X-ray Structure and Circular Dichroism of Pure Rotamers of Bis[guanosine-5′-monophosphate(-1)](N,N,N′,N,′-tetramethylcyclohexyl-1,2-diamine)platinum(II) Complexes That Have R,R and S,S Configurations at the Asymmetric Diamine

The use of a sterically hindered diamine ligand (Me4DACH) has allowed for the first time, the isolation and characterization, both in the solid state (X-ray crystallography) and in solution (circular dichroism), of pure ΔHT rotamers of [Pt(Me4dach)(5′-GMP)2] (compounds 1 and 2 for R,R and S,S configurations of the Me4DACH ligand, respectively). Comparison of the CD spectra obtained for each rotamer, which differ only in the chirality of the Me4DACH ligand (R,R or S,S) or in the chirality of the HT conformation (Δ or Λ), allowed us to conclude that, in the 200-350 nm range, the contributions to the overall CD spectrum that stem from diamine chirality and diamine-induced chirality of platinum dd transitions or from sugar chirality are negligible relative to the exciton chiral coupling that occurs for π-π* transitions of the cis guanines. Accurate molecular structures of 1•10D2O and 2•14D2O (conventional crystallographic agreement indexes R1 convergent to 2.07% and 2.18%, respectively) revealed that the crystallized rotamers have a ΔHT conformation that is in agreement with all previously reported X-ray structures of [Pt(diamine)(nucleos(t)ide) 2] complexes. This conformation allows the 5′-phosphate to be located in proximity to the Me4DACH ligand so that (P)O⋯HC(N) hydrogen-bond interactions exists in both complexes. For both structures, the canting of the guanine planes on the coordination plane is right-handed (R; canting angle (φ) of 80.9° and 73.2°, respectively); this indicates that the canting direction is driven by the HT conformation chirality (Δ for both compounds) and not by the chirality of the carrier ligand (different for the two compounds). Density functional theory analysis of the conformational space as a function of φ indicated a good agreement between the computed and experimental structures. The increase in energy for φ values below 65° and 55° (for 1 and 2, respectively) is mainly due to the short intramolecular contacts between C(8)H and the cis N-Me groups on the same side of the platinum coordination plane.