Effect of intermolecular triplex formation on the yield of cyclobutane photodimers in DNA

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Abstract
We have studied the effect of intermolecular triplexes formation on the yield of cyciobutane photodimers in DNA. DNA duplex within the pyrimidine-purine pyrimidine triplex d(TC)nd(GA)nd(CT)n is protected from the formation of cyclobutane photodimers In the case of the stabilization of this triplex by acid pH, and in the case of supplementary stabilization by Mg2+ or Zn2+. We have studied pH-independent pyrimidine purine-purine tripiexes stabilized by bivalent catlons. In such triplexes, the protection from the formation of [6–4] photodimers is observed, whereas the protection from cyclobutane dlmer formation does not take place. The formation of the d(TC)nd(GA)nd(CT)n triplex leads to an inversion of the intensities of cyclobutane CT and TC photodimers. We observed a sharp decrease in photoreactivity with respect to cyclobutane dimers in the duplex tract d(C)18d(G)18 in the presence of Ba2+, Cd2+, Co2+, Mn2+, Zn2+, and Ni2+, The formation of the d(C)nd(G)nd(G)n triplex leads to ‘antifootprlnting’, i.e. an increase in the yield of cyclobutane photodimers.