The study of triplet excited state behavior of nucleic acids and component mononucleotides is hampered by the very small yields produced by direct photolysis. We have used high energy triplet sensitizers to generate these species in high yield, thus facilitating the study of their photophysical and photochemical behavior. Acetone-sensitized triplet formation of all triplet state nucleotides allowed nucleotide triplet−triplet absorption spectra to be measured. Triplet−triplet absorption coefficients were determined using comparative actinometry. Self-quenching of the nucleotide triplet states was found to occur efficiently with rate constants, k sq > 107 M-1 s-1. The interaction of a variety of ketone triplet sensitizers with mononucleotides has been studied as a function of the relative energies of the sensitizer−nucleotide pair. In all cases, the triplet states of the sensitizers were efficiently quenched by the nucleotides, although different reaction mechanisms were observed depending on the reaction pair under study. Acetone, the sensitizer with the highest triplet energy, sensitized all triplet state nucleotides. Sensitizers with triplet energies, E T > 74 kcal mol-1, sensitized TMP and those with E T < 74 kcal mol-1 did not exhibit any triplet sensitization, although an efficient quenching reaction (k q > 108 M-1 s-1) was observed. Where energy transfer did not take place, sensitizers were quenched by electron transfer from the purines. The quantum yield for this process was determined as 0.31 for GMP and 0.09 for AMP. In DNA, triplet energy transfer from the same sensitizers was probed by determining the relative efficiency of pyrimidine dimer formation in pBR322, an exclusively triplet-mediated reaction under sensitized conditions. Our results allow some conclusions to be drawn on triplet properties and intramolecular energy transfer in DNA. Base triplet energy levels appear to be lower in DNA than in the isolated mononucleotides. In any system where ketone triplet states are generated, electron transfer from a purine should be considered as a significant reaction pathway.

In this paper we report a time-resolved investigation of transients derived from curcumin, which may be intimately involved in the processes leading to its biological activity. Fluorescence and triplet quantum yields are respectively 0.06 and 0.11. The high percentage of internal conversion is proposed to proceed via H-transfer within the thermodynamically favored enol structure of what is formally a 1,3-diketone. The triplet energy (191 +/- 2 kJ mol-1), natural lifetime (1.5 microseconds) and self-quenching rate constant (5.0 x 10(8) L mol-1 s-1) have been determined. Oxygen quenching of the triplet leads to the production of singlet oxygen with unit efficiency. Curcumin quenches the latter species very inefficiently (2.5 x 10(5) L mol-1 s-1). The curcumin radical has been produced via three mechanistically distinct methods. This species is unreactive toward oxygen but is repaired by vitamins C and E and anthralin.

Abstract— Laser flash photolysis of S‐nitroso complexes of glutathione (GSNO) and bovine serum albumin (BSANO) via excitation at 355 nm has been used to investigate the photogeneration of nitric oxide (NO) and subsequent radical reactions. In the case of GSNO, liberation of NO was confirmed by its oxidation of oxyhemoglobin to met hemoglobin. Initial NO release is via homolytic cleavage of the S‐N bond to produce the glutathione thiyl radical, GS, which can subsequently react with (a) ground‐state GSNO (k= 1.7 × 109M−1/i> s−1) to yield additional NO and oxidized glutathione, GSSG; and (b) oxygen (k= 3.0 × 109M−1 s−1) to give the glutathione peroxy radical, GSOO, which subsequently reacts with ground‐state GSNO (k= 3.8 × 108M−1 s−1), also producing additional NO and GSSG. The relative concentrations of oxygen and GSNO in the system determine the major pathway for removal of G'. These secondary reactions occur at such high rates that they preclude radical recombination under low‐intensity irradiation conditions. The quantum yield of overall loss of GSNO thus varies with both GSNO and oxygen concentrations; a value of 0.66 was determined for an aerated solution of GSNO (0.86 mM). In the case of GSNO, therefore, generation of NO is not due solely to homolysis of the S‐N bond; secondary reactions of the radicals formed lead to further NO liberation. In rationalizing the known phototoxicity of GSNO, possible contributions from thiyl and thiyl‐derived radicals should be considered. In contrast to GSNO, direct excitation of BSANO (containing one bound NO group per molecule) led to photodecomposition with a quantum yield of 0.09 but no evidence was obtained for liberation of NO into the bulk medium.