General and specific acid/base catalysis of the hydrolysis and interconversion of ribonucleoside 2′- and 3′-phosphotriesters: kinetics and mechanisms of the reactions of 5′-O-pivaloyluridine 2′- and 3′-dimethylphosphates

Abstract

Kinetics of the hydrolysis and interconversion of 5′-O-pivaloyluridine 2′- and 3′-dimethylphosphates in the pH range 0–9 have been studied. At pH < 2, both reactions are first order in hydronium ion concentration, the hydrolysis being three times as fast as the interconversion. The interconversion, however, becomes hydroxide-ion-catalysed at pH as low as 2.5, whereas the hydrolysis remains pH independent to pH 7, and becomes then base-catalysed. Both reactions are susceptible to general-base catalysis, the Brøsted β values based on carboxylate ions being 0.75. These observations suggest that the monocationic phosphorane intermediate, obtained by the attack of 2′-OH on the phosphotriester monocation, is decomposed to the hydrolysis and isomerization products at a comparable rate. By contrast, the monoanionic phosphorane, obtained by the attack of 2′-O^– on the neutral phosphotriester, predominantly gives isomerization products; the methoxide ion leaves 10⁵ times less readily than the sugar oxyanions, 2′-O^– or 3′-O^– . Accordingly, the pH-independent hydrolysis appears to consist of consecutive specific base/acid catalysis. The buffercatalysed reactions are suggested to proceed by general-base-catalysed attack of 2′-OH on neutral phosphotriester, followed by general-acid-catalysed decomposition of the phosphorane intermediate to either hydrolysis or isomerization products. The mechanisms of the hydrolysis and isomerization of the internucleosidic phosphodiester bonds are discussed on these bases.