The photochemical reactivity of β-Lapachone-3-sulfonic acid (1) towards amino acids, nucleobases or nucleosides has been examined employing the nanosecond laser flash photolysis technique. Excitation (λ = 355 nm) of degassed solutions of 1, in acetonitrile, resulted in the formation of its corresponding triplet excited state. This transient was efficiently quenched by L-tryptophan, L-tryptophan methyl ester, L-tyrosine, L-tyrosine methyl ester and L-cysteine methyl ester (k q ≅ 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 pairs 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 or thymidine, in acetonitrile solution, which is probably due to the π π* character of triplet 1 as well as to its low triplet energy. On the other hand, the rate constant obtained when 1 was quenched by 2'-deoxyguanosine is reasonably fast (k q ≅ 10(9) L mol-1 s-1). The quantum efficiency of singlet oxygen (¹O2) formation from 1 was determined employing time-resolved near-IR emission studies upon laser excitation and showed a considerably high value (ΦΔ= 0.7).
laser flash photolysis; ortho-quinones; triplet excited state; coupled electron
