Protonation de l'adénine, de la purine et de l'adénosine en milieu acide fort

Abstract

In view of a previously proposed enthalpy value for the second protonation of adenine, a calorimetric and nmr spectroscopic study has been made of the protonation of biologically important bases B: adenine, purine, adenosine and, incidentally, that of imidazole and 3-amino pyridine. Heats of solution of the solid bases, and their carbon-13 and proton chemical shifts were determined as a function of HClO4 and H2SO4 concentration. Our results demonstrate that, contrary to previous usage, a strong medium effect precludes the use of the calorimetric results to calculate the thermodynamic functions for the protonation reactions. On the other hand, the carbon-13 nmr data are successfully interpreted on the basis of the Cox and Yates excess acidity method. The values obtained for [Formula: see text] and [Formula: see text] are, respectively: adenine −0.43, −4.23, purine −1.66, < −6, adenosine −1.4 with hydrolytic cleavage of the glycosidic bond. The protonation sites are confirmed as being essentially on the imidazole ring for the second protonation and on the pyrimidine ring (N-3) for the third one.