The photo-oxidation of purine nucleotides adenosine-5'-monophosphate (AMP) and guanosine-5'-monophosphate (GMP) by 3,3',4,4'-benzophenone tetracarboxylic acid (TCBP) has been investigated in aqueous solutions using nanosecond laser flash photolysis (LFP) and time-resolved chemically induced dynamic nuclear polarization (CIDNP). The pH dependences of quenching rate constants and of geminate polarization are measured within a wide range of pH values. As a result, the chemical reactivity of reacting species in different protonation states is determined. In acidic solution (pH < 4.9), the quenching rate constant is close to the diffusion-controlled limit: k(q) = 1.3 x 10(9) M-1 s(-1) (GMP), and k(q) = 1.2 x 10(9) M-1 s whereas in neutral and basic solutions it is significantly lower: k(q) = 2.6 x 10(8) M-1 s(-1) (GMP, 4.9 < pH < 9.4), k(q) = 3.5 x 10(7) M-1 sr(-1) (GMP, pH > 9.4), k(q) = 1.0 x 10(8) M-1 s(-1) (AMP, pH > 6.5). Surprisingly, the strong influence of the protonation state of the phosphoric group on the oxidation of adenosine-5'-monophosphate is revealed: the deprotonation of the AMP phosphoric group (6.5) decreases the quenching rate constant from 5.0 x 10(8) M-1 s(-1) (4.9 < pH < 6.5) to 1.0 x 10(8) M-1 s(-1) (pH > 6.5).