Uracil photodimer

C8H7N5O, 3-(7-Adeninyl)propionic acid lactam, 45B, 447 C8H7N5O, 3-Methyl-3H-imidazo[2,1-i]purine-8(7H)-one, 35B, 254 C8HeCl2HgN404, Uracil-mercuric chloride, 37B, 225 C8H8N4O4, cis-anti Uracil photodimer, 37B, 226 C8H8N4O4, cis-syn Uracil photodimer, 35B, 256... 

In contrast, the photochemistry of uracil, thymine and related bases has a large and detailed literature because most of the adverse effects produced by UV irradiation of tissues seem to result from dimer formation involving adjacent thymine residues in DNA. Three types of reaction are recognizable (i) photohydration of uracil but not thymine (see Section 2.13.2.1.2), (ii) the oxidation of both bases during irradiation and (iii) photodimer formation. 

Quinolone undergoes photochemical addition of tetramethylethylene to give (434) (70AHC(ii)50), l,3-oxazin-4-ones photocycloadd ketene acetals to give (435), and irradiation of 2,6-dimethylpyran-4-one yields the cage dimer (436). 2-Pyranones form [2 + 2] photodimers whose structure is similar to that of uracil dimers (432 or 433). 

The foregoing is of obvious significance in relation to the photochemical behaviour of cyclobutane photodimers of natural pyrimidines such as uracil (III, Scheme 1), thymine, cytosine12,76). Photodissociation of such photodimers to the parent monomers, which proceed with quantum yields ranging from 0.5 to nearly unity, has, indeed, been employed as one of the criteria for identification of such dimers. The validity of this criterion is now, in the light of the behaviour of the dimer electroreduction product of pyrimidone-2, at best somewhat restricted, notwithstanding that the quantum yields for photodissociation of the latter are lower. 

The pyrimidine nucleobases have the highest quantum yields for photoreactivity, with thymine uracil > cytosine. The purine nucleobases have much lower quantum yields for photochemistry, but can be quite reactive in the presence of oxygen. As can be seen from Figure 9-3, thymine forms primarily cyclobutyl photodimers (ToT) via a [2ir + 2tt cycloaddition, with the cis-syn photodimer most prevalent in DNA. This is the lesion which is found most often in DNA and has been directly-linked to the suntan response in humans [65]. A [2Tr + 2Tr] cycloaddition reaction between the double bond in thymine and the carbonyl or the imino of an adjacent pyrimidine nucleobase can eventually yield the pyrimidine pyrimidinone [6 1]-photoproduct via spontaneous rearrangement of the initially formed oxetane or azetidine. This photoproduct has a much lower quantum yield than the photodimer in both dinucleoside monophosphates and in DNA. Finally, thymine can also form the photohydrate via photocatalytic addition of water across the C5 = C6 bond. 

Uracil has a similar photoreactivity to thymine, but in RNA. Although the rate of photoreaction is similar, the photoproduct partitioning is different. While uracil forms the cyclobutyl photodimer and photohydrate, there is no evidence that it forms the pyrimidine-pyrimidinone [6 1] photoproduct. Also, the major photoproduct in... 

As a second example of intersystem crossing mechanism in biochromophores we include here the case of the DNA pyrimidine nucleobases, starting by the uracil molecule [91]. In previous sections we presented a model for the rapid internal conversion of the singlet excited rationalizes the ultrafast decay component observed in these systems, both in the gas phase and in solution. Despite the short lifetimes associated to this state, which is the main contributor to the photophysics of the system, formation of photodimers PyroPyr has been observed for the monomers in solution, as well as in solid state, for oligonucleotides, and DNA [92], Since the sixties, the determination of the mechanism of the photoinduced formation of cyclobutane dimers has been the subject of numerous studies [92, 93-97], One of the most classic models that has been proposed for the photodimerization of Pyr nucleobases in solution invokes photoexcitation of a molecule to a singlet state followed by population of a triplet state by an intersystem crossing mechanism... 

The 1,2-photodimer (194), of some biological significance, was obtained as the main product on irradiation of a frozen aqueous solution of thymine or DNA (followed by hydrolysis in the latter case).275 This dimerization between two adjacent thymine residues is considered to be the major change responsible for the photochemical inactivation of DNA dimerization appeared to occur within one strand (intrastrand).276 Upon irradiation of 1,3-dimethylthymine,277 1,3-dimethyluracil,216,278 uracil,279 and thymidine,280 in frozen aqueous solutions, a mixture of all four possible 1,2-dimerization products (i.e., cis and trans head-to-head, cis and trans head-to-tail) have been produced. A triplet state has recently been proposed as their precursor.216 It has also been shown281 that the trans head-to-head photodimer (195) of 2-quinolone, obtained in ethanol,282 is formed by condensation of the triplet state monomer with another unexcited molecule. 

Pyrimidine bases such as uracil and thymine are known to form photodimers with ejqmsure to UV light (2). It seemed interesting to e]q>lore the photochemical reaction of the pyrimidine bases to the photolithographic process in microelectronics fabrication technology. 

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