DETERMINATION OF SUBSTITUENT EFFECTS ON THE PROTON AFFINITIES OF NATURAL NUCLEOSIDES BY THE KINETIC METHOD

The kinetics of the unimolecular dissociations of proton-bound dimers produced by fast-atom bombardment from nucleosides and reference amine s enables the evaluation of the proton affinities (PA) of ribonucleosi des. The PAs of cytosine, guanosine, adenosine, uridine and deoxyuridi ne have been thus determined. These values and those already available for the corresponding DNA homologues allow the evaluation of the effe ct of the hydroxyl group in position 2' of the sugar moiety, which low ers the PAs of RNA nucleosides by 0.6-1 kcal/mol, and of the methyl gr oup in position 5 of the thymine ring which enhances the basicity of d eoxythymidine over deoxyuridine by 0.6 kcal/mol.