Photodimerization of thymine

An excellent review on organic photochemistry in organized media, including aqueous solvent, has been reported.178 The quantum efficiency for photodimerization of thymine, uracil, and their derivatives increased considerably in water compared with other organic solvents. The increased quantum efficiency is attributed to the preassociation of the reactants at the ground state. 

Ishihara, H. Photodimerization of thymine and uracil on filter paper, Photochem. Photobiol, 2 455-460, 1963. 

In a fluid environment the photodimerization of thymine and its derivatives involves cycloaddition at the 5,6-double bond to form one or more of the four possible stereoisomers of the cyclobutane dimer shown... 

Photodimerization of thymin to butane type dimers is one of the important causes of UV radiation damage to genetic material DNA in its native double helix form. The reaction occur from the triplet state as confirmed by acetone sensitized photodimerization of thymine. These dimers can be monomerized when irradiated by short wavelength radiations. which are absorbed by the dimer only. 

A more important reaction is the photodimerization of thymine (Eq. 23-26), a reaction also observed with uracil. A variety of stereoisomers of the resulting... 

The reactive excited state in the photodimerization of thymine is probably the triplet state Tt the dimerization process can be quenched by triplet acceptor molecules such as ketones and quinones. 

In a series of our interaction studies, the photodimerization of thymine bases along the polymer chain was carried out to estimate the degree of intramolecular selfassociation of the bases (Fig. 14). The photodimerization reaction was also studied for monomeric and dimeric model compounds (Fig. 15)27). The formation of thymine photodimers by UV irradiation onto nucleic add is well-kown and the photodimerization of nucleic add, its model compounds and pyrimidine bases has been reported by many groups, though the work on the synthetic polymers containing thymine bases has hitherto been unknown. 

For the photodimerization of thymine, four isomeric products are assumed to be formed (Fig. 16). It is known that ds-syn and trans-syn isomers are formed by photolysis of l,l -trimethylene-bis-thymine, TpT, and denaturated DNA. The chemical shift of the 5-methyl proton at 1.12 to 1.18 ppm in our case suggests the formation of syn-type isomers. It has been found that when acetone is used as photosensitizer the products exhibit photodimer distribution similar to that obtained without using a sensitizer. 

The photodimerization of thymine derivatives was then studied in the presence of isoprene as the triplet quencher. The data obtained were plotted using the Stern-Volmer equation (3) derived from Scheme 10, in which only the excited triplet state is... 

Photodimerization of thymine in the crystalline form has been reported/ A study has examined theoretically the dimerization of pyrimidine units in DNA. The results illustrate that thymine-thymine dimerization is more likely than other combinations. ... 

Fig. 12.B3 The photodimerization of thymine bases to form either (a) a cyclobutane-thymine dimer or (b) a 6,4 photoproduct.

Photochemical reactions of purines and pyrimidines in light are well known, and lead to mutations. Although various photochemical reactions can occur with both pyrimidines and purines, photodimerization of thymine is of the utmost importance. Photochemical reaction by UV irradiation of thymine results in formation of dimers with cyclobutane rings, which block DNA replication. Photodimerization was reported with the synthetic polymers having pendant thymine bases. In a study with polyacrylates and polymethacrylates with... 

An important photoaddition process responsible for UV-induced molecular lesions in DNA is the photodimerization of thymine as illustrated below. 

Thymine derivatives " are very resistant to UV irradiation in dilute solution because the triplet state is the precursor of the photodimer and intersystem crossing of thymine is very low. The yield of the photodimer in solution is low, but a high yield of photodimer can be obtained with acetone as the sensitizer. In DNA polymers and solid state, however, the photodimerization reaction is fast and proceeds through the singlet state. The photodimerizations of thymine oHgomers and polymers were studied in the presence of isoprene as a triplet quencher. The photodimerization of the thymine, monomeric and dimeric model, were quenched by isoprene, but the photodimerization of a polymer with thymine units was not quenched. This result appears to indicate that the photodimerization of aggregated thymine compounds occurs almost completely from the singlet state. ... 

The photodimerization of thymine in solution gives four isomeric photodimers (Figure 104.2), but the spin-coat film of 1-n-alkylthymine from chloroform solution gave the trans-syn and trans-anti photodimers as the main products (Table 104.1, l-n-tetradecylthymine). On the other hand, the photodimerization of the 2 min annealed film gave only the trans-anti photodimer. As mentioned below, the trans-syn and trans-anti photodimers were obtained by the photodimerization of needles obtained by crystallization from acetonitrile solution. Therefore, the spin-coated film may contain crystals that are the same as the needles from acetonitrile solution. The structure of the thymine in the annealed film may be the same as the plates obtained by crystallization from ethyl acetate solution because the crystals from ethyl acetate gave only the trans-anti photodimer. 

If the photodimerization occurs face-to-face between the stacked thymines, the photodimer should be the trans-syn isomer. However, the photodimer obtained was the trans-anti isomer, suggesting another mechanism of photodimerization. The photocyclization of a diene compound is known to occur by disrotatory motion, although the thermal cychzation occurs by a conrotatory motion. If the photodimerization of thymine bases in the crystal occurred by disrotatory motion, the product should be the... 

Figure 104.37 shows the first and the second photodimerizations of the decyl ester (ESIO) in the thin soHd film (a, b, c) with the data in solution (e) and in polymer film (d). The rate of the photodimerization in chloroform solution was slow but proceeded completely (e). The photodimerization of thymine derivatives in solution is known to proceed by way of the triplet excited state. The short lifetime of the singlet state and the inefficient intersystem crossing are the reasons for the slow reaction rate for photodimerization of thymine in solution. The photodimerization was studied in the polymer film using poly(vinyl acetate) (PVAc) (d). The polymer film was obtained from a chloroform solution of the ester derivatives of thymine and poly(vinyl acetate) by spin coating on a quartz plate. The rate of photodimerization in poly(vinyl acetate) was faster than the rate in chloroform solution. In the polymer matrix, photodimerization occurs from associated thymine bases via the singlet state with a higher quantum yield than that from the triplet-state reaction. 

FIGURE 104.50 The induced strain during the photodimerization of thymine bases in the crystal (illustration). Photosplitting... 

The rate of photodimerization of thymine bases in thin solid films is fast in the amorphous areas. Annealing of the film cause crystallization to give inactive crystals. Alkylthymines with chains around C,o show polymorphism and the thin solid films contain many crystalline forms. Therefore, the films of thymine compound with Cjq chains are very active for photodimerization. 

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

Mochizuki, E., Masuda, K., Wada, T., and Inaki, Y., Photodimerization of thymine derivatives in ionomer matrix, /. Photopolym. Sci Technol, 6,131,1993. 

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