Influence of Sodium Cationization versus Protonation on the Gas-Phase Conformations and Glycosidic Bond Stabilities of 2′-Deoxyadenosine and Adenosine

The influence of noncovalent interactions with a sodium cation on the gas-phase structures and N-glycosidic bond stabilities of 2′-deoxyadenosine (dAdo) and adenosine (Ado), [dAdo+Na]⁺ and [Ado+Na]⁺, are probed via infrared multiple photon dissociation (IRMPD) action spectroscopy and energy-resolved collision-induced dissociation (ER-CID) experiments. ER-CID experiments are also performed on the protonated forms of these nucleosides, [dAdo+H]⁺ and [Ado+H]⁺, for comparison purposes. Complementary electronic structure calculations are performed to determine the structures and relative stabilities of the stable low-energy conformations of the sodium cationized nucleoside complexes and to predict their IR spectra. Comparison between the measured IRMPD action spectra and calculated IR spectra enables the conformations of the sodium cationized nucleosides present in the experiments to be elucidated. The influence of sodium cationization versus protonation on the structures and IR spectra is elucidated by comparison to IRMPD and theoretical results previously reported for the protonated forms of these nucleosides. The influence of sodium cationization versus protonation on the glycosidic bond stability of the adenine nucleosides is determined by comparison of the ER-CID behavior of these systems. All structures present in the experiments are found to involve tridentate binding of Na⁺ to the N3, O4′, and O5′ atoms forming favorable 5- and 6-membered chelation rings, which requires that adenine rotate to a syn configuration. This mode of sodium cation binding results in moderate flexibility of the sugar moiety such that the sugar puckering of the conformations present varies between C2′-endo and O4′-endo. Sodium cationization is found to be less effective toward activating the N-glycosidic bond than protonation for both dAdo and Ado. Both the IRMPD yields and ER-CID behavior indicate that the 2′-hydroxyl substituent of Ado stabilizes the N-glycosidic bond relative to that of dAdo.