= 30 and at 25°K1= 75. For Ca2+ a value K1= 50 at 25° is compatible with the kinetic results.The acylation of ammonia, methylamine, and dimethylamine by acetyl phosphate in amine buffers involves the sequence of nucleophilic reactivity NH3 MeNH2 > Me2NH. It is probable that, in the pH range 7–8·5, this reaction involves acylation by both AcPO4H– and AcPO42–. We calculate that with methylamine AcPO4H– is ca. 200-fold more reactive than AcPO42–.