Amino Proton Exchange Processes in Mononucleosides

Abstract

Amino proton exchange with water could be a useful probe of nucleic acid structure and kinetics. This requires an understanding of exchange processes in the mononucleosides, which has been only partial up till now. We investigate these processes by nuclear magnetization transfer in cytidine, cAMP, cGMP, and endocyclic-methylated derivatives. From the effect of pH on the exchange rate, we disentangle the exchange processes which involve the neutral nucleoside or the nucleoside protonated on an endocyclic nitrogen, and those which operate by basic catalysis (proton transfer to OH- or to water) or by acid catalysis (proton transfer from hydronium). (Cyclic AMP) At basic pH, the amino proton transfers from the neutral nucleoside to OH-. Between pH 7 and pH 5, it transfers from the endocyclic-protonated nucleoside to water. Endocyclic protonation lowers the deprotonation pK of the amino group by about 9 units. This is confirmed by the effect of proton acceptors such as ammonia on amino proton exchange. Below p...