Cytosine photodimerization

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. 

The intramolecular photodimerization and [2 + 2]-photocycloaddition in DNA involves thymine or cytosine as the chromophore. This chemistry has been intensively investigated with regards to DNA damage and repair [131]. Despite the fact that the area is of continuous interest [132], the synthetic applications are limited and are not covered here in detail. However, some preparative aspects of 4-pyrimidinone photocycloaddition chemistry will be addressed. Aitken et al. have prepared a plethora of constrained cyclobutane P-amino acids by intra- or intermolecular [2 + 2]-photocycloaddition to uracil and its derivatives [133, 134]. In a chiral adaptation of this method, the uracil-derived enone 140 was employed to prepare the diastereomeric cyclobutanes 141 in very good yield (Scheme 6.49). The compounds are easily separated and were - despite the relatively low auxiliary-induced diastereoselectivity - well suited to prepare the as-2-aminocyclobutanecarboxylic acids 142 in enantiomerically pure form. Enantioselective access to the corresponding trans-products was feasible by epimerization in a-position to the carboxyl group [135],... 

Of additional, and particular, interest is the finding that the dimer reduction product of pyrimidone-2 is an integral component of a tetrameric photoproduct resulting from the photodimerization of a photoadduct of thymine and pyrimidone-2 77), as shown in Scheme 3. This tetrameric photoproduct has, furthermore, been isolated from an irradiated frozen aqueous solution of cytosine and thymine 8), and, more... 

The CASPT2 results on the photodimerization of two cytosine molecules taking place along the triplet manifold are compiled in Figure 16-14. As can be deduced from this figure that the MEP computations from 3(LE) lead directly to the triplet step-wise intermediate 3(SWI) with out any energy barrier. This intermediate (3(SWI)) is characterized by the formation of a single covalent bond... 

Irradiation of cytosine, cytidine, and 2 -deoxycytidine has recently been reported to produce 4,5 -linked dimers (154 R = H, j3-D-ribose, 2-deoxy-/3-D-ribose, respectively) 210 this reaction is accompanied by deamination. Photodimerization (in aqueous solution) of uracil211 and thymine212 affords the 4,6 -linked dimer (155 R = R = H and Me, respectively) the oxetane (156) (probably formed by 1,2-cycloaddition of a triplet carbonyl to an olefinic bond) is proposed as the precursor.212... 

The results of the refined MO calculations (tt-SCF MO without and with Cl, open-shell tt-SCF MO, tt-UHF, or CNDO/2 Cl methods) differ depending on the approximations used. In some cases, the tt-SCF MO Cl method gives results quite similar for both states to those obtained by tt-HMO method for the first excited state. These results [spin densities, bond orders Pge, and free valences on the C(5)-C(6) bond] all correlate with the greater tendency to photodimerization of uracil and thymine compared with cytosine and do not indicate which of the excited states is involved in the reaction. On the other hand, Danilov et using the tt-SCF MO CI as well as the open-shell tt-SCF... 

Roca-Sanjuan, D., Olaso-Gonzeiez, G.> Gonzalez-Ramirez, L, Serrano-Andres, L.> 8c Merchan, M. (2008b). Molecular basis of DNA photodimerization Intrinsic production of cyclobutane cytosine dimers. Journal of the American Chemical Society, 130,10768-10779. 

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