Protonation of adenine carrying a Pt(II) moiety either at N7, N3, or N1 is possible in solution, but the site of protonation is influenced by the location of the Pt(II) electrophile and to some extent also by the overall charge of the metal entity (+2, +1, 0, -1), hence the other ligands (NH(3), Cl(-), OH(-)) bound to Pt(II). Quantum chemical calculations based on density functional theory (DFT) have been carried out for intrinsic protonation energies of adenine complexes carrying the following Pt(II) species at either of the three ring N atoms: [Pt(NH(3))(3)](2+) (1), trans- [Pt(NH(3))(2)Cl](+) (2a), cis-[Pt(NH(3))(2)Cl](+) (2b), trans-[Pt(NH(3))(2)Cl(2)] (3a), cis-[Pt(NH(3))Cl(2)] (3b), [PtCl(3)](-) (4), trans-[Pt(NH(3))(2)OH](+) (5a), cis-[Pt(NH(3))(2)(OH)](+) (5b), trans-[Pt(NH(3))(OH)(2)] (6a), cis-[Pt(NH(3))(OH)(2)] (6b), and [Pt(OH)(3)](-) (7). The data have been compared with results derived from solution studies (water) and X-ray crystallography, whenever available. The electrostatic effects associated with the charge of the metal entity have the major influence on the calculated intrinsic (gas phase) proton affinities, unlike the condensed phase data. Nevertheless, the relative gas phase trends correlate surprisingly well with condensed phase data; i.e., variation of the pK(a) values measured in solution is consistent with the calculated gas phase protonation energies. In addition to a systematic study of the ring proton affinities, proton transfer processes within the platinated adenine species were often observed when investigating Pt adducts with OH(-) ligands, and they are discussed in more detail. To the best of our knowledge, this is the first study attempting to find a systematic correlation between gas phase and condensed phase data on protonation of metalated nucleobases. The gas phase data provide a very useful complement to the condensed phase and X-ray experiments, showing that the gas phase studies are capable of valuable predictions and contribute to our understanding of the solvent and counterion effects on metal-assisted proton shift processes.

Crosslinking of three different model nucleobases (9-ethyladenine, 9-EtA; 9-ethylguanine, 9-EtGH; 1-methyluracil, 1-MeU) by two linear trans-aPtII (a = NH3 or CH3NH2) entities leads to a flat metal-modified base triplet, trans,trans-[(NH3)2Pt(1-MeU-N3)(mu-9-EtA-N7,N1)Pt(CH3NH2)2(9-EtGH-N7)]3+ (4b). Upon hemideprotonation of the 9-ethylguanine base at the N1 position. 4b spontaneously dimerizes to the metalated nucleobase sextet 5, [(4b)(triple bond)(4b-H)]5+. In this dimeric structure a neutral and an anionic guanine ligand, which are complementary to each other, are joined through three H bonds and additionally by two H bonds between guanine and uracil nucleobases. Four additional interbase H bonds maintain the approximate coplanarity of all six bases. The two base triplets form an exceedingly stable entity (KD = 500 +/- 150 M(-1) in DMSO), which is unprecedented in nucleobase chemistry. The precursor of 4b and several related complexes are described and their structures and solution properties are reported.

Di-and trinuclear complexes of trans-a 2 Pt II (a ) NH 3 or NH 2 CH 3 ) and the purine model nucleobases 9-methyladenine (9-MeA), 9-ethyladenine (9-EtA), 9-methylguanine (9-MeGH), and 9-ethylguanine (9-EtGH) have been prepared and characterized. The following five compounds have been studied using X-ray crystallography: 3a); a ) NH 3 , n ) 1.8 (3b)), and trans,trans,trans-{[Cl(NH 3 ) 2 Pt] 2 (N1-9-MeA-N7) 2 Pt(NH 3 ) 2 }-(ClO 4 ) 4 ‚H 2 O (5). In all diplatinated adenine species (2, 3a, 3b, and 5) the Pt-N(1) and Pt-N(7) vectors are approximately at right angles, therefore making these complexes potential building blocks for molecular squares, rectangles, and meanders. Two open nucleobase quartets, 4 and 6, consisting of three trans-diamineplatinum(II) entities and four purine nucleobases (two bridging 9-MeA′s and two terminal 9-MeGH (4) and 9-EtGH (6), respectively) have been isolated and characterized by elemental analysis and 1 H and 195 Pt NMR spectroscopy. S0020-1669(98)00081-0 CCC: $15.00