Pyrimidines dimers

In turn, an important factor that can damage DNA in nature or at performing molecular genetic studies, is ultraviolet (UV) radiation that is absorbed by this biopolymer at bandwidth maxmum 254 nm. This led to the formation of different DNA photodamages, with increasing of the dose of UV radiation progressed from pyrimidine dimers to single-and double-stranded breaks [Cariello et al., 1988 Lyamichev et al., 1990]. 

Lyamichev, V., Frank-Kamenetskii, M., Soyfer, V. Protection against UV-induced pyrimidine dimerization in DNA by triplex formation. Nature, Vol.344, No. 6266, (1990), pp. 568-570, ISSN 1476-4687... 

A. UV light-induced thymine-thymine (pyrimidine) dimer... 

KATIYAR s K, PEREZ A and MUKHTAR H (2000b) Green tea polyphenol treatment to hiunan skin prevents formation of ultraviolet light B-induced pyrimidine dimers in DNA , Clin Cancer Res, 6 (10), 3864-9. 

Zhang RB, Eriksson LA (2006) A triplet mechanism for the formation of cyclobutane pyrimidine dimers in UV-irradiated DNA. J Phys Chem B 110 7556-7562... 

Fig. 2 Effect of the laser intensity on the quantum yield of formation of oxidized bases (j)ox) and pyrimidine dimeric photoproducts (0dim)...

Scheme 1 UV-light induced formation of the two major photo lesions in DNA. T=T cyclobutane pyrimidine dimer. (6-4)-photo product (6-4)-lesion, formed after ring opening of an oxetane intermediate, which is the product of a Paterno-Buchi reaction...

Scheme 2 Mechanism of repair of cyclobutane pyrimidine dimers (CPD) by a CPD photolyase. 8-HDF 8-hydroxy-5-deazaflavin, ET electron transfer. FADH reduced and de-protonated flavin-coenzyme...

In order to investigate the single electron donation process from a reduced flavin to a pyrimidine dimer or oxetane lesion, the photolyase model compounds 1-4 depicted in Scheme 4 were prepared [41, 42]. The first model compounds 1 and 2 contain a cyclobutane uracil (1) or thymine (2) dimer covalently connected to a flavin, which is the active electron donating subunit in photolyases. These model systems were dissolved in various solvents... 

Scheme 4 The four pyrimidine dimer and oxetane model compounds 1-4 with either a reduced and deprotonated flavin, or a pyrene as the electron donor. Depiction of the detected reaction products 5-7...

These experiments proved that a light-excited, reduced flavin is indeed able to photoreduce cyclobutane pyrimidine dimers and that these dimers undergo a spontaneous cycloreversion. The quantum yield of about 0=5% clarified that the overall dimer splitting process is highly efficient, even in these simple model systems ((]) photolyase 70%). 

Whatever the reason may be behind the strict necessity to deprotonate the flavin donor, the reduced and deprotonated flavin was established in these model studies to be an efficient electron donor, able to reduce nucle-obases and oxetanes. In the model compounds 1 and 2 the pyrimidine dimer translates the electron transfer step into a rapidly detectable chemical cycloreversion reaction [47, 48], Incorporation of a flavin and of a cyclobutane pyrimidine dimer into DNA double strands was consequently performed in order to analyse the reductive electron transfer properties of DNA. 

Incorporation of a flavin electron donor and a thymine dimer acceptor into DNA double strands was achieved as depicted in Scheme 5 using a complex phosphoramidite/H-phosphonate/phosphoramidite DNA synthesis protocol. For the preparation of a flavin-base, which fits well into a DNA double strand structure, riboflavin was reacted with benzaldehyde-dimethylacetale to rigidify the ribityl-chain as a part of a 1,3-dioxane substructure [49]. The benzacetal-protected flavin was finally converted into the 5 -dimethoxytri-tyl-protected-3 -H-phosphonate ready for the incorporation into DNA using machine assisted DNA synthesis (Scheme 5a). For the cyclobutane pyrimidine dimer acceptor, a formacetal-linked thymine dimer phosphoramidite was prepared, which was found to be accessible in large quantities [50]. Both the flavin base and the formacetal-linked thymidine dimer, were finally incorporated into DNA strands like 7-12 (Scheme 5c). As depicted in... 

Scheme 5 a Flavin-H-phosphonate and formacetal-linked thymine dimer phospho-ramidite used for the synthesis of the flavin and dimer containing DNA-strands 7-12. b Representation of a reduced flavin- and formacetal-linked cyclobutane pyrimidine dimer containing DNA strand, which upon irradiation (hv) and electron transfer (ET) performs a cycloreversion (CR) of the dimer unit, c Depiction of the investigated oligonucleotides... 

Flavin-cyclobutane pyrimidine dimer and flavin-oxetane model compounds like 1-3 showed for the first time that a reduced and deprotonated flavin is a strong photo-reductant even outside a protein environment, able to transfer an extra electron to cyclobutane pyrimidine dimers and oxetanes. There then spontaneously perform either a [2n+2n cycloreversion or a retro-Paternd-Buchi reaction. In this sense, the model compounds mimic the electron transfer driven DNA repair process of CPD- and (6-4)-photolyases. 

Incorporation of an artificial flavin nucleobase and of a cyclobutane pyrimidine dimer building block into DNA DNA double strands, DNArPNA hybrid duplexes, and DNA-hairpins, provided compelling evidence that an excess electron can hop through DNA to initiate dimer repair even at a remote site. The maximum excess electron transfer distance realised so far in these defined Donor-DNA-Acceptor systems is 24 A. New experiments are now in progress to clarify whether even larger transfer distances can be achieved. 

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... 

Freeman, S. E., et al., Wavelength dependence of pyrimidine dimer formation in DNA of human skin irradiated in situ with ultraviolet light, Proc. Natl. Acad. Sci. U S A 86,... 

Yarosh, D. et al., Pyrimidine dimer removal enhanced by DNA repair liposomes reduces the incidence of UV skin cancer in mice, Cancer Res. 52,4224-4231, 1992. 

Vink, A. A. et al., The inhibition of antigen-presenting activity of dendritic cells resulting from UV irradiation of murine skin is restored by in vitro photorepair of cyclobutane pyrimidine dimers, Proc. Natl. Acad, Sci. USA 94, 5255-5260, 1997. 

B.M. Sutherland used in vitro experiments with human neonatal foreskin fibroblasts in buffer solution to show that DNA-pyrimidine dimer formation under a sunlamp was sensitized by p-aminobenzoic acid. Cells illuminated in the presence of the acid had a 10-fold higher frequency of transformation to anchorage-independant growth than cells irradiated in buffer alone [41]. 

Answer B, Nucleotide excision repair of thymine (pyrimidine) dimers is deficient in XP... 

An ab initio study on the structure and splitting of the uracil dimer anion-radical (see Scheme 3.72, R = H) gives preference to the one-step mechanism (Voityuk and Roesch 1997). Anion-radical anions of the pyrimidine dimers cleave with rate constants in excess of 10 s ... 

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