Synthesis and structural characterisation of a new form of bis(acyclovir)(ethylenediamine)platinum(II) - Correlation between the puckering of the carrier ligand and the canting of the nucleobases

A new conformer of the [Pt(en)(acv)(2)](2+) cation (en = ethylenediamine; a cv = acyclovir = 9-[(2-hydroxyethoxy)methyl]guanine) has been prepared by s low crystallisation of the [Pt(en)(acv)(2)]SO4. 2.5H(2)O salt. X-ray diffra ction studies have shown that the guanine moieties have a Head-to-Head conf ormation (HH) with the two C(8)-H vectors pointing towards the same side wi th respect to the platinum coordination plane; the dihedral angles formed b y the purine planes and the coordination plane are 58.3(2)degrees and 41.5( 2)degrees, respectively. There is a correlation between the canting of the purine bases (right or left handed) and the puckering of the ethylenediamin e chelate ring (delta or lambda). Right-handed canting is associated with l ambda puckering of the chelate ring and left-handed canting with delta puck ering of the chelate ring, so that in both cases one of the two purines for ms an O(6)... H-N hydrogen bond with a "quasi equatorial" NH of the ethylen ediamine. The "quasi equatorial" character of the NH appears to be a common feature for intramolecular H-bonds between guanine bases and cis-amines. T he purine not involved in the H-bond has the six-membered ring portion lean ing towards the cis purine rather than towards the cis amine and may give r ise to a weak O(6)... Pt attractive interaction. Such an interaction is sup ported by density functional (DFT) molecular orbital calculations carried o ut on the model systems [Pt(NH3)(4)(CH2O)](2+) and Head-to-Tail cis-[Pt(NH3 )(2){N(=CH2)-C(=CH2)-C(=O)-NH2}(2)](2+). The exocyclic chains Linked to N(9 ) each have two different orientations, and all four are stretched away fro m the metal centre as well as from the purine N(1)H and N(2)H-2 protons. Th e acv and the en NH protons are instead involved in H-bonds with the sulfat e anion.