IDENTIFICATION AND CHARACTERIZATION OF A NOVEL LINKAGE ISOMERIZATION IN THE REACTION OF TRANS-DIAMMINEDICHLOROPLATINUM(II) WITH 5'-D(TCTACGCGTTCT)

The oligonucleotide 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 initially formed isomer and could be characterized 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 ~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 isomer ranging from 129 (at 30 °C) to 3.6 h (at 62 °C), with activation parameters ΔH* = 91 ± 2 kJ/mol and ΔS‡ = −58 ± 8 J/(mol•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. © 1990, American Chemical Society. All rights reserved.