Identification and characterization of a novel linkage isomerization in the reaction of trans-diamminedichloroplatinum(II) with 5'-d(TCTACGCGTTCT)

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Abstract
The olingonucleotide 5''-d (TCTACGCGTTCT) reacts with trans-diamminedichloroplatinum (II) to yield primarily trans [Pt(NH3)2 {d (TCTACGCGTTCT)-N7-G(6),N7-G(8)}], containing the desired trans-[Pt(NH3)2{d(GCG}] 1,3-cross-link. A key element of the platination reaction is the use of low pH to suppress coordination at A(4). The product was fully characterized by pH-dependent NMR titrations, enzymatic degradation analysis, and 195Pt NMR spectroscopy. Interestingly, the 1,3-cross-linked adduct is unstable at neutral pH, rearranging unexpectedly to form the linkage isomer trans-[Pt(NH3)2{d-(TCTACGCGTTCT)-N3-C(5),N7-G(8)}]. This rearrangement product is more stable than the intially formed isomer and could be characterised by pH-dependent NMR titrations, enzymatic degradation analysis, liquid secondary ion mass spectrometric analysis of an enzymatically digested fragment, 195Pt NMR spectroscopy, and modified maxam-Gilbert footprinting experiments. By contrast, the 1,3-intrastrand cross-linked isomer rearranges during the course of both pH titration and enzymatic degradation experiments to form the 1,4-adduct. The equilibrium constant for this rearrangement is .apprx. 3, favoring, the 1,4-adduct. Kinetic studies of the linkage isomerization reaction reveal t1/2 values for the first order disappearance of the 1,3-intrastrand cross-linked iosomer ranging from 129 (at 30.degree.C) to 3.6 h (at 62.degree.C), with activation parameters .DELTA. H* = 91 .+-. 2kJ/mol and .DELTA. S* = -58 .+-. 8 J/ (mol .cntdot. K). Mechanistic implications of these kinetic results as well as the general relevance of this linkage isomerization reaction to platinum-DNA chemistry are briefly discussed.