Riboflavin (VB2) photosensitized oxidation of 2′-deoxyguanosine-5′-monophosphate (dGMP) in aqueous solution:: a transient intermediates study

The use of time-resolved 337 nm and 248 nm laser flash photolysis with transient absorbance detection has shown direct evidence of electron transfer from dGMP to the triplet state of riboflavin ((RF)-R-3*). dGMP was used as a DNA model system in order to study the damaging potential of photoexcited riboflavin. The evidence obtained was that: (1) formation of radical anion of riboflavin (RF.-/RFH.) matched the decay of (RF)-R-3*; (2) the decay of (RF)-R-3* was pseudo-first-order with the concentrations of dGMP, the bimolecular reaction rate constant was determined to be 6.6 x 10(8) dm(3) mol(-1) s-(1); (3) after the complete decay of 3RF*, the transient absorption spectra of the deprotonated radical cation of dGMP [dGMP(-H)(.)] was observed in aerated condition for the first time; (4) the addition of N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) to the experimental system resulted in the formation of the TMPD radical cation via scavenging dGMP radical cation; and (5) the free energy change (Delta G) from dGMP to (RF)-R-3* was calculated to be -43.7 kJ mol(-1) with the Rehm-Weller equation, which indicates that photo-oxidation of dGMP attacked by triplet state of riboflavin is thermodynamically possible. Electron-transfer from dGMP to the oxidized radicals of riboflavin is found, which may be another pathway of DNA damage in vivo and in vitro. The direct observation of oxidized guanine radical has provided unambiguously direct initial proof for a photosensitization process of dGMP, with riboflavin as photosensitizer.