Characterization, reactivity and photosensitizing properties of the triplet excited state of alpha-lapachone

alpha-Lapachone is a natural 1,4-naphthoquinone with promising biological activity. The fused dihydropyran ring present in its structure, acting as formal 2-alkoxy and 3-alkyl substituents to the quinone moiety, endows this compound with milder redox properties and lower toxicity, when compared with other bioactive 1,4-quinones. Its photochemistry, here reported, seems to originate from the triplet state, which shows pipi* character. Triplet quenching in acetonitrile solution with added hydrogen-atom donors such as 1,4-cyclohexadiene or 2-propanol is inefficient, independent of solvent polarity, and leads to formation of the semiquinone radical. With phenol and indole, quenching rate constants are two orders of magnitude higher, but smaller than the value for triethylamine. In the first two cases the semiquinone radical can be detected by laser flash photolysis and in the last case, the anion radical derived from alpha-lapachone is readily detected. The semiquinone radical can also be observed in the quenching of triplet alpha-lapachone by 2'-deoxyguanosine and by the methyl esters of L-tryptophan and L-tyrosine, whereas for L-cysteine methyl ester the quenching rate constant is very slow. Triplet alpha-lapachone is not quenched by thymine, thymidine, 2'-deoxycytosine or 2'-deoxyadenosine; this is probably due to its pipi* character and low energy, which prevents oxetane formation and triplet-triplet energy transfer, respectively. Steady-state photolysis of aerated solutions of these compounds in the presence of alpha-lapachone does not show evidence of decomposition, whereas similar experiments with 2'-deoxyguanosine result in efficient consumption of the nucleoside. Singlet oxygen is formed from triplet alpha-lapachone, and a quantum yield of 0.68 is measured.