Competitive Reactions of Interstrand and Intrastrand DNA-Pt Adducts: A Dinuclear-Platinum Complex Preferentially Forms a 1,4-Interstrand Cross-Link Rather than a 1,2 Intrastrand Cross-link on Binding to a GG 14-Mer Duplex

A study of the kinetics and mechanism of the reaction between the dinuclear Pt complex [{trans-PtCl(NH3)2}2(μ-NH2(CH2)6NH2)]2+ (1) and the 14-mer duplex 5′-d(ATACATG(7)G(8)TACATA)-3′⋅5′-d(TATG(25)TACCATG(18)TAT)-3′ is reported. [1H,15N]-HSQC NMR was used to follow the reaction at 298 K, pH 5.4. The product is primarily the 5′–5′ 1,4-interstrand cross-link between G(8) and G(18) bases and exists in two conformational forms. No evidence for the possible 1,2-intrastrand G(7)G(8) adduct was seen, confirming the preferential formation of interstrand cross-links by these dinuclear complexes. An initial electrostatic association of 15N-1 with the duplex is indicated by changes in its 1H/15N chemical shifts, followed by aquation of 1 to form the monoaqua monochloro species 2, with a rate constant of 4.00±0.03×10−5 s−1. Monofunctional binding to the duplex occurs primarily at G(8), the 3′ base of the nucleophilic GG grouping, with a rate constant of 1.5±0.7 M−1 s−1. Changes in the 1H/15N shifts indicate there is an electrostatic interaction between the unbound {PtN3Cl} group of the monofunctional adduct and the duplex. No peaks for a transient aquated monofunctional species are seen and closure of 3 to form the 1,4-G(8)G(18) interstrand cross-link (5) was treated as direct, with a rate constant of 4.47±0.06×10−5 s−1. The G(8)G(18) cross-link was confirmed from analysis of the NOESY NMR spectrum of the final product. Structural perturbations for the 1,4-interstrand cross-link extend over approximately four base-pairs and are similar to those found for a 1,4-interstrand cross-link with a shorter 8-mer -GTAC- sequence. A major distortion was evident for the 5′T (T(17)) adjacent to the platinated G(18), consistent with the findings from the use of chemical probes to investigate the conformation of 1,4-interstrand cross-links.