Nucleic acids photohydration

All these bases absorb around 260 nm. Thymine and cytosine are most sensitive to irradiation. Two most important types of photochemical reactions that have been observed for these pyrimidine bases are photohydration and photodimerization. In vivo systems, interactions between protons and nucleic acids can also be initiated by radiations of wavelength dlorter than 300 nm. 

Photochemical reactions of the purines and pyrimidines assume special significance because of the high molar extinction coefficients of the nucleic acids present in cells. Light is likely to be absorbed by nucleic acids and to induce photoreactions that lead to mutations.190 Both pyrimidines and purines undergo photochemical alterations, but purines are only about one-tenth as sensitive as pyrimidines. Photohydration of cytidine (Eq. 23-25) is observed readily. The reaction is the photochemical analog of the hydration of a,P-unsaturated carboxylic acids. Uracil derivatives also undergo photohydration. 

The photochemical stability of the nucleic acid bases under UV irradiation is no longer a major concern after the formation of the ozone layer, and the role of the state in the form of modern life is significantly reduced. However, Kohler s group suspects that the tt state might still be involved in the formation of photohydrates and pyrimidine/pyrimidine photoproducts, while the formation of... 

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