Second-sphere ‘communication’ between two cis-bound guanine nucleotides. Factors influencing conformations of dynamic adducts of cis-type platinum anticancer drugs with guanine nucleotides as deduced by circular dichroism spectroscopy

Adducts of Pt anticancer drugs bound to N7 of guanine are fluxional, regardless of whether the guanine is part of a larger species like DNA or of a small analog like 3′- or 5′-guanosine monophosphate (3′- or 5′-GMP). This dynamic motion problem makes it impossible for NMR methods to reveal the solution conformation of the simple adducts studied here, cis-PtA2G2 (G=guanine derivative, A=NH3, and A2=trimethylenediamine (tn), or ethylenediamine (en)). Each adduct has three possible atropisomers (Δ and ΛHT (head-to-tail) and HH (head-to-head) forms). Recently, we demonstrated using NMR methods that the dynamic motion problem could be minimized by the bulky CCC ligands of (CCC)PtG2 complexes (CCC=chirality-controlling chelate ligand). We identified the atropisomers and determined their conformation. We showed that the chirality of the major HT form correlated with the sign of the CD peaks of the mixture. In the present study, CD spectroscopy was used to evaluate the conformation of the dominant HT atropisomer of dynamic cis-PtA2G2 adducts. All simple cis-PtA2G2 adducts examined had the Δ type CD signal for G=3′-GMP and the Λ type CD signal for 5′-GMP. The CD signal type did not change when the phosphate group was protonated (pH 3), but the intensity decreased. However, the sign changed and the intensity decreased when the guanine ring NH (N1H) was deprotonated (pH ∼10). Interestingly, the 1-methylguanosine 3′-monophosphate (1-Me-3′-GMP) and 1-methylguanosine 5′-monophosphate (1-Me-5′-GMP) adducts had a Λ and Δ CD signal, respectively. 1-Me-3′-GMP was synthesized by phosphorylation of 1-methylguanosine (1-MeGuo) using sodium cyclo-triphosphate hexahydrate. For cis-PtA2G2 adducts of G’s lacking a 3′ or 5′-phosphate group (e.g. guanosine, guanosine 2′-monophosphate, and 9-ethylguanine), the CD signals were always weak. The inosine 3′-monophosphate adduct, cis-Pt(NH3)2(3′-IMP)2, has a very strong CD similar to that of the 3′-GMP adduct. These results indicate that the stability of the dominant atropisomer of cis-PtA2G2 complexes depends on the presence and the position of the phosphate group and on the G N1H groups, but not the G amino group. We conclude that intramolecular phosphate–N1H hydrogen bonds formed between two cisG nucleotides favor the ΔHT and ΛHT conformers for the 3′-GMP and 5′-GMP adducts, respectively. The dominance of one chiral form accounts for the characteristic enhanced CD signal.