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, ksq > 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 pa...

Interaction of Triplet Photosensitizers with Nucleotides and DNA in Aqueous Solution at Room Temperature

Photochemical and gamma -ray induced reactions of purines with amines are described which lead to the appropriate 8-substituted purines. The photoproducts were obtained in yields of up to 100% by the use of a variety of photosensitizers, including peroxides. A free radical mechanism is proposed for these reactions. It is suggested that such reactions may be relevant to the problem of the nature DNA-protein cross-links induced by ultraviolet radiation. (auth)

Ultraviolet and $gamma$-ray-induced reactions of nucleic acid constituents. Reactions of purines with amines

. Transient absorption spectra produced by 266-nm ns laser flash photolysis of aqueous solutions of adenine, 2′-deoxyadenosine, 2′-deoxyadenosine-5′-phosphate, adenosine-5′-phosphate, 2′-deoxyadenylyl-(3′-5′)-2-deoxyadenosine, 7-methyladenine and 9-methyladenine have been measured under different pH conditions. The transients observed after excitation consisted mainly of the hydrated electron, the radical cation, the radical anion and/or neutral radicals resulting from its protonation, and a minor contribution from the excited triplet state. The photoionization occurs mainly through an excited singlet state. For adenine, the cation and anion radical absorption spectra were determined using laser and pulse radiolysis techniques. The triplet state presence was established through sensitization experiments. The hydrated electron decays following a pseudo-first-order reaction with the neutral base.

Characterization of the transient species in the 266-nm laser photolysis of adenine and its derivatives

The low-intensity steady-state (254 nm), microsecond flash and nanosecond (266 nm) laser photolysis of some guanine (Gua) derivatives in aqueous solution were studied. A photodestruction yield between 10 23 and 10 22 at a base concentration of 75 mM was determined for 254 nm irradiation at room temperature using high-performance liquid chromatography. This yield decreases with increasing purine concentration. For a similar concentration of the purine bases (2 6 1) 3 10 25 M, the yield increases as follows: Gua 9-ethylguanine , deoxyguanosine guanosine (Guo) , guanosine 59-monophosphate. At concentrations higher than 2 3 10 24 M the Gua derivatives’ photodestruction yield seems to converge to a limiting value of the order of 10 24 . This behavior is explained in terms of self-quenching and aggregation effects which deactivate the excited states of the bases. The yields of electron photoejection have been determined in the nanosecond laser photolysis (0.083) and in the lowintensity steady-state (5.8 3 10 23 ) for Guo. Competition experiments using electron scavengers suggest that the electron adducts of the bases are one of the principal species participating in the photodestruction mechanism of these monomeric Gua. Close to 75% of the total destruction yield has contributions from initial reactions of the photojected electron at neutral pH. The quantum yield of photodestruction of Guo increases when the pH is increased as follows: 4.7 3 10 23

Part I. Photochemical and Photophysical Studies of Guanine Derivatives: Intermediates Contributing to its Photodestruction Mechanism in Aqueous Solution and the Participation of the Electron Adduct

The 266 nm nanosecond laser photolysis of various purine and pyrimidine derivatives results in their photoionization (PI) as one of the primary photochemical pathways. Electron photoejection occurs through a combination of one- and two-photon mechanisms. The PI values depend on the substituents attached to the chromophore of the base. The net PI of the purine bases at 266 nm are of the same order of magnitude (10(-2)) as those of the pyrimidine bases under similar experimental conditions. The monophotonic component is approximately one-third of the net PI yield of the bases. A nonrelaxed singlet excited state intermediate is tentatively proposed for this pathway. It is proposed that this state is significantly stabilized by water solvation, transforming it into a charge transfer to solvent state from which the hydrated electron evolves.

Photoionization of DNA and RNA bases, nucleosides and nucleotides through a combination of one- and two-photon pathways upon 266 nm nanosecond laser excitation.

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The flash photolysis of purine in acetonitrile and in water at different pH was studied. The transients produced on flash excitation of degassed aqueous solutions have been identified as the triplet excited state, the hydrated electron, a purine radical cation and radical anion on the basis of quenching experiments and comparison to transients observed in low temperature photolysis.

V. Rivera

Papers

Intermediates in the room temperature flash photolysis and low temperature photolysis of purine solutions