Purines, pyrimidines, and imidazoles. Part XXXVIII. A kinetics study of the decarboxylation of 5-amino-1-β-D-ribofuranosylimidazole-4-carboxylic acid 5′-phosphate and related compounds

Abstract

The kinetics and mechanism of decarboxylation of 5-amino-1-β-D-ribofuranosylimidazole-4-carboxylic acid 5′-phosphate and related compounds have been investigated by several techniques including direct measurement of the volume of carbon dioxide evolved, loss of u.v. absorption on decarboxylation, and a pH stat technique. The results may be interpreted in terms of a first-order decarboxylation of both the acid and the zwitterion, whereas the anion is stable and does not decarboxylate. Evidence of catalysis by hydrogen ions at low pH values and of general base of nucleophilic catalysis was also found. Activation energies and related thermodynamic parameters were evaluated and the results are discussed in mechanistic terms. Transition metal ions were found to inhibit the reaction, presumably by complex formation. The reaction was not affected by the presence of pyridoxal, but formaldehyde produced complete loss of absorption at room temperature within a few minutes.