Protein metalation by metal-based drugs: reactions of cytotoxic gold compounds with cytochrome?c and lysozyme

Protein metalation processes are crucial for the mechanism of action of several anticancer metallodrugs and warrant deeper characterisation. We have explored the reactions of three cytotoxic gold(III) compounds??namely [(bipy^2Me)₂Au₂(??-O)₂][PF₆]₂ (where bipy^2Me is 6,6??-dimethyl-2,2??-bipyridine) (Auoxo6), [(phen^2Me)₂Au₂(??-O)₂][PF₆]₂ (where phen^2Me is 2,9-dimethyl-1,10-phenanthroline) (Au₂phen) and [(bipy^dmb-H)Au(OH)][PF₆] [where bipy^dmb-H is deprotonated 6-(1,1-dimethylbenzyl)-2,2??-bipyridine] (Aubipyc)??with two representative model proteins, i.e. horse heart cytochrome?c and hen egg white lysozyme, through UV?Cvisible absorption spectroscopy and electrospray ionisation mass spectrometry (ESI MS) to characterise the inherent protein metalation processes. Notably, Auoxo6 and Au₂phen produced stable protein adducts where one or more ??naked?? gold(I) ions are protein-coordinated; very characteristic is the case of cytochrome?c, which upon reaction with Auoxo6 or Au₂phen preferentially forms ??tetragold?? adducts with four protein-bound gold(I) ions. In turn, Aubipyc afforded monometalated protein adducts where the structural core of the gold(III) centre and its +3 oxidation state are conserved. Auranofin yielded protein derivatives containing the intact auranofin molecule. Additional studies were performed to assess the role played by a reducing environment in protein metalation. Overall, the approach adopted provides detailed insight into the formation of metallodrug?Cprotein derivatives and permits trends, peculiarities and mechanistic details of the underlying processes to be highlighted. In this respect, electrospray ionisation mass spectrometry is a very straightforward and informative research tool. The protein metalation processes investigated critically depend on the nature of both the metal compound and the interacting protein and also on the solution conditions used; thus, predicting with accuracy the nature and the amounts of the adducts formed for a given metallodrug?Cprotein pair is currently extremely difficult.     

Overcoming cisplatin resistance using gold(III) mimics: anticancer activity of novel gold(III) polypyridyl complexes

Gold(III) compounds have been recognized as anticancer agents due to their structural and electronic similarities with currently employed platinum(II) species. An added benefit to gold(III) agents is the ability to overcome cisplatin resistance. This work identified four gold(III) compounds, [Au(Phen)Cl₂]PF₆, [Au(DPQ)Cl₂]PF₆, [Au(DPPZ)Cl₂]PF₆, and [Au(DPQC)Cl₂]PF₆, (Phen = 1,10-phenanthroline, DPQ = dipyrido[3,2-d:2′,3′-f]quinoxaline, DPPZ = dipyrido[3,2-a:2′,3′-c] phenazine, DPQC = dipyrido[3,2-d:2′,3′-f] cyclohexyl quinoxaline) that exhibited anticancer activity in both cisplatin sensitive and cisplatin resistant ovarian cancer cells. Two of these compounds, [Au(DPQ)Cl₂]PF₆ (AQ) and [Au(DPPZ)Cl₂]PF₆ (AZ), displayed exceptional anticancer activity and were the focus of more intensive mechanistic study. At the molecular level, AQ and AZ formed DNA adducts, generated free radicals, and upregulated pro-apoptotic signaling molecules (p53, caspases, PARP, death effectors). Taken together, these two novel gold(III) polypyridyl complexes exhibit potent antitumor activity in cisplatin resistant cancer cells. These activities may be mediated, in part, by the activation of apoptotic signaling.     

Mechanistic studies on two dinuclear organogold(III) compounds showing appreciable antiproliferative properties and a high redox stability

Two dinuclear oxo-bridged organogold(III) compounds, namely [(N,N,C)(2)Au(2)(μ-O)][PF(6)](2) (with N,N,CH = 6-(1-methylbenzyl)-2,2'-bipyridine, Au(2)O1; or 6-(1,1-dimethylbenzyl)-2,2'-bipyridine, Au(2)O2), were previously prepared and characterised. Their solution chemistry under physiological-like conditions has been investigated here as well as their in vitro antiproliferative properties. Notably, these compounds reveal a marked redox stability even in the presence of effective biological reductants such as ascorbic acid and glutathione. The two dinuclear gold(iii) compounds were evaluated for cytotoxic actions against a representative panel of 12 human tumor cell lines, in comparison to respective mononuclear parent compounds [(N,N,C)AuOH][PF(6)], and appreciable biological activity could be highlighted. The reactions of Au(2)O1 and Au(2)O2 with a few model proteins were studied and the ability to form metallodrug-protein adducts monitored through ESI MS methods. Typical adducts were identified where the protein is associated to monometallic gold fragments; in these adducts gold remains in the oxidation state +3 and conserves its organic ligand. A direct comparison of the biological profiles of these binuclear organogold(III) compounds with those previously reported for a series of dinuclear oxo-bridged complexes [(N,N)(2)Au(2)(μ-O)(2)][PF(6)](2) (N,N = 6(6')-substituted 2,2'-bipyridines) named Auoxo's was carried out. It emerges that the greater cytotoxicity of the latter is mainly due to the greater oxidising power of their gold(III) centres and to propensity to generate gold(i) species; in contrast, the here described bimetallic organogold(III) complexes manifest a far higher redox stability in the biological milieu coupled to lower, but still significant, antiproliferative properties. Different molecular mechanisms are thus hypothesised for these two classes of dinuclear gold(III) agents.     

Reactions of the cationic monobut-2-yne complex [WI(CO)(NCMe)(dppm)(η2-MeC2Me)][BF4] {dppm = Ph2P(CH2)PPh2} with carbon monoxide and tButylisonitrile

The compound [WI(CO)(NCMe)(dppm)(η²-MeC₂Me)][BF₄] reacts with carbon monoxide and ^tbutylisonitrile in CH₂Cl₂ at room temperature to give the substituted products [WI(CO)₂(dppm)(η²-MeC₂Me)][BF₄] (1) and [WI(CO)(CN^tBu)(dppm)(η²-MeC₂Me)][BF₄] (2) in good yield. The new complexes were fully characterised by elemental analysis, infrared, ¹H and ¹³C NMR spectroscopy. ¹³C NMR spectroscopy suggests that the but-2-yne ligand is donating four electrons to the tungsten in these complexes.     

Synthesis, characterization, and aqueous chemistry of cytotoxic Au(III) polypyridyl complexes

This work reports the synthesis, characterization, and aqueous chemistry of a series of cytotoxic [Au(polypyridyl)Cl₂]PF₆ complexes {(where polypyridyl = dipyrido[3,2-f:2′,3′-h] quinoxaline (DPQ), dipyrido[3,2-a:2′,3′-c] phenazine (DPPZ) and dipyrido[3,2-a:2′,3′-c](6,7,8,9-tetrahydro) phenazine (DPQC))}. The crystal structure of [Au(DPQ)Cl₂]PF₆ was determined as example of the series and exhibits the anticipated square planar geometry common for d⁸ coordination complexes. The crystals of the complex belong to the space group P2₁/n with a = 7.624(2) Å, b = 18.274(5) Å, c = 14.411(14) Å, β = 98.03(3)°, and Z = 4. In ¹H NMR studies of these compounds in the presence of aqueous buffer, all four complexes rapidly converted to the dihydroxy species [Au(polypyridyl)(OH)₂] in a stepwise fashion. However, the [Au(polypyridyl)]³⁺ fragment believed to impart cytotoxicity in human ovarian cancer cell lines (A2780) remained intact and appeared stable for days. It was also noted that these Au(III) complexes were readily reduced in the presence of the common biological reducing agents, reduced glutathione and sodium ascorbate. How solution and redox stability may affect the biological activity of these novel Au(III) complexes is discussed.     

First example of photomagnetic effects in ionic pairs [Ni(bipy)3]2[Mo(CN)8] · 12H2O

Ionic triads formed by [Ni^II(bipy)₃]²⁺ (bipy = 2,2′-bipyridyl) and diamagnetic [M^IV(CN)₈]^4? (M = Mo and W) were prepared and structurally characterized. The two compounds are isostructural and their structure consists of a three-dimensional hydrogen-bonded framework where cation–anion interactions occur through short contacts M–CN?H–C(bipy). Before irradiation, the Mo analogue behaves as paramagnet with small intermolecular interactions between the [Ni^II(bipy)₃]²⁺ cations. Upon irradiation with visible light, it exhibits a reversible photomagnetic effect, which is interpreted in terms of the formation of paramagnetic [Mo^V(CN)₈]^3? and [Ni^II(bipy)₂(bipy^?)]⁺ due to the outer-sphere electron transfer.     

Arene-Ru(II)-chloroquine complexes interact with DNA, induce apoptosis on human lymphoid cell lines and display low toxicity to normal mammalian cells

The complexes [Ru(η⁶-p-cymene)(CQ)Cl₂] (1), [Ru(η⁶-benzene)(CQ)Cl₂] (2), [Ru(η⁶-p-cymene)(CQ)(H₂O)₂][BF₄]₂ (3), [Ru(η⁶-p-cymene)(en)(CQ)][PF₆]₂ (4), [Ru(η⁶-p-cymene)(η⁶-CQDP)][BF₄]₂ (5) (CQ = chloroquine base; CQDP = chloroquine diphosphate; en = ethylenediamine) interact with DNA to a comparable extent to that of CQ and in analogous intercalative manner with no evidence for any direct contribution of the metal, as shown by spectrophotometric and fluorimetric titrations, thermal denaturation measurements, circular dichroism spectroscopy and electrophoresis mobility shift assays. Complexes 1-5 induced cytotoxicity in Jurkat and SUP-T1 cancer cells primarily via apoptosis. Despite the similarities in the DNA binding behavior of complexes 1-5 with those of CQ the antitumor properties of the metal drugs do not correlate with those of CQ, indicating that DNA is not the principal target in the mechanism of cytotoxicity of these compounds. Importantly, the Ru-CQ complexes are generally less toxic toward normal mouse splenocytes and human foreskin fibroblast cells than the standard antimalarial drug CQDP and therefore this type of compound shows promise for drug development.     

Solution chemistry and cytotoxic properties of novel organogold(III) compounds

The solution behaviour of some novel organogold(III) compounds was investigated, and their cytotoxic properties evaluated against a few human tumour cell lines (A2780/S, A2780/R, MCF7, HT29 and A549). Specifically, the following compounds were considered: [Au(bipy(dmb)-H)(2,6-xylidine-H)][PF(6)] (AuXyl) and [Au(bipy(dmb)-H)(p-toluidine-H)][PF(6)] (AuTol) (in which bipy(dmb)=6-(1,1-dimethylbenzyl)-2,2'-bipyridine), [Au(py(dmb)-H)(AcO)(2)] (AuPyAcO) (in which py(dmb)=2-(1,1-dimethylbenzyl)-pyridine) and [Au(pz(Ph)-H)Cl(3)]K (AuPzCl) (in which pz(Ph)=1-phenylpyrazole). The solution chemistry of these compounds, under physiological-like conditions, was investigated through UV-vis absorption and (1)H NMR spectroscopies. Significant cytotoxic effects in vitro were observed in selected cases.     

Structural and solution chemistry, protein binding and antiproliferative profiles of gold(I)/(III) complexes bearing the saccharinato ligand

A series of new gold(I) and gold(III) complexes based on the saccharinate (sac) ligand, namely M[Au(sac)₂] (with M being Na⁺, K⁺ or NH₄⁺), [(PTA)Au(sac)], K[Au(sac)₃Cl] and Na[Au(sac)₄], were synthesized and characterized, and some aspects of their biological profile investigated. Spectrophotometric analysis revealed that these gold compounds, upon dissolution in aqueous media, at physiological pH, manifest a rather favourable balance between stability and reactivity. Their reactions with the model proteins cytochrome c and lysozyme were monitored by mass spectrometry to predict their likely interactions with protein targets. In the case of disaccharinato gold(I) complexes, cytochrome c adducts bearing four coordinated gold(I) ions were preferentially formed in high yield. In contrast, [(PTA)Au(sac)] (PTA = 1,3,5-triaza-7-phosphaadamantane) turned out to be poorly effective, only producing a mono-metalated adduct in very low amount. In turn, the gold(III) saccharinate derivatives were less reactive than their gold(I) analogues: K[Au(sac)₃Cl] and Na[Au(sac)₄] caused moderate protein metalation, again with evidence of formation of tetragold adducts. Finally, the above mentioned gold compounds were challenged against the reference human tumor cell line A2780S and its cisplatin resistant subline A2780R and their respective cytotoxic profiles determined. [(PTA)Au(sac)] turned out to be highly cytotoxic whereas moderate cytotoxicities were observed for the gold(III) complexes and only modest activities for disaccharinato gold(I) complexes. The implications of these results are thoroughly discussed in the light of current knowledge on gold based drugs.     

Mono-, bi- and trinuclear bis(diphenylarsino)methane gold complexes. Crystal and molecular structure of [{(C6F5)3Au(Ph2AsCH2AsPh2)}2Ag(OClO3)]dd]Dedicated to Professor Rafael Usón on his 60th birthday.

Addition of bis(diphenylarsino)methane (dpam) to neutral or cationic gold(l) or gold(III) complexes containing weakly coordinating ligands leads to the formation of mononuclear {R₃Au(dpam), R₂ClAu(dpam), [R₂Au(dpam)]ClO₄ (RC₆F₅)} or binuclear complexes {RAu(dpam)AuR, [Au₂(dpam)₂](ClO₄)₂}. Mixed gold(III)gold(I) compounds can be synthesized either by oxidation of the gold(I) complexes or from mononuclear gold(III) derivatives. Reaction of R₃Au(dpam)with ClAu(tht), [Au(tht)₂]ClO₄ or AgClO₄ leads to the trinuclear complexes [{R₃Au(dpam)}₂Au]X (X=[AuCl₂] or ClO₄) or [{R₃Au(dpam)}₂Ag(OClO₃)], respectively. The structure of the silver complex has been determined by X-ray crystallography.     

A mass spectrometric investigation of the binding of gold antiarthritic agents and the metabolite [Au(CN)2]− to human serum albumin

Electrospray ionisation (ESI) mass spectrometry was used to examine the reactions of the clinically used antiarthritic agent [Au(S₂O₃)₂]³⁻, and AuPEt₃Cl, a derivative of another clinically used agent auranofin, with human serum albumin (HSA) obtained from a human volunteer. Both compounds reacted readily with HSA to form complexes containing one or more covalently attached gold fragments. In the case of AuPEt₃Cl, binding was accompanied by the loss of the chloride ligand, while for [Au(S₂O₃)₂]³⁻ the mass spectral data indicated binding of Au(S₂O₃) groups. Experiments performed using HSA with Cys34 blocked by reaction with iodoacetamide were consistent with reaction of both gold compounds with this amino acid. Separate blocking experiments using diethylpyrocarbonate and AuPEt₃Cl also provided evidence for histidine residues acting as lower-affinity binding sites for this gold compound. ESI mass spectra of solutions containing [Au(S₂O₃)₂]³⁻ or [Au(CN)₂]⁻, and HSA, provided evidence for the formation of protein complexes in which intact gold molecules were non-covalently bound. In the case of [Au(S₂O₃)₂]³⁻, these non-covalent complexes proved to be transitory in nature. However, for [Au(CN)₂]⁻ a non-covalent complex containing a single gold molecule bound to HSA was found to be stable, and constituted the main adduct formed in solutions containing low-to-medium Au-to-HSA ratios. Evidence was also obtained for the formation of a covalent adduct in which a single Au(CN) moiety was bonded to Cys34 of the protein. AuPEt₃Cl reacted to a much lower extent with HSA that had Cys34 modified by formation of a disulfide bond to added cysteine, than with unmodified HSA. This suggests that the extent of modification of the protein in vivo may have an important influence on the transport and bioavailability of gold antiarthritic drugs.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Binuclear cyclopalladated pincer compounds bridged by ditopic nitrogenated ligands

The binuclear cyclopalladated compounds [(SCS)Pd-pz-Pd(SCS)][BF₄]₂ (pz = pyrazine) 2a, [(SCS)Pd-bipy-Pd(SCS)][BF₄]₂ (bipy = 4,4′-bipyridine) 2b, [(SCS)Pd-dcb-Pd(SCS)][BF₄]₂ (dcb = 1,4-dicyanobenzene) 2c and [(SCS)Pd-tmeda-Pd(SCS)][BF₄]₂ (tmeda = N,N,N′,N′-tetramethylethylenediamine) 2d (SCS = {C₆H₃-2,6-(CH₂SC₆H₄F-4)₂}) were synthesized by a substitution reaction between the pincer unit [Pd(C₆H₃-2,6-(CH₂SC₆H₄F-4)₂)Cl] 1 and the corresponding bidentate nitrogenated ligands. The topology of the bridging ligand between both pincer units induces the aggregation of the organometallic cations in the solid state. The X-ray diffraction molecular structures of complexes 2a and 2d are also reported.     

Metal-metal stacking patterns between and with [Pt(tpy)X] cations

A comparative study of metallophilic interactions of [Pt(tpy)X]⁺ cations (tpy = 2,2′:6′,2″-terpyridine) in the presence of two different types of anions, (i) []⁻ anions that form double salts and (ii) simple p-block anions, is reported. Single-crystal X-ray diffraction data, solution-state ¹⁹⁵Pt NMR spectra, and variable temperature solid-state luminescence spectra are reported. Three [Pt(tpy)Cl]Y derivatives (Y = SbF₆, 1, SbF₆·CH₃CN, 4, PF₆, 2) and the [Pt(tpy)Br]PF₆ analog, 3, as well as two new double salts [Pt(tpy)CN][Au(CN)₂], 5, and [Pt(tpy)CN]₂[Au(C₆F₅)₂](PF₆), 6, have been synthesized and characterized. Structural analysis shows consistent patterns in Pt···Pt interactions that vary slightly depending on the coordinating halogen or pseudo-halogen X, counter anion Y, and lattice solvent. Metallophilic interactions are seen between [Pt(tpy)X]⁺ cations with all types of X ligands, but only with π-accepting X′ ligands from []⁻ anions are Pt?Au metallophilic interactions seen to be favored over Pt?Pt interactions. The [Au(CN)₂]⁻ anion consistently forms Pt···Au metallophilic contacts, unlike [Au(C₆F₅)₂]⁻. The ¹⁹⁵Pt NMR chemical shifts are ∼−2750 ppm for π-donor ligands and near −3120 ppm for π-acceptor ligands in [Pt(tpy)X]PF₆ compounds. Luminescence data show an unusual blue shift in [Pt(tpy)CCPh][Au(C₆F₅)₂] versus [Pt(tpy)CCPh]PF₆ ascribed to an intermolecular charge transfer.     

Hydrogen bonding: further evidence for the cause of the color change of nitrosylpentaamminechromium (III) compounds. Crystal structures of [Cr(NO)(NH3)5](PF6)2 and [Cr(NO)(NH3)5]Cl(PF6)

The crystal structures of [Cr(NO)(NH₃)₅](PF₆)₂ (red) and [Cr(NO)(NH₃)₅]Cl(PF₆) (brown) have been determined. The [Cr(NO)(NH₃)₅]²⁺(A) complex cations in these compounds have a slightly distorted octahedral geometry with a strictly linear Cr-N-O arrangement (from symmetry). The short interatomic distances (2.399 Å × 4) between the O (nitrosyl) and H (ammonia in adjacent complex cations) atoms in A(PF₆)₂ indicate the existence of hydrogen bonds, while the interatomic distances (3.258 Å × 8) between those in ACl(PF₆) are much longer, and the hydrogen bonds should be weak in spite of the presence of the smaller counter anion of chloride ion in ACl(PF₆). Comparisons of the five crystal structures of A(PF₆)₂, ACl₂, ACl(ClO₄), ACl(PF₆), and A(ClO₄)₂ have led to the conclusion that the existence of the strong hydrogen bonds gives red crystals of A(PF₆)₂, while the absence of hydrogen bonds results in the formation of green crystals of A(ClO₄)₂ (O ? H, 3.595 Å × 2). The color change of the crystals (from red to green) with the change of outer sphere anions is attributed to the change of the strength of the hydrogen bonding between the complex cations.     

Iminophosphorane-organogold(III) complexes induce cell death through mitochondrial ROS production

Gold compounds are being investigated as potential antitumor drugs. Some gold(III) derivatives have been shown to induce cell death in solid tumors but their mechanism of action differs from that of cisplatin, since most of these compounds do not bind to DNA. We have explored cellular events triggered by three different iminophosphorane-organogold(III) compounds in leukemia cells (a neutral compound with two chloride ligands [Au{κ²-C,N-C₆H₄(PPh₂ = N(C₆H₅)-2}Cl₂] 1, and two cationic compounds with either a dithiocarbamate ligand [Au{κ²-C,N-C₆H₄(PPh₂ = N(C₆H₅)-2}(S₂CN-Me₂)]PF₆2, or a water-soluble phosphine and a chloride ligand [Au{κ²-C,N-C₆H₄(PPh₂ = N(C₆H₅)-2}(P{Cp(m-C₆H₄-SO₃Na)₂}₃) Cl]PF₆3). All three compounds showed higher toxicity against leukemia cells when compared to normal T-lymphocytes. Compounds 1 and 2 induced both necrosis and apoptosis, while 3 was mainly apoptotic. Necrotic cell death induced by 1 and 2 was Bax/Bak- and caspase-independent, while apoptosis induced by 3 was Bax/Bak-dependent. Reactive oxygen species (ROS) production at the mitochondrial level was a critical step in the antitumor effect of these compounds.     

In vitro antitumour activity of water soluble Cu(I), Ag(I) and Au(I) complexes supported by hydrophilic alkyl phosphine ligands

Hydrophilic, monocationic [M(L)₄]PF₆ complexes (M = Cu or Ag; L: thp = tris(hydroxymethyl)phosphine, L: PTA = 1,3,5-triaza-7-phosphaadamantane, L: thpp = tris(hydroxypropyl)phosphine) were synthesized by ligand exchange reaction starting from [Cu(CH₃CN)₄]PF₆ or AgPF₆ precursors at room temperature in the presence of an excess of the relevant phosphine. The related [Au(L)₄]PF₆ complexes (L = thp, PTA or thpp) were synthesized by metathesis reactions starting from [Au(L)₄]Cl at room temperature in the presence of equimolar quantity of TlPF₆. The three series of complexes [M(L)₄]PF₆ were tested as cytotoxic agents against a panel of several human tumour cell lines also including a defined cisplatin resistant cell line. These investigations have been carried out in comparison with the clinically used metallodrug cisplatin and preliminary structure-activity relationships are presented. The best results in terms of in vitro antitumour activity were achieved with metal-thp species and, among the coinage metal complexes, copper derivatives were found to be the most efficient drugs. Preliminary studies concerning the mechanism of action of these [M(L)₄]PF₆ species pointed to thioredoxin reductase as one of the putative cellular targets of gold and silver complexes and provided evidence that copper derivatives mediated their cytotoxic effect through proteasome inhibition.     

Synthesis and structural characterization of gold(III) dioximates with anions [AuCl4] and [AuCl2]

Two complexes of Au(III) with dimethylglyoxime of compositions [Au^III(HDMG)₂][Au^IIICl₄] (1) and [Au^III(HDMG)₂][Au^ICl₂] (2) were synthesized and characterized by X-ray structural analysis. It was shown that in [Au^III(HDMG)₂]⁺ cation Au(III) has a square-planar environment, and the oxygen atoms of oxime groups are joined by intramolecular H-bond. The secondary Au?Au and Au?Cl interactions in the crystal are discussed.     

Isokinetic Relationship in the Oxidation of Cuprous Stellacyanin by Cobalt(III) Complexes

Rate parameters have been obtained for the oxidation of cuprous stellacyanin by cobalt(III) ions of the form cis(N)-[CoN₂O₄]^?, including cis(N)-[Co(NTA)(gly)]^?, cis(N)-[Co(IDA)₂]^?, [Co(en)(ox)₂]^?(μ 0.5 M(phosphate), pH 7.0), and Co(EDTA)^?(μ 0.1 M(NaCl), pH 7.2, 0.001 M phosphate). An excellent isokinetic correlation between the activation parameters ΔH^≠ and ΔS^≠ exists for the reactions of aminopolycarboxylatocobalt(III) ions with reduced stellacyanin (β = 300 ± 12 K; correlation coefficient = 0.995). It is concluded that enthalpy-entropy compensation in these reactions may be understood in terms of differing orientations preferred by the various oxidants in forming precursor complexes with the reduced blue protein. While ΔH^≠ and ΔS^≠ values for electron transfer from stellacyanin to cis(N)-[CoN₂O₄]^? ions vary over ranges of 10.7 kcal/mol and 34 cal/mol-deg, respectively, room temperature rate constants are relatively constant (3.6–34.5 M^?1 sec^?1), as expected from Marcus theory for outer sphere electron transfer.     

Metal–drug synergy: new ruthenium(II) complexes of ketoconazole are highly active against Leishmania major and Trypanosoma cruzi and nontoxic to human or murine normal cells

In our ongoing search for new metal-based chemotherapeutic agents against leishmaniasis and Chagas disease, six new ruthenium–ketoconazole (KTZ) complexes have been synthesized and characterized, including two octahedral coordination complexes—cis,fac-[Ru^IICl₂(DMSO)₃(KTZ)] (1) and cis-[Ru^IICl₂(bipy)(DMSO)(KTZ)] (2) (where DMSO is dimethyl sulfoxide and bipy is 2,2′-bipyridine)—and four organometallic compounds—[Ru^II(η⁶-p-cymene)Cl₂(KTZ)] (3), [Ru^II(η⁶-p-cymene)(en)(KTZ)][BF₄]₂ (4), [Ru^II(η⁶-p-cymene)(bipy)(KTZ)][BF₄]₂ (5), and [Ru^II(η⁶-p-cymene)(acac)(KTZ)][BF₄] (6) (where en is ethylenediamine and acac is acetylacetonate); the crystal structure of 3 is described. The central hypothesis of our work is that combining a bioactive compound such as KTZ and a metal in a single molecule results in a synergy that can translate into improved activity and/or selectivity against parasites. In agreement with this hypothesis, complexation of KTZ with Ru^II in compounds 3–5 produces a marked enhancement of the activity toward promastigotes and intracellular amastigotes of Leishmania major, when compared with uncomplexed KTZ, or with similar ruthenium compounds not containing KTZ. Importantly, the selective toxicity of compounds 3–5 toward the leishmania parasites, in relation to human fibroblasts and osteoblasts or murine macrophages, is also superior to the selective toxicities of the individual constituents of the drug. When tested against Trypanosoma cruzi epimastigotes, some of the organometallic complexes displayed activity and selectivity comparable to those of free KTZ. A dual-target mechanism is suggested to account for the antiparasitic properties of these complexes.     

Short hydrogen bond: Spectral studies and X-ray structure of bis(alpha-picoline N-oxide)-hydronium tetrachloroaurate(III)

Spectral and the X-ray diffraction studies of the title compound [H(2-picNO)₂] [AuCl₄] (2-picNO = alpha-picoline N-oxide) were done to investigate the short hydrogen bond in the dimeric cation [H(2-picNO)₂]⁺. The compound exhibits several narrow transmission bands overlaid on a broad continuous absorption characteristic of a short hydrogen bond; it crystallizes with four molecules in a monoclinic unit cell of space group P2₁/n with lattice parameters a = 22.376(9), b = 9.874(6), c = 7.957(5) Å and β = 94.98(6)°. The structure was solved using 2623 data points collected on a CAD4 to the final value for R(F_o) = 0.039. The structure consists of discrete [AuCl₄]^? and [H(2-picNO)₂]⁺ ions. Contrary to all known examples, the dimeric cation in this compound has a short hydrogen bond having no symmetry constraints making the present structural analysis unique in this series of compounds. The O—O distance is 2.393(6) Å and the dihedral angle between the two planar 2-picNO moieties in the cation is 44.1°. The [AuCl₄]^? anion is planar with an average AuCl distance of 2.268(2) Å. The infrared spectrum and the structure of the compound is compared with the related structures.