Thymine dimerisation

If the mechanistic information just discussed in Fig. 7.1 is to be really useful then it must be an intrinsic property of the chromophores themselves, i.e. the two ethylene molecules. For example, it is the key feature in intra-strand thymine dimerisation and these dimers can disrupt the function of DNA and trigger complex biological responses, including apoptosis, immune suppression, and carcinogenesis. One can identify the geometry corresponding to the point E in Fig. 7.1 as well as the computed directions [20] Xi and X2 for the 2-1-2 cyclo-addition reaction of two thymine molecules. 

Thymine and other bases in nucleic acids undergo dimerisation when irradiated with ultraviolet light, and this can result in DNA mutation or even cell death. 

Solid state NMR spectroscopy has been used to analyse the dimerisation of the 1,4-dihydropyridine derivative (107) that yields the two dimers (108) and (109) on irradiation. This report complements an earlier account of the same dimerisation. The reaction is claimed as a useful synthetic entry into derivatives of 3,9-diazatetraasteranes. The photodimerisation of the thymine derivative (110) does not take place to any great extent on direct irradiation in solution but when acetone-sensitisation is used all four possible (2+2)-cycloadducts are obtained. Only one of these dimers is obtained from the irradiation in the crystalline phase and this was identified as the cis,anti,cis dimer (111). Interestingly, when the thymine (110) is crystallised from acetonitrile both needles and plates are obtained but only the needle form of the crystalline compound gives the dimer (111). ... 

Intramolecular photocycloaddition occurs with thymine derivatives and related compounds. Thus, the bis thymine dimer (116) is formed on irradiation at 254 nm of (117). Zinc complexes of 1,4,7,10-tetraazacyclododecane inhibit the intramolecular photodimerisation of the thymidilyl thymidine (118) and the same complexes are active in cleaving cyclobutane systems (119). Conventional (2+2)-cycloaddition does not occur on irradiation of (120) but instead the main product is the cytosine hydrate accompanied by the (6-4)-photoproduct (121). Dimerisation is reported to occur on irradiation in an acidic medium. 

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