Proton Transfer Accompanied by the Oxidation of Adenosine

Despite numerous experimental and theoretical studies, the proton transfer accompanying the oxidation of 2'-deoxyadenosine 5'-monophosphate 2'-deoxyadenosine 5'-monophosphate (5'-dAMP, A) is still under debate. To address this issue, we have investigated the oxidation of A in acidic and neutral solutions by using transient absorption (TA) and time-resolved resonance Raman (TR³ ) spectroscopic methods in combination with pulse radiolysis. The steady-state Raman signal of A was significantly affected by the solution pH, but not by the concentration of adenosine (2-50 mm). More specifically, the A in acidic and neutral solutions exists in its protonated (AH⁺ (N1+H⁺ )) and neutral (A) forms, respectively. On the one hand, the TA spectral changes observed at neutral pH revealed that the radical cation (A^.+ ) generated by pulse radiolysis is rapidly converted into A^. (N6-H) through the loss of an imino proton from N6. In contrast, at acidic pH (<4), AH^.2+ (N1+H⁺ ) generated by pulse radiolysis of AH⁺ (N1+H⁺ ) does not undergo the deprotonation process owing to the pK_a value of AH^.2+ (N1+H⁺ ), which is higher than the solution pH. Furthermore, the results presented in this study have demonstrated that A, AH⁺ (N1+H⁺ ), and their radical species exist as monomers in the concentration range of 2-50 mm. Compared with the Raman bands of AH⁺ (N1+H⁺ ), the TR³ bands of AH^.2+ (N1+H⁺ ) are significantly down-shifted, indicating a decrease in the bond order of the pyrimidine and imidazole rings due to the resonance structure of AH^.2+ (N1+H⁺ ). Meanwhile, A^. (N6-H) does not show a Raman band corresponding to the pyrimidine+NH₂ scissoring vibration due to diprotonation at the N6 position. These results support the final products generated by the oxidation of adenosine in acidic and neutral solutions being AH^.2+ (N1+H⁺ ) and A^. (N6-H), respectively.