Thymine photodimerization

Blancafort L, Migani A (2007) Modeling thymine photodimerizations in DNA mechanism and correlation diagrams. J Am Chem Soc 129 14540-14541... 

In the category of more elaborate calculations are an SCF—PMO treatment of the reaction of cyclopentadiene with cyclopropene 33>, and calculations including overlap on the photodimerization of linear steroidal dienones. 31> A very fine, detailed theoretical study of thymine photodimerization has been recently presented. 47> Other PMO calculations on photocycloadditions of biochemical interest have also been published. 48>49>... 

It should be noted here that thymine photodimerization may occur by a non-concerted mechanism, involving free radical intermediates. Indeed, photoproducts other than cis-syn dimer, such as the next most abundant thymine dimer, so-called 6 4 adduct, were observed in irradiated DNA. However, the quantum yield of cis-syn photodimer formation (r/j 0.02) is more than an order of magnitude higher than that of the 6 4 adduct ( 0.0013) which in turn is an order of magnitude higher than the quantum yields for other thymine isomers [68]. This specificity can lead to the conclusion that the thymine photodimerization occurs predominantly via concerted 2 + 2 cycloaddition mechanism. A time-resolved study of thymine dimer formation demonstrated that thymine cyclobutane dimers are formed on a timescale of less than 200 nsec, while the 6 4 adduct is formed on a timescale of few milliseconds [69]. The delay in the formation of the latter was attributed to the mechanism of its formation through a reactive intermediate. 

Theoretical Studies of Thymine-Thymine Photodimerization Using Ground State Dynamics to Model Photoreaction... 

Hariharan M, McCullagh M, Schatz GC, Lewis FD (2010) Conformational control of thymine photodimerization in single-strand and duplex DNA containing locked nucleic add TT steps. J Am Chem Soc 132 12856-12858... 

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