Cytosine photochemistry

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. 

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... 

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. 

Figure 16-3. Global picture of the photochemistry of uracil (U), thymine (T), and cytosine (C) as suggested by the CASPT2 calculations. (Reproduced from Ref. [47] with permission from the American Chemical Society)...

COMPUTATIONAL MODELING OF CYTOSINE PHOTOPHYSICS AND PHOTOCHEMISTRY FROM THE GAS PHASE TO DNA... 

Scheme based on CASPT2 calculations of the dimerization photochemistry of the cytosine dimer after UV-llght absorption... 

Malone, R. J., Miller, A. M., Kohler, B. (2003). Singlet excited-state lifetimes of cytosine derivatives measured by femtosecond transient absorption. Photochemistry and Photobiology, 77(2), 158-164. 

Schreiber, M., Gonzalez, L. (2007). The role of Ag(I) ions in the electronic spectroscopy of adenine-cytosine mispairs A MS-CASPT2 theoretical study. Journal of Photochemistry and Photobiology A Chemistry, 190, 301. 

Besides typical ESIPT compounds, other tautomeric systems have also been investigated by ultrafast spectroscopy. However, the number of studies is less. Cytosine was investigated by femtosecond ionization techniques to learn about the relevance of tautomerism for the photochemistry and photostabiHty of DNA [57, 58]. It was found that the isolated cytosine molecule exhibits a rather fast IC. It... 

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