Photoproducts, DNA

CHvio, P. and Fourrey, J.L., (6—4) photoproduct DNA photolyase mechanistic studies using s -(6-4) photoproducts, Chem. Commun., 2203,1996. 

The formation of final oxidation products such as 8-oxodGuo 44, 8-oxo-dAdo 48, 5-HMdUrd 12 and 5-FordUrd 13 is monitored using the highly specific method of HPLC-MS/MS [17]. Application of this powerful technique as described in the next section has also been extended to the quantitative measurement of the main DNA photoproducts that arise from the reaction... 

The HPLC-MS/MS assay was also successfully applied to the measurement of UV-induced dimeric pyrimidine photoproducts [123, 124]. The latter lesions were released from DNA as modified dinucleoside monophosphates due to resistance of the intra-dimer phosphodiester group to the exonuclease activity during the hydrolysis step [125, 126]. The hydrolyzed photoproducts exhibit mass spectrometry and chromatographic features that allow simultaneous quantification of the three main classes of photolesions, namely cyclobutane dimers, (6-4) photoproducts, and Dewar valence isomers, for each of the four possible bipyrimidine sequences. It may be added that these analyses are coupled to UV detection of normal nucleosides in order to correct for the amount of DNA in the sample and obtain a precise ratio of oxidized bases or dimeric photoproducts to normal nucleosides. 

DNA strongly absorbs UV radiation, especially mid-range UVB (290 to 320 nm) radiation. Two major DNA lesions are induced following UV exposure, pyrimidine dimers and 6-4 pyrimidine-pyrimidone photoproducts. Because the action spectrum (induction of a biological activity as a function of wavelength) for erythema closely matches the action spectrum for pyrimidine dimer formation, DNA is believed to be the chromophore for sunburn.6 Pyrimidine dimer formation, or more properly, the failure to adequately repair dimers after solar irradiation is also the primary cause of sunlight-induced skin cancer formation.7-8... 

These results seem to suggest that by irradiation in alkylamines DNA (single stranded) can undergo a selective release of thymine, accompanied by strand scission. This has indeed experimentally been verified. Irradiation of calf thymus DNA in 5% aqueous n-butylamine gave a photoproduct that on heating yields 1-n-butylthymine (81MI1). 

The ultraviolet irradiation of DNA produces two photoproducts from thymine. The major product is identical with thymine ice-dimer (cis-syn, LXXVa), and accounts for 91 per cent of the DNA-derived photoproducts [541, 542]. A uracil-thymine dimer is also obtained in smaller quantity, which arises from deamination of a cytosine-thymine dimer [543, 544]. 

The cis-syn thymine dimer from DNA is believed to be formed from intra-strand dimerization of adjacent thymine residues. Photoaddition of two unsaturated molecules in the solid state can arise only if they are initially located in proximity in the crystal lattice [545, 546]. The formation of interstrand dimers would require gross distortion of the helical structure of DNA in order for the bases to approach the limiting distance (c. 4 A.) Hence, such dimers would be formed in only very small amount. However, the composition of the photoproducts may differ under varying experimental conditions [547-550]. 

Only a limited number of reliable prediction tools are currently available for photoinduced toxicity. This is not surprising since establishing phototoxic potential is a complex task. Phototoxicity can be the consequence of various mechanisms such as photogeneration of reactive oxygen species, production of toxic photoproducts or sensitization of DNA damage by energy transfer. In addition, so far, there are no available universal descriptors (indicators) to predict the photodynamic potency of chemicals. 

Matsunaga, T. Hieda, K. Nikaido, O. Wavelength dependent formation of th5mine dimers and (6-4) photoproducts in DNA by monochromatic ultraviolet light ranging from 150 to 365 nm. Photochem. Photobiol. 1991, 54 (3), 403-410. 

A particular dimer of thymine was the first heat-stable photoproduct isolated from photolysis of nucleic acid derivatives, and still appears to be the principal isolable product from photolysis of most examples of biologically active DNA.8,10 Investigation of this product has drawn much attention in recent years. Most of this work has been carried out on polynucleotides and nucleic acids and will be dealt with in those sections. The remaining work has been largely carried out with... 

The formation of dimers other than thymine dimer in irradiated DNA has been recently reported by Setlow and Carrier,87 using E. coli DNA containing either labeled thymine (14C or tritium) or labeled cytosine (tritium). Three photoproducts could be isolated by chromatography of the acid hydrolysates of the irradiated DNA and were... 

Fig. 36. Percentage of radioactivity of the labeled base, in E. coli DNA that appears in the indicated photoproduct as a function of dose of 280-nm radiation. (O) TT/T ( ) UT/C or UT/T (A) CC/C. Each point represents the average of 5 determinations (Setlow and Carrier87).

Observed Photoproducts in DNA after a Large Ultraviolet Dose (4 x 104 ergs/ mm2, 280 mp.) and Their Relations to Dinucleotide Frequencies... 

The correlation between definite photoproducts and biological action of ultraviolet light discussed in this section is confined to a particular action of ultraviolet light, namely an inactivating or lethal action. Microorganisms are killed, DNA synthesis in cells is inhibited, transforming DNA is inactivated, template activity of DNA is reduced, and phage and viruses are inactivated because the pyrimidine dimer which is formed in-strand is apparently able to provide a block which hinders DNA or RNA replication. 

Psoralens must be photoactivated by long-wavelength ultraviolet light in the range of 320-400 nm (ultraviolet A [UVA]) to produce a beneficial effect. Psoralens intercalate with DNA and, with subsequent UVA irradiation, cyclobutane adducts are formed with pyrimidine bases. Both monofunctional and bifunctional adducts may be formed, the latter causing interstrand cross-links. These DNA photoproducts may inhibit DNA synthesis. The major long-term risks of psoralen photochemotherapy are cataracts and skin cancer. 

Photoinduced electron transfer (PET) has attracted considerable interest and has been intensively studied as a fundamental step in mechanistic and synthetic organic photochemistry and appears to be involved in key biological processes. Cyclore version of oxetans by PET is important for the photoenzymatic repair of the photoproducts of the DNA dipyrimidine sites by photolyase244. The oxidative version of this reaction has been achieved using cyanoaromatics, chloranil, or (thia)pyrylium salts as electron-transfer photosensitizers245 246. 

The year 2003 is the tenth anniversary of the first Femtochemistry Conference and the fiftieth anniversary of Watson and Crick s celebrated discovery of the DNA double helix [1], Remarkable progress has been made in both fields femtosecond spectroscopy has made decisive contributions to Chemistry and Biology, and advances in the elucidation of static nucleic acid structures have profoundly transformed the biosciences. However, much less is known about the dynamical properties of these complex macromolecules. This is especially true of the dynamics of the excited electronic states, including their evolution toward the photoproducts that are the end result of DNA photodamage [2],... 

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