Thymine, reversible photodimerization

Also, water-soluble poly(vinyl alcohol) was modified by thymine derivatives. This yielded a material that reversibly photodimerizes upon UV irradiation. Photocrosslinking experiments of this polymer were carried out in the film state on quartz plates by irradiation with light at 280 nm. The poly(vinyl alcohol) containing thymine units were found to be more light sensitive than uracil units in a similar polymer. In addition, photodimerization reaction of an ester derivative of the thymene substituted poly(vinyl alcohol) was found to be faster than that of an acetal derivatives, but the sensitivity was lower, because... 

FIGURE 104.3 Reversible photodimerization and UV absorption of thymine derivatives. 

Reversible photodimerizations of l- -alkylthymine in single crystals depend on the crystal structures. However, the orientation of the thymine bases in the single crystal does not correspond to the isomer of the photodimer formed because thymines undergo disrotatory motion during the photodimerization process in the single crystals. The most important factor of the crystal structure for the photodimerization is the distance between the thymine base and the terminal methyl group of alkyl chain because this can influence the rotation of the thymine bases. 

Mochizuki, E., Yasui, N., Kai, Y, Inaki, Y, Wang, Y, Saito, T, Tohnai, N., and Miyata, M., Reversible photodimerization of ester derivatives of thymine having long alkyl chain in solid film. Polymer /., 32, 492, 2000. 

An unusual photochemical reaction of 2-pyridones, 2-aminopyridinium salts and pyran-2-ones is photodimerization to give the so-called butterfly dimers. These transformations are outlined in equations (13) and (14). Photodimerization by [2+2] cyclization is also a common and important reaction with these compounds. It has been the subject of particular study in pyrimidines, especially thymine, as irradiation of nucleic acids at ca. 260 nm effects photodimerization (e.g. equation 15) this in turn changes the regular hydrogen bonding pattern between bases on two chains and hence part of the double helix structure is disrupted. The dimerization is reversed if the DNA binds to an enzyme and this enzyme-DNA complex is irradiated at 300-500 nm. Many other examples of [2+2] photodimerization are known and it has recently been shown that 1,4-dithiin behaves similarly (equation 16) (82TL2651). 

Takemoto, K. and Inaki, Y, Photodimerization of thymine-containing polymers applicability to reversible photoresists, /. Macromol. Sci. Chem., A25, 757,1988. 

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