DNA-binding and molecular mechanics modelling studies of the bulky chiral platinum(II) complex [PtCl 2 (mepyrr)] (mepyrr = N-methyl-2-aminomethylpyrrolidine) Connie I. Diakos, Ronald R. Fenton, Trevor W. Hambley * Centre for Heavy Metals Research, School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Received 9 August 2006; received in revised form 7 September 2006; accepted 7 September 2006 Available online 23 September 2006 Abstract Detailed studies were carried out on the binding of the enantiomers of [PtCl 2 (mepyrr)] (mepyrr = N-methyl-2-aminomethylpyrroli- dine) to dG, d(GpG) and a 52-mer oligonucleotide. The pyrrolidine ligand structure was found to be neither sufficiently rigid nor bulky to enforce a single chirality at the exocyclic amine site in this complex, resulting in the presence of diastereomers that complicated the binding studies. Reaction of the (GpG) dinucleotide with R- and S-[PtCl 2 (mepyrr)] resulted in formation of four [Pt{d(GpG)}(mepyrr)] isomers for each enantiomer as a consequence of the existence of two orientational isomers and two diastereomers. These isomers formed in different amounts most likely as a consequence of the unequal formation of the diastereomers together with stereoselectivity induced by interactions between the dinucleotide and the mepyrr ligand. The [PtCl 2 (mepyrr)] complexes displayed stereoselectivity and enanti- oselectivity in their reactions with a 52-mer duplex designed to allow formation of only GpG intrastrand adducts. All four bifunctional adducts formed for each enantiomer, providing further evidence of the lack of directing ability of the ligand in formation of the 1,2-intra- strand adduct. Significant amounts of monofunctional species remained in these assays suggesting that the introduction of the methyl substituent to the exocyclic amine inhibited ring-closure to the bifunctional adduct. This was not sufficient to achieve enantiospecificity, but in the case of the R-enantiomer, one of the bifunctional adducts formed in only small amounts. Crown Copyright Ó 2006 Published by Elsevier Inc. All rights reserved. Keywords: Pt complexes; DNA; Molecular modelling; HPLC; Enantioselectivity 1. Introduction The primary DNA-binding site for all mononuclear bifunctionally-binding platinum(II) complexes studied thus far is two adjacent guanines on the same strand of DNA (GpG) [1–8]. Increasingly, evidence supports the hypothe- sis that this adduct is primarily responsible for the anti- cancer action of these drugs [9,10]. Therefore, a detailed understanding of the factors controlling binding at the GpG site is highly desirable. Of the compounds that have so far been studied in detail [6,8,11–13], only two bind ste- reoselectively to DNA as a result steric interactions alone and these have provided substantial insights into the importance of such interactions [8,13]. Therefore, we have extended our investigations to other platinum complexes with bulky ligands. We have used asymmetric compounds that generate two stereoisomers of the GpG adduct, as any observed stereoselectivity can reveal the factors that control platinum binding to DNA [13–15]. The synthesis of the N-methyl-2-aminomethylpyrroli- dine (mepyrr) ligand and its platinum(II) complex has been described elsewhere [16]. The ligand features a five-mem- bered chelate ring containing a chiral (stereogenic) carbon atom, from which extends a methylene bridge to an exocy- clic nitrogen atom (Scheme 1). The substitution of a methyl group onto this exocyclic nitrogen results in it becoming chiral upon coordination to platinum(II). The synthesis 0162-0134/$ - see front matter Crown Copyright Ó 2006 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2006.09.012 * Corresponding author. Tel.: +61 2 9351 2830; fax: +61 2 9351 3329. E-mail address: t.hambley@chem.usyd.edu.au (T.W. Hambley). www.elsevier.com/locate/jinorgbio Journal of Inorganic Biochemistry 100 (2006) 1965–1973 JOURNAL OF Inorganic Biochemistry