The concentration and temperature dependences of the UV and CD spectra of the ohgonucleotide( etcosamer) in aqueous solution at pH 7 in the presence of 0.5 M NaCl were studied. At less than 10m6 M, the eicosamer was shown to form in solution a hairpin with parallel orientation of chains (parallel hairpin). From thermal denaturation profiles [A2&T)] the thermodynamic parameters, AH", As" and T, for parallel hairpin formation were calculated to be -9Ok8 kJ/mol. -300+20 J mol-' K-* and 40.5"C, respectively. The CD spectra of the parallel double helix differed from those of B-form DNA and had characteristic features: decreasing magnitude of the positive maximum at 265 nm and a negative peak at 285 nm.
Interaction of DNA with the analogs of the antibiotic distamycin A having different numbers of pyrrolcarboxamide groups and labeled with dansyl was studied. The binding isoterms of the analogs to synthetic polydeoxyribonucleotides were obtained. Analysis of the experimental data leads to the following conclusions: (1) the free energy of binding of the analogs to poly(dA).poly(dT) depends linearly on the number of amide groups in the molecule of the analog whereas attachment of each pyrrolcarboxamide group produces changes of 2 kcal/mole in the free energy; (2) attachment of a pyrrolcarboxamide unit to the GC pair results in the free energy change of 0.95 kcal/mole; (3) the binding of analogs to poly(dA).poly(dT) is a cooperative process, presumbly, dependent on conformational changes induced by the binding of analogs to DNA.
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