Aspects of the stereochemistry of bis-guanosine-5′-monophosphate diamine platinum complexes

Abstract

The conformations, in aqueous solution, of some [PtA2(GMP)2]2− complexes (A = NH3, or A2 - ethylenediamine, 1,3-propylenediamine R,R-, S,S- and meso-1,2-diaminocyclohexane and R,R-, S,S- and meso-2,3-diaminobutane; GMP = N(7)-coordinated guanosine-5′-monophosphate) have been investigated by means of circular dichroism (CD) spectroscopy. There is only one 1H NMR detectable isomer for the majority of the complexes, whose CD spectra suggest a ⋀HT conformation with a counter-clockwise tilt around the Pt-N(7) bonds, which produces a right-handed, propeller-like, spatial arrangement of the guanine planes. The derivatives of the S,S-diamines, which show two NMR observable isomers, display CD spectra of opposite sign, presumably originated by a clockwise tilt around Pt-N(7) in the more abundant conformer. Addition of urea to the solutions of these latter compounds produces only one conformer (NMR evidence) with the same CD spectra of the derivatives of the R,R-diamines, whose NMR and CD spectra are unaffected by urea. It is concluded that the conformations of the various [PtA2(GMP)2]2− depend on a competition between amine-purine hydrogen bonds and purine-purine interactions.