Mechanisms and energetics for N-glycosidic bond cleavage of protonated 2′-deoxyguanosine and guanosine

Abstract

Experimental and theoretical investigations suggest that hydrolysis of N-glycosidic bonds generally involves a concerted S_N2 or a stepwise S_N1 mechanism. While theoretical investigations have provided estimates for the intrinsic activation energies associated with N-glycosidic bond cleavage reactions, experimental measurements to validate the theoretical studies remain elusive. Here we report experimental investigations for N-glycosidic bond cleavage of the protonated guanine nucleosides, [dGuo+H]⁺ and [Guo+H]⁺, using threshold collision-induced dissociation (TCID) techniques. Two major dissociation pathways involving N-glycosidic bond cleavage, resulting in production of protonated guanine or the elimination of neutral guanine are observed in competition for both [dGuo+H]⁺ and [Guo+H]⁺. The detailed mechanistic pathways for the N-glycosidic bond cleavage reactions observed are mapped via electronic structure calculations. Excellent agreement between the measured and B3LYP calculated activation energies and reaction enthalpies for N-glycosidic bond cleavage of [dGuo+H]⁺ and [Guo+H]⁺ in the gas phase is found indicating that these dissociation pathways involve stepwise E1 mechanisms in analogy to the S_N1 mechanisms that occur in the condensed phase. In contrast, MP2 is found to significantly overestimate the activation energies and slightly overestimate the reaction enthalpies. The 2′-hydroxyl substituent is found to stabilize the N-glycosidic bond such that [Guo+H]⁺ requires ∼25 kJ mol⁻¹ more than [dGuo+H]⁺ to activate the glycosidic bond.