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Supercoiling-induced structural transition of the d(C24GC21).d(G21CG24) sequence in plasmid DNA in the presence of Mg2+ at neutral pH results in alterations of efficiencies of not only single-quantum (pyrimidine[6-4]pyrimidone adducts) but also two-quantum (alkali-sensitive lesions of dG residues) photomodifications of nucleoside residues within this sequence. The generation of both types of photoreactions was achieved by the application of high-intensity laser UV radiation (intensity approximately 10(11) W/m2, pulse duration approximately 10(-8) s, lambda = 266 nm) for irradiation of a plasmid DNA. The modification extent sufficient for analysis of photoreaction efficiency distributions along both strands of the insert (photofootprinting) was obtained by the action of a single nanosecond pulse of laser UV radiation. The pattern of a laser photofootprinting is consistent with the d(C).d(G).d(G) triplex formation in the presence of Mg2+ within the insert and shows some details of this triplex structure.