Pyrimidine dimers, photosensitized

Traynor, N.J. and Gibbs, N.K. (1999) The phototumorigenic fiuoroqumolone lomefioxacin photosensitizes pyrimidine dimer formation in hrunan keratinocytes in vitro. Photochemistry and Photobiology, 70, 957-959. 

The photosensitized monomerization of thymine dimers has been achieved in the presence of reduced flavin,and pyrimidine dimers undergo cleavage by light-utilizing enzymes which are thought to act by a photoinduced electron transfer mechanism. ... 

A variety of pyrimidine dimer-cleaving photochemical model systems have been developed to aid in the study and elucidation of the DNA-PL reaction mechanism (752, 158, 159). Direct excitation of pyrimidine dimers does not occur on ultraviolet or visible irradiation, suggesting that enzyme cofactors might be involved in a photosensitization process 146). Model systems utilizing free flavin derivatives have recently been described 137, 160). Rokita and Walsh have demonstrated that lumiflavin, 5-deazariboflavin, and 8-methoxy-7,8-didemethyl-7V °-ethyl-5-deazaflavin (Scheme 31) are effective photosensitizers for the thymine dimer cleavage reaction 160). These reactions utilized cis-syn-thymine dimer as substrate with irradiation at the Xmax of the flavin derivative, under strict anaerobic conditions and high pH. Flavin derivatives that contain electron-rich substituents at the 8-position, such as 8-hydroxy-5-deazariboflavin,... 

Based on the model studies discussed above, an analogous mechanism for the DNA-PL-catalyzed cleavage of pyrimidine dimers would occur via initial photoactivation of one or both of the enzyme cofactors, followed by electron transfer between the activated photosensitizer and enzyme-bound dimer. The studies by Joms indicated that only reduced flavin is required to initiate thymine dimer cleavage in the model reaction 162). This is consistent with the observation that the quantum yield for the blue semiquinone radical form of E. coli DNA-PL in vitro was found to be much lower than the in vivo values. Treatment of the isolated enzyme with dithionite to give fully reduced flavin cofactor then results in a 12- to 15-fold increase in the quantum yield, suggesting that the flavin cofactor is in the reduced oxidation state in vivo 163). 

Scheme 35. Scission of pyrimidine dimers by a singlet photosensitization mechanism (154).

Hollander s theory [41] of pair substitution, they concluded that an open chain or extended radical cation 28 + must precede a closed radical cation 291 + in which spin and charge are localized in an aromatic moiety with the reverse sequence of intermediates, which would perhaps appear more natural from the point of view of chemical intuition, the experimental polarization pattern could not be simulated. Based on their analysis, they obtained a rate constant of 4 x 10s s 1 for the transformation 28 + -> 29 +. Related cycloreversions of biological significance (photosensitized splitting of pyrimidine dimers) are dealt with in Section V.G.2. 

Nucleic Acids. Efforts in this area have focused almost exclusively on the photosensitized splitting of pyrimidine dimers [140] (see Chart XXI, which shows the dimer D and the monomer M of dimethyluracil). Motivation of these studies has been to gain insight into the mechanism of photorepair of DNA by DNA photolyase. The operating principle of this enzyme is photoinduced electron transfer. However, the key question whether excited DNA photolyase donates or accepts the electron is still unresolved. 

J.C. Sutherland and Griffin incorporated tritium-labelled thymine into DNA and irradiated it with 313 nm light in buffered saline in the presence of p-aminobenzoic acid for up to 12 min. After separation of the products by TLC, the radioactivity was associated with a fraction which had the characteristic RF of dimers. When the hydrolysate from a 313 nm irradiated sample was re-irradiated at 254 nm and then chromatographed, the radioactivity had the mobility of thymine monomer. This is characteristic of pyrimidine cyclobutyl dimers which were known to be photosynthesized at 313 nm and photodegrad-ed to monomers at 254 nm. Although not degraded itself, the p-aminobenzoic acid clearly acted as a photosensitizer for the DNA-damaging thymine dimerization [40]. 

Tab. 4.5.1. Most important redox-photosensitizers, radicals and radical ions used in the studies on the oxidative repair of pyrimidine cyclobutane dimers.

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