X-ray-Induced Photoreduction of Hg(II) in Aqueous Frozen Solution Yields Nearly Monatomic Hg(0).

We use X-ray-induced photochemistry, which is well known to cause changes in a number of systems, to reduce Hg(II) to Hg(0) in frozen aqueous solution with added glycerol maintained at 10 K. X-ray absorption spectroscopy was used to monitor the extent of the reaction and to characterize the species. An analysis of the extended X-ray absorption fine structure (EXAFS) of the photochemical product indicated a nearly monatomic Hg(0) species bound only by long, weak bonds to oxygens at ∼3.5 Å. The results of the EXAFS analysis agree quantitatively with the results of density functional theory calculations using the meta-GGA approximation with the M11-L functional. This is the first structural characterization of nearly monatomic Hg(0) bound by hard ligands similar to those expected in aqueous environmental systems. We conclude that Hg(0) is expected to exist in solution as a nearly monatomic entity.

Binding of Copper and Cisplatin to Atox1 Is Mediated by Glutathione through the Formation of Metal-Sulfur Clusters.

Copper is an essential nutrient required for many biological processes involved in primary metabolism, but free copper is toxic due to its ability to catalyze formation of free radicals. To prevent toxic effects, in the cell copper is bound to proteins and low molecular weight compounds, such as glutathione, at all times. The widely used chemotherapy agent cisplatin is known to bind to copper-transporting proteins, including copper chaperone Atox1. Cisplatin interactions with Atox1 and other copper transporters are linked to cancer resistance to platinum-based chemotherapy. Here we analyze the binding of copper and cisplatin to Atox1 in the presence of glutathione under redox conditions that mimic intracellular environment. We show that copper(I) and glutathione form large polymers with a molecular mass of approximately 8 kDa, which can transfer copper to Atox1. Cisplatin also can form polymers with glutathione, albeit at a slower rate. Analysis of simultaneous binding of copper and cisplatin to Atox1 under physiological conditions shows that both metals are bound to the protein through copper-sulfur-platinum bridges.

Combined EXAFS and DFT structure calculations provide structural insights into the 1:1 multi-histidine complexes of Cu(II) , Cu(I) , and Zn(II) with the tandem octarepeats of the mammalian prion protein.

The metal-coordinating properties of the prion protein (PrP) have been the subject of intense focus and debate since the first reports of its interaction with copper just before the turn of the century. The picture of metal coordination to PrP has been improved and refined over the past decade, but structural details of the various metal coordination modes have not been fully elucidated in some cases. In the present study, we have employed X-ray absorption near-edge spectroscopy as well as extended X-ray absorption fine structure (EXAFS) spectroscopy to structurally characterize the dominant 1:1 coordination modes for Cu(II) , Cu(I) , and Zn(II) with an N-terminal fragment of PrP. The PrP fragment corresponds to four tandem repeats representative of the mammalian octarepeat domain, designated as OR4 , which is also the most studied PrP fragment for metal interactions, making our findings applicable to a large body of previous work. Density functional theory (DFT) calculations have provided additional structural and thermodynamic data, and candidate structures have been used to inform EXAFS data analysis. The optimized geometries from DFT calculations have been used to identify potential coordination complexes for multi-histidine coordination of Cu(II) , Cu(I) , and Zn(II) in an aqueous medium, modelled using 4-methylimidazole to represent the histidine side chain. Through a combination of in silico coordination chemistry as well as rigorous EXAFS curve-fitting, using full multiple scattering on candidate structures derived from DFT calculations, we have characterized the predominant coordination modes for the 1:1 complexes of Cu(II) , Cu(I) , and Zn(II) with the OR4 peptide at pH 7.4 at atomic resolution, which are best represented as square-planar [Cu(II) (His)4 ](2+) , digonal [Cu(I) (His)2 ](+) , and tetrahedral [Zn(II) (His)3 (OH2 )](2+) , respectively.

Synthesis and structural characterization of copper-cuprizone complexes.

Copper(II) coordination by bis(cyclohexanone)oxalyldihydrazone (also known as cuprizone), resulting in the formation of an intensely coloured blue complex, was first reported over 70 years ago. The cuprizone reaction has been employed in colourimetric tests for the presence of trace levels of copper. Cuprizone administration in C57BL/6 mice also leads to demyelination over time - a consequence that appears to be due to copper dyshomeostasis - and this has led to use of cuprizone as the leading method for toxicant-induced generation of an animal model of demyelination since its first use in the 1960s. Despite broad interest in cuprizone and its ability to bind copper there have been relatively few studies to structurally characterize the copper coordination properties of this ligand. In the absence of an aqueous medium, such as neat alcohol, copper and cuprizone exclusively form an amorphous green precipitate. Under aqueous conditions, where a large excess of cuprizone (relative to copper) is present, the blue complex that is synonymous with copper-cuprizone coordination is predominantly formed. The blue and green copper-cuprizone products demonstrate poor solubility and present challenges for conventional structure characterization methods, such as X-ray crystallography or nuclear magnetic resonance spectroscopy. By combining mass spectrometry, X-ray absorption spectroscopy, computational chemistry, and other techniques, a self-consistent picture of the copper coordination structures of the blue and green complexes is revealed - confirming that the blue complex is in the Cu(III) state, containing two hydrolyzed cuprizone ligands per metal centre, while the green complex represents an extended oligomeric complex, comprised of repeating Cu(II) centres that lie 4.8 Å apart and are bridged by unhydrolyzed cuprizone donors.

Cryoprotectants Severely Exacerbate X-ray-Induced Photoreduction.

Approximately 11% of enzymes contain a transition metal ion that is essential for catalytic function. Such metalloenzymes catalyze much of the most chemically challenging and biologically essential chemistry carried out by life. X-ray-based methods, predominantly macromolecular crystallography (MX) and also X-ray absorption spectroscopy (XAS), have proved essential for determining structures of transition metal ion-containing active sites in order to deduce enzyme catalytic mechanisms. However, X-ray irradiation can induce change in both the oxidation state and structure of the metal, which is problematic in structure determination. We present an XAS study of whether cryoprotectants such as polyethylene glycol (PEG) or glycerol, routinely added to MX or XAS samples to improve data quality, affect photoreduction. Our data demonstrate a remarkable 10-fold exacerbation in rate of photoreduction of Cu(II) to Cu(I) when alcohol or ether cryoprotectants are present. Our results suggest that widespread use of cryoprotectants may increase the potential for erroneous structures.

Cytotoxic 5-aryl-1-(4-nitrophenyl)-3-oxo-1,4-pentadienes mounted on alicyclic scaffolds.

The 5-aryl-1-(4-nitrophenyl)-3-oxo-1,4-pentadienyl pharmacophore was incorporated into four series of compounds 1-4. Compounds 1a-g comprised a cluster of 3-arylidene-1-(4-nitrophenylmethylene)-2-oxo-3,4-dihydro-1H-naphthalenes while the analogues 2a-g consisted of a group of 6-arylidene-2-(4-nitrophenylmethylene)cyclohexanones. Three other compounds prepared in this study were 1-(4-nitrophenylmethylene)-3-(3,4,5-trimethoxyphenylmethylene)-2-oxo-2,3-dihydro-1H-indene 3a as well as two 5-arylidene-2-(4-nitrophenylmethylene)cyclopentanones 4a,b. The compounds were evaluated against human Molt 4/C8 and CEM T-lymphocytes as well as murine L1210 cells. In general, the compounds in series 1 displayed marked cytotoxicity having IC50 values in the 1-5 microM range while the related cyclohexyl analogues in series 2 were slightly less potent (IC50 figures were mainly 5-10 microM). The relative locations of two aryl rings present in all four series were considered to contribute significantly to bioactivity and may have accounted for the virtual absence of cytotoxic properties in series 3 and 4. Most of the compounds were administered intraperitoneally to mice using doses up to and including 300 mg/kg. No mortalities were noted. The inhibiting effect of most of the compounds towards Helicobacter pylori is noteworthy. The modes of action of representative compounds include the induction of apoptosis while some compounds weakly inhibited tubulin polymerisation and human N-myristoyltransferase.

Cytotoxic and anticonvulsant aryloxyaryl Mannich bases and related compounds.

A series of 1-(4-aryloxyphenyl)-3-diethylamino-1-propanone hydrochlorides 3a-3e and related compounds 3f, 3g and 4a-4d were synthesised. In addition, a group of 4-(4-aryloxyphenyl)-3-(4-aryloxyphenylcarbonyl)-1-ethyl-4-piperidinol hydrochlorides 6a-6e were prepared which incorporated most of the structural features of 3a-3e. All of these compounds displayed cytotoxic properties towards murine L1210 cells as well as human Molt 4/C8 and CEM T-lymphocytes. A number of these compounds possessed noteworthy potencies towards seven human colon cancer cell lines. Some correlations were noted between the IC(50) values generated in the different screens and the sigma, pi and molar refractivity constants of the aryl substituents as well as with the volumes and solvent accessible surface areas of various basic groups. Molecular modelling of representative compounds revealed structural features, which may have contributed to the varying potencies noted. In general, the compounds in series 6 were well tolerated when administered to mice. Anticonvulsant properties were demonstrated by a number of compounds in the maximal electroshock (MES) screen when administered intraperitoneally to mice while 4c and 6e afforded protection in the MES test when given orally to rats.

Cytotoxic analogues of 2,6-bis(arylidene)cyclohexanones.

A series of 2,6-bis(arylidene)cycloalkanones (1) and related compounds containing one or two substituents at the four position of the cyclohexyl ring were prepared and shown to display cytotoxic activity towards murine P388 and L1210 cells as well as human Molt 4/C8 and CEM T-lymphocytes. In some of the series of compounds, positive correlations were noted between the potencies of the enones and the magnitude of the Hammett sigma values of the aryl substituents. Four representative compounds were cytotoxic to a number of human tumours in vitro, particularly towards colon cancer and leukemic cells. A noteworthy feature of the compounds prepared in this study is that, in general, they were well tolerated when administered to rodents. A number of lead molecules emerged from this investigation as well as guidelines for future expansion of these series of compounds.

Cytotoxic N-[4-(3-aryl-3-oxo-1-propenyl)phenylcarbonyl]-3,5-bis(phenylmethylene)-4-piperidones and related compounds.

A series of 4-carboxychalcones 1 were prepared and coupled to 3,5-bis(phenylmethylene)-4-piperidone (2) giving rise to a novel series of N-[4-(3-aryl-3-oxo-1-propenyl)phenylcarbonyl]-3,5-bis(phenylmethylene)-4-piperidones (3). Molecular simplification of the amides 3 led to the formation of the corresponding N-(3-aryl-1-oxo-2-propenyl)-3,5-bis(phenylmethylene)-4-piperidones (4). A cytotoxic evaluation of the compounds in series 1-4 utilized murine P388 and L1210 cells as well as human Molt 4/C8 and CEM T-lymphocytes. In general, the compounds displayed significant toxicity; the IC(50) values of 54% of the enones were less than 10 microM when all four screens were considered and less than 1 microM for all members of series 3 in the P388 assay. Various correlations were established between the potencies of the compounds in series 1, 3 and 4 and the Hammett sigma, Hansch pi and molecular refractivity constants of the aryl substituents. Several torsion angles and interatomic distances of five representative compounds in series 3 and 4 were determined by X-ray crystallography, some of which contributed to the observed bioactivity. The marked cytotoxicity and lack of murine toxicity of most of the compounds described in this study, as well as their selective toxicity towards different tumour cell lines, revealed that development of the enones 2-4 as novel candidate antineoplastic agents should be pursued.

The solution structure of the copper clioquinol complex.

Clioquinol (5-chloro-7-iodo-8-hydroxyquinoline) recently has shown promising results in the treatment of Alzheimer's disease and in cancer therapy, both of which also are thought to be due to clioquinol's ability as a lipophilic copper chelator. Previously, clioquinol was used as an anti-fungal and anti-protozoal drug that was responsible for an epidemic of subacute myelo-optic neuropathy (SMON) in Japan during the 1960s, probably a myeloneuropathy arising from a clioquinol-induced copper deficiency. Previous X-ray absorption spectroscopy of solutions of copper chelates of clioquinol suggested unusual coordination chemistry. Here we use a combination of electron paramagnetic, UV-visible and X-ray absorption spectroscopies to provide clarification of the chelation chemistry between clioquinol and copper. We find that the solution structures for the copper complexes formed with stoichiometric and excess clioquinol are conventional 8-hydroxyquinolate chelates. Thus, the promise of clioquinol in new treatments for Alzheimer's disease and in cancer therapy is not likely to be due to any novel chelation chemistry, but rather due to other factors including the high lipophilicity of the free ligand and chelate complexes.

E,E,E-1-(4-Arylamino-4-oxo-2-butenoyl)-3,5-bis(arylidene)-4-piperidones: a topographical study of some novel potent cytotoxins.

A series of E,E,E-3,5-bis(arylidene)-1-(4-arylamino-4-oxo-2-butenoyl)-4-piperidones 4 (phenylidene) and 5 (4-nitrophenylidene) were prepared in order to explore the structural features of the N-acyl group which affects the cytotoxic potency. Evaluation toward human Molt 4/C8 and CEM T-lymphocytes revealed that many of the IC(50) figures were submicromolar and lower than melphalan. Marked inhibitory potencies toward murine leukemia L1210 cells were also noted. When evaluated against a panel of human tumor cell lines, three representative compounds in series 4 displayed selective toxicity to leukemia and colon cancer cell lines and were significantly more potent than the reference drug melphalan. Molecular modeling of representative compounds in both series 4 and the analogs, in which the configuration of the olefinic double bond was changed from E to Z (series 3), revealed that the torsion angles of the arylidene aryl rings and locations of the terminal arylaminocarbonyl groups may have contributed to the greater cytotoxic properties displayed in 3. Compounds 4c (3,4-dichlorophenylamino), d (4-methylphenylamino) and 5c (3,4-dichlorophenylamino), d (4-methylphenylamino) inhibited the activity of human N-myristoyltransferase by approximately 50% at concentrations of 50-100 microM. The compounds in series 4 and 5 were well tolerated in a short-term toxicity study in mice.