title: Effect of C5-Methylation of Cytosine on the UV-Induced Reactivity of Duplex DNA: Conformational and Electronic Factors.
abstract: C5-methylation of cytosines is strongly correlated with UV-induced mutations detected in skin cancers. Mutational hot-spots appearing at TCG sites are due to the formation of pyrimidine cyclobutane dimers (CPDs). The present study, performed for the model DNA duplex (TCGTA)3·(TACGA)3 and the constitutive single strands, examines the factors underlying the effect of C5-methylation on pyrimidine dimerization at TCG sites. This effect is quantified for the first time by quantum yields ϕ. They were determined following irradiation at 255, 267, and 282 nm and subsequent photoproduct analysis using HPLC coupled to mass spectrometry. C5-methylation leads to an increase of the CPD quantum yield up to 80% with concomitant decrease of that of pyrimidine(6-4) pyrimidone adducts (64PPs) by at least a factor of 3. The obtained ϕ values cannot be explained only by the change of the cytosine absorption spectrum upon C5-methylation. The conformational and electronic factors that may affect the dimerization reaction are discussed in light of results obtained by fluorescence spectroscopy, molecular dynamics simulations, and quantum mechanical calculations. Thus, it appears that the presence of an extra methyl on cytosine affects the sugar puckering, thereby enhancing conformations of the TC step that are prone to CPD formation but less favorable to 64PPs. In addition, C5-methylation diminishes the amplitude of conformational motions in duplexes; in the resulting stiffer structure, ππ* excitations may be transferred from initially populated exciton states to reactive pyrimidines giving rise to CPDs.
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, F-91191 Gif-sur-Yvette, France.
authors.forename: Akos
authors.initials: A
authors.lastname: Banyasz
authors.affiliation: Istituto Biostrutture e Bioimmagini - CNR , 80134 Napoli, Italy.
authors.forename: Luciana
authors.initials: L
authors.lastname: Esposito
authors.affiliation: Université Grenoble Alpes, INAC, LCIB, LAN & CEA, INAC, SCIB, LANF-38000 Grenoble, France.
authors.forename: Thierry
authors.initials: T
authors.lastname: Douki
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, F-91191 Gif-sur-Yvette, France.
authors.forename: Marion
authors.initials: M
authors.lastname: Perron
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, F-91191 Gif-sur-Yvette, France.
authors.forename: Clément
authors.initials: C
authors.lastname: Lepori
authors.affiliation: Istituto Biostrutture e Bioimmagini - CNR , 80134 Napoli, Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Improta
authors.affiliation: LIDYL, CEA, CNRS, Université Paris Saclay, F-91191 Gif-sur-Yvette, France.
authors.forename: Dimitra
authors.initials: D
authors.lastname: Markovitsi
mesh_terms: D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid; D003596:Cytosine / Q000737:chemistry*; D004247:DNA / Q000737:chemistry*; D019281:Dimerization; D008745:Methylation; D056004:Molecular Dynamics Simulation; D009690:Nucleic Acid Conformation; D011740:Pyrimidine Dimers / Q000032:analysis / Q000737:chemistry; D011789:Quantum Theory; D013050:Spectrometry, Fluorescence; D053719:Tandem Mass Spectrometry; D014466:Ultraviolet Rays
chemical_list: D011740:Pyrimidine Dimers; D003596:Cytosine; D004247:DNA