Photosensitizing properties of triplet beta-lapachones in acetonitrile solution

The photochemical reactivity of beta-lapachone (1), nor-beta-lapachone (2) and 1,2-naphthoquinone (3) towards amino acids and nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (lambda = 355 nm) of degassed solutions of 1-3, in acetonitrile, resulted in the formation of their corresponding triplet excited states. These transients were efficiently quenched by l-tryptophan, l-tryptophan methyl ester, l-tyrosine, l-tyrosine methyl ester and l-cysteine (k(q) approximately 10(9) L mol(-1) s(-1)). For l-tryptophan, l-tyrosine and their methyl esters new transients were formed in the quenching process, which were assigned to the corresponding radical pair resulting from an initial electron transfer from the amino acids or their esters to the excited quinone, followed by a fast proton transfer. No measurable quenching rate constants could be observed in the presence of thymine and thymidine. On the other hand, efficient rate constants were obtained when 1-3 were quenched by 2'-deoxyguanosine (k(q) approximately 10(9) L mol(-1) s(-1)). The quantum efficiency of singlet oxygen ((1)O(2)) formation from 1 to 3 was determined employing time-resolved near-IR emission studies upon laser excitation and showed considerably high values in all cases (Phi(Delta) = 0.6), which are fully in accord with the pipi character of these triplets in acetonitrile.