Conformer Distribution in (<i>cis</i>-1,4-DACH)bis(guanosine-5′-phosphate)platinum(II) Adducts: A Reliable Model for DNA Adducts of Antitumoral Cisplatin

In [PtCl2(cis-1,4-DACH)] (DACH = diaminocyclohexane), the N−Pt−N bite angle (≥97°, as determined by X-ray diffraction analysis) is much larger than those found in other Pt complexes with bidentate diamines or in cisplatin (~91°). Hence, the possibility exists that in (cis-1,4-DACH)PtG2 adducts, rotation of the Gʼs around the Pt−N7 bonds is slowed enough to allow observation of different conformers. In accord with this prevision, decreasing the temperature to 238 K enabled us to observe different conformers of (cis-1,4-DACH)Pt(5′-GMP)2 (GMP = guanosine monophosphate). This observation is the first case in which such conformers for a platinum derivative with primary diamines and untethered guanines have been resolved and represents the closest model to clinically effective cisplatin obtained to date. We also found that the presence of the 1,4-DACH ligand increased the intensity of the circular dichroism signal stemming from the dominance of an HT conformer (ΔHT in the adduct with 3′-GMPs and ΛHT in the adduct with 5′-GMPs).