Quenching Enhancement of the Singlet Excited State of Pheophorbide‐a by DNA in the Presence of the Quinone Carboquone

Abstract

Changes in the emission fluorescence intensity of pheophorbide‐a (PHEO) in the presence of carboquone (CARBOQ) were used to obtain the association constant, the number of CARBOQ molecules interacting with PHEO, and the fluorescence quantum yield of the complex. Excitation spectra of mixtures of PHEO and CARBOQ in ethanol (EtOH) show an unresolved doublet in the red‐most excitation band of PHEO, indicating the formation of a loose ground‐state complex. The 1:1 CARBOQ–PHEO complex shows a higher fluorescence quantum yield in EtOH (0.41 ± 0.02) than in buffer solution (0.089 ± 0.002), which is also higher than that of the PHEO monomer (0.28). Quenching of the PHEO fluorescence by DNA nucleosides and double‐stranded oligonucleotides was also observed and the bimolecular quenching rate constants were determined. The quenching rate constant increase as the oxidation potential of the DNA nucleoside increases. Larger quenching constants were obtained in the presence of CARBOQ suggesting that CARBOQ enhances DNA photo‐oxidation, presumably by inhibiting the back–electron‐transfer reaction from the photoreduced PHEO to the oxidized base. Thus, the enhanced DNA‐base photosensitized oxidation by PHEO in the presence of CARBOQ may be related to the large extent by which this quinone covalently binds to DNA, as previously reported.