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 (TR 3 ) 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 m m ). More specifically, the A in acidic and neutral solutions exists in its protonated ( A H + (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), A H .2+ (N1+H + ) generated by pulse radiolysis of A H + (N1+H + ) does not undergo the deprotonation process owing to the p K a value of A H .2+ (N1+H + ), which is higher than the solution pH. Furthermore, the results presented in this study have demonstrated that A , A H + (N1+H + ), and their radical species exist as monomers in the concentration range of 2–50 m m . Compared with the Raman bands of A H + (N1+H + ), the TR 3 bands of A H .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 A H .2+ (N1+H + ). Meanwhile, A . (N6−H) does not show a Raman band corresponding to the pyrimidine+NH 2 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 A H .2+ (N1+H + ) and A . (N6−H), respectively.