The mechanisms and products of photosensitized and singlet oxygen oxidation of nucleic acid model compounds have been investigated using kinetics analysis and product isolation and identification. A competitive kinetics analysis is presented as a simple yet quantitative method for the determination of the role of singlet oxygen in homogeneous chemical reactions. This method is compared to other techniques that have been used in the past including determination of the solvent isotope effect and the effect of singlet oxygen quencliers and variations in oxygen and substrate concentration. The results of competitive kinetics experiments indicate that the photooxidation of dGuo (2'-deoxyguanosine) at pH 7-10 in buffered aqueous solution sensitized by Rose Bengal, Methylene Blue, Thionine or several other dyes proceeds predominantly by a pathway requiring intermediate formation of singlet oxygen. In contrast, dGuo photooxidation under the same conditions sensitized by flavin or pterin derivatives proceeds predominantly by pathways that do not involve singlet oxygen. Most of the primary products of dGuo photooxidation are very unstable and consequently are difficult, if not impossible, to isolate and characterize. However, two compounds have now been isolated that correspond to the addition of a single molecule of oxygen to the purine ring of guanine with the purine ring system still intact. Some of the products formed in the singlet oxygen oxidation of 2'deoxyguanosine appear to be different than some of the products formed in flavin photosensitized reactions in agreement with kinetic results that indicate that the flavin sensitized reactions proceed by mechanisms that do not involve intermediacy of singlet oxygen. If the unique nature of the products of 2'-deoxyguanosine oxidation by singlet oxygen can be firmly established it may be possible to detect singlet oxygen reactions in v i m by product analysis.' Deceased.

A product isolated from thymine irradiated with ultraviolet light in frozen aqueous solution undergoes dehydration on heating with acids. As judged by elemental analysis, mass, ultraviolet, infrared, and nuclear magnetic resonance spectra, the most probable structures for this compound and its dehydration product, respectively, are 5-hydroxy-6-4'-[5''-methylpyrimidin-2'-one]-dihydrothymine and 6-4'-[5'-methylpyrimidin-2'-one]-thymine. Apparently, this compound is a thymine-thymine adduct and presumably is formed through the rearrangement of an initial photoproduct. Both compounds are closely related to 6-4'-[pyrimidin-2'-one]-thymine which has been isolated from acid hydrolyzates of ultraviolet-irradiated DNA and supposedly is derived from cytosine-thymine adduct. Formation of such adducts between pyrimidine bases is apparently a common photoreaction and may be important to the study of the photochemistry and photobiology of nucleic acids.

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Two new products were isolated from uracil irradiated with ultraviolet light in frozen aqueous solution. As judged by mass, nuclear magnetic resonance, and ultraviolet and infrared spectra, one is a photopolymer, U(3) and the structure of the other is probably 6-4'-[ pyrimidin-2'-one]-uracil. Formations of these products between pyrimidine bases are apparently common photoreactions, and may be important to the study of the photochemistry and photobiology of nucleic acids.

Thymine trimer was isolated from a frozen aqueous solution of thymine which was irradiated with ultraviolet light and was presumably formed through the rearrangement of an initial oxetane photoproduct. X-ray diffraction analysis of a single crystal has confirmed the trimeric diol structure and has established the stereoconfiguration of the molecule. The possible importance of the diol structure in photobiology is pointed out.