Factors Affecting DNA-DNA Interstrand Cross-Links in the Antiparallet 3' - 3' Sense. A Comparison with the 5' - 5' Directional Isomer

Reported herein is a study of the unusual 3′–3′ 1,4-GG interstrand cross-link (IXL) formation in duplex DNA by a series of polynuclear platinum anticancer complexes. To examine the effect of possible preassociation through charge and hydrogen-bonding effects the closely related compounds [{trans-PtCl(NH3)2}2(μ-trans-Pt(NH3)2{NH2(CH2)6NH2}2)]4+ (BBR3464, 1), [{trans-PtCl(NH3)2}2(μ-NH2(CH2)6NH2)]2+ (BBR3005, 2), [{trans-PtCl(NH3)2}2(μ-H2N(CH2)3NH2(CH2)4)]3+ (BBR3571, 3) and [{trans-PtCl(NH3)2}2{μ-H2N(CH2)3-N(COCF3)(CH2)4}]2+ (BBR3571-COCF3, 4) were studied. Two different molecular biology approaches were used to investigate the effect of DNA template upon IXL formation in synthetic 20-base-pair duplexes. In the “hybridisation directed” method the monofunctionally adducted top strands were hybridised with their complementary 5′-end labelled strands; after 24 h the efficiency of interstrand cross-linking in the 5′–5′ direction was slightly higher than in the 3′–3′ direction. The second method involved “postsynthetic modification” of the intact duplex; significantly less cross-linking was observed, but again a slight preference for the 5′–5′ duplex was present. 2D [1H, 15N] HSQC NMR spectroscopy studies of the reaction of [15N]-1 with the sequence 5′-d{TATACATGTATA}2 allowed direct comparison of the stepwise formation of the 3′–3′ IXL with the previously studied 5′–5′ IXL on the analogous sequence 5′-d(ATATGTACATAT)2. Whereas the preassociation and aquation steps were similar, differences were evident at the monofunctional binding step. The reaction did not yield a single distinct 3′–3′ 1,4-GG IXL, but numerous cross-linked adducts formed. Similar results were found for the reaction with the dinuclear [15N]-2. Molecular dynamics simulations for the 3′–3′ IXLs formed by both 1 and 2 showed a highly distorted structure with evident fraying of the end base pairs and considerable widening of the minor groove.