Ultrafast Guanine Oxidation by Photoexcited Cationic Porphyrins Intercalated into DNA

The photophysical and photochemical properties of a cationic porphyrin [bis(arginyl)porphyrin (BAP)] complexed to [poly(dG-dC)]₂ and [poly(dA-dT)]₂ have been investigated and compared to those of free BAP in aqueous solution. A drastic enhancement of the quantum yield of nonradiative deactivation of the lowest excited singlet state of BAP is observed upon intercalation between GC base pairs in [poly(dG-dC)]₂ but not upon complexation with [poly(dA-dT)]₂. Both picosecond time-resolved fluorescence and femtosecond transient absorption measurements give evidence for the occurrence of an ultrafast direct electron transfer (k ≥ 1.25 × 10¹³ s^-1) from guanine to the lowest excited singlet state of BAP followed by an efficient back electron transfer (k = 8.3 × 10¹² s^-1). A classical nonadiabatic Marcus model for this reverse electron transfer explains the experimental observations which allow one to estimate the electronic coupling energy (230 cm^-1) involved for BAP−[poly(dG-dC)]₂ complexes.