A theoretical model for the binding of cis-Pt(NH3)2(+2) to DNA.

The binding of cis-Pt(NH3)2B1B2 to the bases B1 and B2, i.e., guanine (G), cytosine (C), adenine (A), and thymine (T), of DNA is studied theoretically. The components of the binding are analyzed and a model structure is proposed for the intrastrand binding to the dB1pdB2 sequence of a kinked double helical DNA. Quantum mechanical calculations of the ligand binding energy indicates that cis-Pt(NH3)2(+2) (cis-PDA) binds to N7(G), N3(C), O2(C), O6(G), N3(A), N7(A), O4(T) and O2(T) in order of decreasing binding energy. Conformational analysis provides structures of kinked DNA in which adjacent bases chelate to cis-PDA. Only bending toward the major groove allows the construction of acceptable square planar complexes. Examples are presented for kinks of -70 degrees and -40 degrees at the receptor site to orient the base pairs for ligand binding to B1 and B2 to form a nearly square planar complex. The energies for complex formation of cis-PDA to the various intra-strand base sites in double stranded DNA are estimated. At least 32 kcal/mole separates the energetically favorable dGpdG.cis-PDA chelate from the dCpdG.cis-PDA chelate. All other possible chelate structures are much higher in energy which correlates with their lack of observation in competition with the preferred dGpdG chelate. The second most favorable ligand energy occurs with N3(C). A novel binding site involving dC(N3)pdG(N7) is examined. Denaturation can result in an anti----syn rotation of C about its glycosidic bond to place N3(C) in the major groove for intrastrand binding in duplex DNA. This novel intrastrand dCpdG complex and the most favored dGpdG structure are illustrated with stereographic projections.