Purines photochemistry

Pliitzer C, Hiinig I, Kleinermanns K, Nir E, de Vries MS (2003) On the photochemistry of purine nucleobases. Chem Phys Chem 4 838-842... 

The photochemistry of the polynucleotides has been elucidated primarily by studies of the photochemical behavior of the individual pyrimidine and purine bases (the ribose and phosphate groups would not be expected to undergo photochemical reactions in this wavelength range). These studies have shown the pyrimidines (cytosine and thymine) to be roughly ten times more sensitive to UV than the purines (adenine and guanine.) Thus we would expect most of the photochemistry of the nucleic acids to result from the action of light on the pyrimidines. 

Although a signal was detected also in irradiated pyrimidine solutions under similar conditions, it was attributed to dissociation products of the pyrimidine molecule and not to photoionization. If the purine solutions contained ethanol, then the signal of the CH3CHOH radical could also be detected this did not happen in ethanolic pyrimidine solutions. The possible importance of photoionization as the primary process in the photochemistry of nucleic acid derivatives has been supported by Kearns, from a study of photoionization in anthracene single crystals.115 He concluded that anthracene photoionization occurred via the interaction of two singlet excitons. 

Phosphorescence and photochemistry of aromatic amino acids have been reported.481-483 Triplet states of nucleic acids have also been detected. For example, the phosphorescence of DNA equals the sum of the slow emissions from deoxyadenosine and deoxyguanosine monophosphates present, indicating that only the purine bases phosphoresce.484... 

Ekpenyong Kl, Shetlar MD (1979) Photochemical reactions of cytosine N-methyl analogs in alcoholic solutions. Photochem Photobiol 30 455-461 Elad D (1976) Photoproducts of purines. In Wang SY (ed) Photochemistry and photobiology of nucleic acids. Academic Press, New York, pp 357-380 Elad D, Rosenthal I (1969) Photochemical alkylation of caffeine with amino-acids. Chem Commun 905-906... 

In conclusion, the application of the Suzuki—Miyaura reaction of 6-chloropurine derivatives with substituted phenylboronic acids is a facile and effective approach for the synthesis of a series of specifically substituted 6-phenylpurine bases and nucleosides. In comparison with the previously known methods25-30 using other types of organometallic reagents or photochemistry, this method is more effective and selective, and therefore, further applications in the synthesis of 6 C-substituted purine derivatives may be expected. 

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. 

Very few reports of the excited-state structural dynamics of the purine nucleobases have appeared in the literature. This lack of research effort is probably due to a number of factors. The primary factor is the lack of photochemistry seen in the purines. Although adenine can form photoadducts with thymine, and this accounts for 0.2% of the photolesions found upon UVC irradiation of DNA [67], the purines appear to be relatively robust to UV irradiation. This lack of photoreactivity is probably due to the aromatic nature of the purine nucleobases. A practical issue with the purine nucleobases is their insolubility in water. While adenine enjoys reasonable solubility, it is almost an order of magnitude lower than that of thymine and uracil, the two most soluble nucleobases [143], Guanine is almost completely insoluble in water at room temperature [143],... 

The photochemistry of purines is more complex. First, because of the involvement of at least three low-lying excited states, one nNTT (A) or n0iT (G), and two tttt states, typically labeled La) or HL) (the HOMO-LUMO and bright... 

The distinguishing feature of the photochemistry of purines and their derivatives is their relatively higher resistance to UV irradiation compared with pyrimidine derivatives. With an increase of the dose of UV irradiation, a decrease of absorption of the purines at or ven its complete disappearance, is observed. Purine oligo- and polynucleotides exhibit greater photochemical stability than mononucleotides. 

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