HYDROGEN DEUTERIUM EXCHANGE OF NUCLEOTIDES IN THE GAS-PHASE/

Gas-phase hydrogen/deuterium exchange reactions have been performed on the 5'- and 3'-nucleotide monophosphates and on the 3',5'-cyclic nucl eotides, Following negative mode electrospray ionization and transport to a Fourier transform ion cyclotron resonance cell, each nucleotide was reacted with gaseous D2O for up to 600 s. Extensive deuterium exch ange was observed for the 3'- and 5'-nucleotides in negative ion mass spectra with relative rates of exchange following the trend 5'-dCMP > 5'-dAMP > 5'-dTMP much greater than 5'-dGMP and 3'-dGMP > 3'-dAMP appr oximate to 3'-dCMP approximate to 3'-dTMP. At least two classes of exc hanging protons are observed. The more facile class is assigned to the amino protons of the bases, with a slower class attributed to the pho sphate and/or hydroxyl proton. Overall, the 3'-nucleotides exchange mo re quickly than the 5'-oligonucleotides The cyclic nucleotides did not undergo deuterium exchange, suggesting that a charged phosphate group proximate to the base is required to catalyze the exchange reaction. Exchange through tautomerization of the bases is not observed, althoug h molecular modeling suggests an energy barrier of <30 kcal.