Abstract
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 (kq ≅ 109 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 (kq ≅ 109 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).
Highlights
Quinones show several biological and pharmacological activities,[1,2,3,4,5,6,7,8,9] with their mechanism of action beingVol 21, No 6, 2010Neto-Ferreira et al.topoisomerase IIa-mediated DNA breaks,[20] and inhibition of poly(ADP-ribose) polymerase-1.21 More recently, Boothman and co-workers[23] have clearly demonstrated that b-lapachone, an ortho-quinone, activates a novel apoptotic response in a number of cell lines
In this work we show results of the laser flash photolysis studies on the reactivity of b-lapachone-3-sulfonic acid (1) towards biological substrates such as amino acids and their methyl esters, nucleic bases and nucleosides, as well as its ability to form singlet oxygen, O2 (1Dg)
It is worth noting that in the quenching experiments samples were excited at 355 nm (3rd harmonic of a Nd-YAG laser), since at this wavelength the only absorbing species is the b-lapachone-3-sulfonic acid
Summary
Quinones show several biological and pharmacological activities,[1,2,3,4,5,6,7,8,9] with their mechanism of action being. Topoisomerase IIa-mediated DNA breaks,[20] and inhibition of poly(ADP-ribose) polymerase-1.21 More recently, Boothman and co-workers[23] have clearly demonstrated that b-lapachone, an ortho-quinone, activates a novel apoptotic response in a number of cell lines. The characterization and reactivity of triplet b-lapachone-3-sulfonic acid (1) has been recently reported by us. Hydrogen abstraction rate constant for the triplet 1 towards 2-propanol or 1,4-cyclohexadiene is quite low (105 L mol-1 s-1), which has been associated with its pp* character. 4-methoxyphenol or indole quenches the triplet excited state of 1 with a rate constant of 109 L mol-1 s-1. Triplet 1 reacts with electron donors, such as triethylamine, at an almost diffusion-controlled rate, yielding the corresponding long-lived anion radical
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