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. [1H, 15N] HSQC NMR spectroscopy has been used to follow the stepwise binding of the dinuclear Pt complex [{trans‐PtCl(NH3)2}2(μ‐NH2(CH2)6NH2)]2+ (1) to a 14‐mer duplex (I), illustrated here, that offers three possible intra‐ and interstrand bifunctional binding modes. The major adduct is the 5′–5′ 1,4‐interstrand cross‐link between G(8) and G(18) bases which exists in two conformational forms.