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

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Abstract
In view of a previously proposed enthalpy value for the second protonation of adenine, a calorimetric and NMR spectroscopic study was 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 13C and 1H chemical shifts were determined as a function of HClO4 and H2SO4 concentration. Contrary to previous usage, a strong medium effect precludes the use of the calorimetric results to calculate the thermodynamic functions for the protonation reactions. The 13C NMR data are successfully interpreted on the basis of the Cox and Yates excess acidity method. The values obtained for pKBH22+ and pKBH33+ 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.

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