Differential interactions of the Mg²⁺ complexes of chromomycin A₃ and mithramycin with Poly(dG-dC)•Poly(dC-dG) and Poly(dG)•Poly(dC)

The interaction of the two anticancer antibiotics, chromomycin A3 and mithramycin, with the polynucleotides poly(dG-dC).poly(dC-dG), representative of B-DNA, and poly(dG)•poly(dC), representative of A-DNA, in the presence of Mg2+ is studied by spectroscopic techniques such as absorbance, fluorescence, and dircular dichroism (CD). The studies were done with both drug•Mg2+ complexes, I and II, having 1:1 and 2:1 stoichiometries with respect to drug and Mg2+, respectively [Aich, P., Sen, R., & Dasgupta, D. (1992) Biochemistry 31, 2988−2997]. The objective of the present work is 2-fold. First, an attempt is made to understand the structural basis of the ligand−DNA interaction, particularly the role of DNA backbone conformation with its groove size and the accessibility of the 2-amino group in the minor groove of guanosine. Second, the role of the antibiotic saccharide moieties in the association with DNA was studied. For this purpose, the spectroscopic characterization of the binding was done followed by the evaluation of binding parameters and associated thermodynamics. Analysis of the observed thermodynamics for the ligand−DNA interactions in terms of the different structures of the polynucleotides was done. The salient results are as follows. Complex I does not discriminate significantly among the A- and B-forms of DNA when it binds to them in an entropy-driven process. On the other hand, complex II for both drugs recognizes B- and A-forms of DNA in different ways. This observation implies that the sequence specificity shown by this complex is a sequel to the difference in the parameters such as groove size and accessibility of the guanosine amino group. Another important finding is that binding with the same polynucleotide is not comparable for the complex II of the two drugs. It emphasizes the involvement of the sugar moieties, when the drug•Mg2+ complex binds to DNA. The presence of an acetoxy group in the sugars of chromomycin A3 imparts some distinctive specific features of the association of the chromomycin dimer•Mg2+ complex with DNA. Finally, the results are compared with those available from NMR studies of different drug−oligonucleotide complexes under conditions where complex II is the ligand