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Thephoto-oxidation of purine nucleotides adenosine-5′-monophosphate(AMP) and guanosine-5′-monophosphate (GMP) by 3,3′,4,4′-benzophenonetetracarboxylic acid (TCBP) has been investigated in aqueous solutionsusing nanosecond laser flash photolysis (LFP) and time-resolved chemicallyinduced dynamic nuclear polarization (CIDNP). The pH dependences ofquenching rate constants and of geminate polarization are measuredwithin a wide range of pH values. As a result, the chemical reactivityof reacting species in different protonation states is determined.In acidic solution (pH < 4.9), the quenching rate constant is closeto the diffusion-controlled limit: kq=1.3 × 109M–1s–1(GMP), and kq= 1.2 × 109M–1s–1(AMP), whereas in neutraland basic solutions it is significantly lower: kq= 2.6 × 108M–1s–1(GMP, 4.9 < pH < 9.4), kq= 3.5× 107M–1s–1(GMP,pH > 9.4), kq= 1.0 × 108M–1s–1(AMP, pH > 6.5).Surprisingly,the strong influence of the protonation state of the phosphoric groupon the oxidation of adenosine-5′-monophosphate is revealed:the deprotonation of the AMP phosphoric group (6.5) decreases thequenching rate constant from 5.0 × 108M–1s–1(4.9 < pH < 6.5) to 1.0 × 108M–1s–1(pH > 6.5). [ABSTRACT FROM AUTHOR]

Thekinetics of the photooxidation of aromatic amino acids histidine (His),tyrosine (Tyr), and tryptophan (Trp) by 3,3′,4,4′-benzophenonetetracarboxylicacid (TCBP) has been investigated in aqueous solutions using time-resolvedlaser flash photolysis and time-resolved chemically induced dynamicnuclear polarization. The pH dependence of quenching rate constantsis measured within a large pH range. The chemical reactivities offree His, Trp, and Tyr and of their acetylated derivatives, N-AcHis, N-AcTyr, and N-AcTrp, toward TCBP triplets are compared to reveal the influenceof amino group charge on the oxidation of aromatic amino acids. Thebimolecular rate constants of quenching reactions between the triplet-excitedTCBP in the fully deprotonated state and tryptophan, histidine, andtyrosine with a positively charged amino group are kq= 2.2 × 109M–1s–1(4.9 < pH < 9.4), kq= 1.6 × 109M–1s–1(6.0 < pH < 9.2), and kq= 1.5× 109M–1s–1(4.9< pH < 9.0), respectively. Tryptophan, histidine, and tyrosinewith a neutral amino group quench the TCBP triplets with the correspondingrate constants kq= 8.0 × 108M–1s–1(pH > 9.4), kq= 3.0 × 108M–1s–1(pH > 9.2), and kq= (4.0–10.0) × 108M–1s–1(9.0 < pH < 10.1) that are close tothose for the N-acetylated derivatives. Thus, it has been establishedthat the presence of charged amino group changes oxidation rates bya significant factor; i.e., His with a positively charged amino groupquenches the TCBP triplets 5 times more effectively than N-AcHis and His with a neutral amino group. The efficiency of quenchingreaction between the TCBP triplets and Tyr and Trp with a positivelycharged amino group is about 3 times as high as that of both Tyr andTrp with a neutral amino group, N-AcTyr and N-AcTrp. [ABSTRACT FROM AUTHOR]