Dinuclear platinum complexes form a novel intrastrand adduct with d(GpG), an anti-syn conformation of the macrochelate as observed by NMR and molecular modeling

Dinuclear platinum complexes form a unique array of DNA adducts including (Pt,Pt) interstrand and (Pt,Pt) intrastrand cross-links. A (Pt,Pt) intrastrand adduct between two adjacent guanines is the structural analog of the major adduct formed by cis-DDP. In this study, we examined the kinetics of formation and structure of the (Pt,Pt) intrastrand adduct by following the interaction of [{trans-PtCl(NH3)(2)}(2){mu-H2N(CH2)(n)NH2}](2+) (1,1/t,t, n = 2-6) with d(GpG) using NMR spectroscopy and by molecular modeling. Initial coordination, to either the 5'-G or the 3'-G, is relatively fast compared to the second binding step, ring closure to the macrochelate adduct (i.e., [1,1/t,t]-d(GpG)-N7(1),N7(2)). The rate of ring closure depends on the chain length of the diamine linker. Complexes linked by a longer diamine chain (n = 4-6) react faster and produce a higher yield of macrochelate compared to the shorter n = 2, 3 diamine linkers. The structure of the (Pt,Pt) intrastrand adduct is significantly different from the cis-DDP-d(GpG) chelate. The major difference is the presence of a syn-orientated G base (observed for 1,1/t,t-(GpG) macrochelates of n = 3 and n = 6). For the macrochelate of [1,1/t,t, n = 3], i.e., [{trans-PtCl(NH3)(2)}(2){mu-H2N(CH2)(3)NH2}]{d(GpG)-N7(1),N7(2)}, the sugar conformation of the 5'-G (G1) is 28% S, whereas for 3'-G (G2) this value is much higher (69% S). In the corresponding n = 6 macrochelate these values are 55% S and 30% S for G(1) and G(2), respectively. The orientation of the two G bases in the [1,1/t,t]-d(GpG) chelates is best described as tectonic or ''stepped head-to-head''. The structure helps explain the flexible bending in DNA induced by the dinuclear platinum complexes in contrast to the rigid directed bend into the major groove caused by cis-DDP.