Luminescent Ir(III) complex exclusively made of polypyridine ligands capable of intercalating into calf-thymus DNA.

Efficient intercalation of a luminescent Ir(III) complex exclusively made of polypyridine ligands in natural and synthetic biopolymers is reported for the first time. The emission of the complex is largely enhanced in the presence of [poly(dA-dT)(2)] and strongly quenched in the presence of [poly(dG-dC)(2)]. By comparing the emission decays in DNA and in synthetic polynucleotides, it is proposed that the emission quenching of the title compound by guanine residues in DNA is no longer effective over a distance of four dA-dT base pairs.

Characterization of the cell growth inhibitory effects of a novel DNA-intercalating bipyridyl-thiourea-Pt(II) complex in cisplatin-sensitive and -resistant human ovarian cancer cells.

The cellular effects of a novel DNA-intercalating agent, the bipyridyl complex of platinum(II) with diphenyl thiourea, [Pt(bipy)(Ph(2)-tu)(2)]Cl(2), has been analyzed in the cisplatin (cDDP)-sensitive human ovarian carcinoma cell line, 2008, and its -resistant variant, C13* cells, in which the highest accumulation and cytotoxicity was found among six related bipyridyl thiourea complexes. We also show here that this complex causes reactive oxygen species to form and inhibits topoisomerase II activity to a greater extent in the sensitive than in the resistant line. The impairment of this enzyme led to DNA damage, as shown by the comet assay. As a consequence, cell cycle distribution has also been greatly perturbed in both lines. Morphological analysis revealed deep cellular derangement with the presence of cellular masses, together with increased membrane permeability and depolarization of the mitochondrial membrane. Some of these effects, sometimes differentially evident between the two cell lines, might also be related to the decrease of total cell magnesium content caused by this thiourea complex both in sensitive and resistant cells, though the basal content of this ion was higher in the cDDP-resistant line. Altogether these results suggest that this compound exerts its cytotoxicity by mechanisms partly mediated by the resistance phenotype. In particular, cDDP-sensitive cells were affected mostly by impairing topoisomerase II activity and by increasing membrane permeability and the formation of reactive oxygen species; conversely, mitochondrial impairment appeared to play the most important role in the action of complex F in resistant cells.

Luminescence properties of Pt(II) complexes containing polypyridine ligands with extended aromatic moieties.

The luminescence properties of eleven Pt(II) complexes containing polypyridine ligands with extended aromatic moieties have been studied, both in acetonitrile fluid solution at 298 K and in butyronitrile rigid matrix at 77 K. For comparison purposes, also the phosphorescence properties of three free ligands at 77 K in butyronitrile have been investigated. The absorption spectra of all the compounds exhibit intense bands (epsilon in the range 10(4)-10(5) M(-1) cm(-1)) in the UV region, which are attributed to spin-allowed ligand-centered (LC) transitions, and moderately intense bands (epsilon in the range 10(3)-10(4) M(-1) cm(-1)) in the visible region, which receive contribution from both spin-allowed LC transitions and spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. At low energy, less intense spin-forbidden MLCT bands are also present. At 77 K in rigid matrix, all the studied compounds exhibit structured and long-lived (lifetimes from 840 micros on the millisecond timescale) luminescence, which is attributed to triplet LC states in all cases. At room temperature in fluid solution the luminescence lifetime of all the compounds is largely shortened (nanosecond timescale), and most of the emission spectra are unstructured and red-shifted. For species exhibiting structured emission spectra even at room temperature, low luminescence quantum yields are always obtained (Phi < 10(4)), and their emission is assigned to triplet LC states, which mainly deactivate to the ground state by thermal-activated surface crossing to a closely-lying metal-centered (MC) triplet state. Compounds exhibiting unstructured emission show relatively high emission quantum yields (about 0.1) and their emission is assigned to a mixed LC/MLCT state.

Studies on the anti-proliferative effects of novel DNA-intercalating bipyridyl-thiourea-Pt(II) complexes against cisplatin-sensitive and -resistant human ovarian cancer cells.

Six bipyridyl complexes of platinum(II) with thiourea, with different substituents on thiourea moiety [Pt(bipy)(R,R'NCSNR'',R''')(2)]Cl(2) (bipy=2,2'-bipyridine: R=R'=R''=R''' =H; R=Me, R'=R''=R'''=H; R=n-Bu, R'=R''=R'''=H; R=Et, R'=H, R''=Et, R'''=H; R=p-tolyl, R'=R''=R'''=H; R=phenyl, R'=H, R''=phenyl, R'''=H), rationally designed to intercalate into DNA, have been tested against a cisplatin (cDDP)-sensitive human ovarian carcinoma cell line (2008) and its -resistant variant (C13( *)). We show here that the anti-proliferative efficacy of these drugs was dependent on molecular structure, since it increased with ancillary ligand bulkiness and hydrophobicity of substituents on thiourea moiety. In particular, the presence of two phenyl groups on thiourea moiety confers an outstanding cytotoxicity. The increasing cell growth inhibition along the series of complexes partially paralleled with drug accumulation, particularly in resistant cells, but not with drug intercalation into DNA since all compounds exerted comparable ethidium bromide displacement ability. The cDDP-resistant phenotype seems, at least in part, to be involved in the action of these compounds, since the level of cross-resistance established for most complexes appeared to be in agreement with the observed impairment of drug accumulation in the resistant subline. These findings indicate that resistance to alkylating agents such as cDDP confers low level of cross-resistance to this class of DNA intercalators, which, however, depending on substituents on thiourea moiety may present remarkable cell growth inhibition even of resistant cells.

DNA and RNA noncovalent interaction of platinum(II) polypyridine complexes.

A comparative investigation of the noncovalent interaction of the platinum(II) polypyridine complexes [Pt(dipy)(n-Rpy)2]2+ and [Pt(4,4'-Me2dipy)(2-Rpy)2]2+ (dipy = 2,2'-dipyridine; Me = CH3; n = 2-4; R = H or CH3) with double-helical DNA (calf thymus) and RNA [poly(A).poly(U)] has been conducted. With the exception of [Pt(dipy)(2-Mepy)2]2+, all of the complexes interact strongly, by intercalation, with both nucleic acids giving rise to large changes in the electronic spectra and induced circular dichroism signals; in addition, viscosity experiments on rodlike DNA and RNA show that both biopolymers elongate upon interaction with the complexes. The binding constant values, KB, determined at 25 degrees C, indicate that, at 0.101 M ionic strength, the affinity for poly(A).poly(U) is strongly dependent on the complexes nature, while for DNA it is leveled off. [Pt(dipy)(2-Mepy)2]2+ binds to DNA but does not interact appreciably with poly(A).poly(U).

Luminescence of a Pt(II) complex in the presence of DNA. Dependence of luminescence changes on the interaction binding mode.

Luminescence intensity changes of a Pt(II) complex which is known to bind externally to DNA at low [DNA]/[complex] ratio and to intercalate at high [DNA]/[complex] ratio are studied in the presence of calf thymus DNA. External binding is demonstrated to induce luminescence enhancement whereas intercalation leads to luminescence quenching. The reasons for this behaviour are discussed.

Role of the coulombic interaction in ligand-induced biopolymer aggregation.

The interaction mechanisms responsible for the binding between metal complexes and biopolymers in aqueous solution, as well as the consequent aggregation process of biopolymers themselves, involve many factors, from geometrical aspects and hydrophobic contributions, as examples, to the electrostatic potential. In this paper aqueous solutions of a polynucleotide, polyadenylic acid (PolyA), which mimics the helix arrangement of RNA or single-stranded DNA but has a simpler structure, are used as a model system. The role of the electrostatic interactions in the binding process between some platinum(II) complexes and PolyA and in the aggregation among PolyA molecules is investigated, by means of elastic and quasielastic light scattering and electrophoretic mobility. The results show that the presence of large, planar aromatic moiety in the dicationic platinum(II) complexes is essential for the binding with PolyA and suggest that the consequent lowering of the local electrostatic barrier between PolyA molecules can be involved in triggering the aggregation process.

DNA interaction of platinum(II) complexes with 1,10-phenanthroline and extended phenanthrolines.

The interaction with DNA of the platinum(II) square planar complexes [Pt(N-N)(py)(2)](2+) (N-N = 1,10-phenanthroline (phen), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq), dipyrido[3,2-a:2',3'-c]phenazine (dppz), benzodipyrido[b:3,2-h:2'3'-f]phenazine (bdppz)) has been investigated by means of absorption, circular and linear dichroism spectroscopy, DNA melting, and viscosity. In the presence of excess [DNA] all the complexes intercalate to the double helix. For those with the most extended phenanthrolines the binding mode depends on the [DNA]/[complex] ratio (q); at low q values the substances bind externally to DNA probably self-aggregating along the double helix. When the DNA concentration is large enough, the aggregate breaks up and the complex intercalates within the nucleobases. The complexes self-aggregate, without added DNA, in the presence of a large salt concentration.

The intercalation to DNA of bipyridyl complexes of platinum(II) with thioureas.

The non-covalent interaction of the complexes [Pt(bpy)(R,R'NCSNR'',R''')(2)]Cl(2) (bpy=2,2'-bipyridine; R=R'=R''=R'''=H; R=Me, R'=R''=R'''=H; R=n-Bu, R'=R''=R'''=H; R=p-tolyl, R'=R''=R'''=H; R=Et, R'=H, R''=Et, R'''=H) with calf thymus DNA has been studied at pH 7 and 25 degrees C. The processes give rise to: (i) reversible bathochromic shifts and strong hypochromicity of the absorption bands of the complexes, (ii) induced circular dichroism and (iii) an increase both in the melting temperature and viscosity of the DNA comparable to that observed for other well known metallointercalators. The binding constants, K(B), have been determined spectrophotometrically using the McGhee von Hippel equation. Plot of logK(B) vs -log[Na(+)] for the complex with unsubstituted thiourea gives a straight line with a slope value close to that expected for a dicationic intercalator. The binding affinity of the various complexes for DNA is independent of the thiourea nature; this suggests that the intercalation occurs through stacking of the bpy moiety while the ancillary ligands lie outside the nucleobases far away from the sugar phosphate backbone. The data show also that the electronic effects of the ligand substituents are not transmitted to the intercalating unit.

Ambivalent intercalators for DNA: L-shaped platinum(II) complexes.

Novel platinum(II) square planar coordination complexes, in which two heteroaromatic ligands are held by the metal in an unusual L-shaped geometry orthogonal to each other, have been synthesized, and their interaction with DNA was investigated with absorption and linear dichroism spectroscopy. As a rule, the ligand that is coplanar with the coordination square of Pt is found to be oriented perpendicular relative to the DNA helix axis when bound, suggestive of its intercalation between the base pairs of DNA. However, when this coplanar ligand is replaced by two pyridines, the opposite ligand, orthogonal to the coordination square, is instead preferentially intercalated. This behavior shows that these new complexes do indeed show some properties of true ambintercalators, i.e., compounds that can bind by intercalation of either of two distinct aromatic moieties.

Stacking Surface Effect in the DNA Intercalation of Some Polypyridine Platinum(II) Complexes.

The interaction of the platinum(II) polypyridine complexes [Pt(bipy)(2)](2+), [Pt(quaterpy)](2+), [Pt(terpy)(n-Rpy)](2+) and [Pt(bipy)(py)(2)](2+) (bipy = 2,2'-bipyridine; terpy = 2,2':6',2' '-terpyridine; quaterpy = 2,2':6',2' ':6' ',2' "-quaterpyridine; n-R = H, 2-CH(3), or 4-CH(3) ) with double-helix DNA has been studied with a variety of experimental techniques. Induced circular dichroism, strong hypochromism and red shifts of the absorption maxima of the complexes, increase in melting temperature and viscosity of DNA, and inhibition of the reaction of the complexes with thiourea in the presence of DNA, characterize the processes. Intercalation, implying the whole molecule or part of it, is the suggested binding mode. The binding constants, K(B), determined spectrophotometrically at 25 degrees C, pH 7, and I = 0.15 M, using the McGhee-von Hippel approach, increase in the order [Pt(bipy)(py)(2)](2+)< [Pt(terpy)(py)](2+)< [Pt(quaterpy)](2+), on increasing aromatic planar surface extension. The steric interference with double helix of the methyl group in [Pt(terpy)(2-Mepy)](2+) destabilizes the interaction by reducing the stacking surface.

Noncovalent Interactions of Platinum(II) Square Planar Complexes Containing Ligands Out-of-Plane with DNA.

The interaction of the complexes [Pt(bipy)(4-Rpy)(2)](2+) and [Pt(4,4'-Ph(2)bipy)(4-Rpy)(2)](2+) (Ph = phenyl; bipy = 2,2'-bipyridine; R = H, CN, CH(3), NH(2)) with DNA has been studied with a series of techniques. The processes give rise to (i) lengthening of rodlike DNA and unwinding of closed circular DNA and (ii) an increase in the DNA melting temperature comparable with that observed for known intercalators. In addition, the reaction of the complexes [Pt(bipy)(py)(2)](2+) and [Pt(4,4'-Ph(2)bipy)(py)(2)](2+) is inhibited by the presence of DNA. These results have been interpreted by assuming that the substances intercalate in spite of the presence of ligands out of plane. The crystal structure determined for [Pt(4,4'-Ph(2)bipy)(3,5-Me(2)py)(2)](2+) by X-ray analysis shows that also one of the phenyl rings is twisted with respect to the square plane. Binding constants, K(B), determined spectrophotometrically at 25 degrees C and pH 7 using the McGhee-von Hippel approach, increase for both series of complexes on increasing pK(a) of coordinated pyridines and are larger for those with 4,4'-Ph(2)bipy. The increasing affinity for DNA on increasing electron density of the interacting moiety is accounted for by assuming that London dispersion forces play a major role in the processes.