The use of conductivity measurements in organic solvents for the characterisation of coordination compounds

With their bright, photostable fluorescence, semiconductor quantum dots (QDs) show promise as alternatives to organic dyes for biological labeling. Questions about their potential cytotoxicity, however, remain unanswered. While cytotoxicity of bulk cadmium selenide (CdSe) is well documented, a number of groups have suggested that CdSe QDs are cytocompatible, at least with some immortalized cell lines. Using primary hepatocytes as a liver model, we found that CdSe-core QDs were indeed acutely toxic under certain conditions. Specifically, we found that the cytotoxicity of QDs was modulated by processing parameters during synthesis, exposure to ultraviolet light, and surface coatings. Our data further suggest that cytotoxicity correlates with the liberation of free Cd2+ ions due to deterioration of the CdSe lattice. When appropriately coated, CdSe-core QDs can be rendered nontoxic and used to track cell migration and reorganization in vitro. Our results provide information for design criteria for the use of ...

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Probing the Cytotoxicity Of Semiconductor Quantum Dots.

Nitric oxide (NO.), a potentially toxic molecule, has been implicated in a wide range of biological functions. Details of its biochemistry, however, remain poorly understood. The broader chemistry of nitrogen monoxide (NO) involves a redox array of species with distinctive properties and reactivities: NO+ (nitrosonium), NO., and NO- (nitroxyl anion). The integration of this chemistry with current perspectives of NO biology illuminates many aspects of NO biochemistry, including the enzymatic mechanism of synthesis, the mode of transport and targeting in biological systems, the means by which its toxicity is mitigated, and the function-regulating interaction with target proteins.

https://science.sciencemag.org/content/sci/258/5090/1898.full.pdf

Biochemistry of nitric oxide and its redox-activated forms

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

The first mononuclear molybdenum(V) complex with a terminal sulfido ligand: dichloro[hydrotris(3,5-dimethyl-1-pyrazolyl)borato]sulfidomolybdenum

Sulfite oxidizing enzymes (SOEs) are physiologically vital and occur in all forms of life. During the catalytic cycle the five-coordinate square-pyramidal oxo-molybdenum active site passes through the Mo(V) state, and intimate details of the structure can be obtained from pulsed EPR spectroscopy through the hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of nearby magnetic nuclei (e.g., 1H, 2H, 17O, 31P) of the ligands. By employing spectrometer operational frequencies ranging from ∼4 to ∼32 GHz, it is possible to make the nuclear Zeeman interaction significantly greater than the hfi and nqi, and thereby simplify the interpretations of the spectra. The SOEs exhibit three general types of Mo(V) structures which differ in the number of nearby exchangeable protons (one, two or zero). The observed structure depends upon the organism, pH, anions in the medium, and method of reduction. One type of structure has a single exchangeable Mo–OH proton approximately in the equatorial plane and a large isotropic hfi (e.g., low pH form of chicken SOE, low pH form of plant SOE reduced by Ti(III)); the second type has two exchangeable protons with distributed orientations out of the equatorial plane and very small (or zero) isotropic hfi (e.g., high pH form of chicken SOE, high pH form of plant SOE reduced by sulfite); the third type has no nearby exchangeable protons and a coordinated oxyanion (e.g., phosphate inhibited chicken SOE, low pH form of plant SOE reduced by sulfite). An additional structural conclusion is that the orientation angle of any exchangeable equatorial ligand (OH, OH2, PO43−) is not uniquely fixed, but is distributed around its central value by up to ±20° (depending on pH, the type of the ligand and the type of enzyme). An unexpected finding was that the axial oxo group of SOEs exchanges with 17O in solutions enriched in H217O. The first determination of oxo 17O nqi parameters for a well-characterized model compound, [Mo17O(SPh)4]−, clearly demonstrated that 17O nqi parameters can distinguish between oxo and OH2 ligands.

Investigation of the coordination structures of the molybdenum(v) sites of sulfite oxidizing enzymes by pulsed EPR spectroscopy.

The active sites of molybdenum- and tungsten-containing enzymes

The titration of chicken liver sulfite oxidase (SO) with the one-electron reductant Ti(III) citrate, at pH 7.0, results in nearly quantitative selective reduction of the Mo(VI) center to Mo(V), while the b-type heme center remains in the fully oxidized Fe(III) state. The selective reduction of the Mo(VI/V) couple has been established from electronic and EPR spectra. The elec- tronic spectrum of the Fe(III) heme center is essentially unchanged during the titration, and the continuous wave (CW)-EPR spectrum shows the appearance of the well-known Mo(V) signal due to the low pH (lpH) form of SO. Further confirmation of the selective formation of the Mo(V)/Fe(III) form of SO is provided by the � 1:1 ratio of the integrated intensities of the Mo(V) and low- spin Fe(III) EPR signals after addition of one equivalent of Ti(III). The selective generation of the Mo(V)/Fe(III) form of SO is unexpected, considering that previous microcoulometry and flash photolysis investigations have indicated that the Mo(VI/V) and Fe(III/II) couples of SO have similar reduction potentials at pH 7. The nearly quantitative preparation of the one-electron re- duced Mo(V)/Fe(III) form of SO by reduction with Ti(III) has enabled the interaction between these two paramagnetic metalcenters, which are l inked by a flex- ible loop with no secondary structure, to be investigated for the first time by variable-frequency pulsed electron- electron double resonance (ELDOR) spectroscopy. The ELDOR kinetics were obtained from frozen solutions at 4.2 K at severalmicrowave frequencies by pumping on the narrow Mo(V) signaland observing the effect on the Fe(III) primary echo at both higher and lower fre- quencies within the microwave C-band region. The ELDOR data indicate that freezing the solution of one- electron reduced SO produces localized regions where the concentration of SO approaches that in the crystal structure, which results in the interpair interactions be- ing the dominant dipolar interaction. However, thor- ough analysis of the ELDOR decay curves and simulations suggests a distribution of intramolecular Mo ... Fe distances, consistent with the proposalof mul - tiple conformations in solution for the flexible loop that connects the Mo and heme domains of SO.

Pulsed ELDOR spectroscopy of the Mo(V)/Fe(III) state of sulfite oxidase prepared by one-electron reduction with Ti(III) citrate

The nature of the catalytic centres of the oxomolybdoenzymes is considered with particular reference to the results of recent protein crystallographic studies. The different nature of these centres, with one or two molecules of a special pyranopterin (molybdopterin) ligating the metal through a dithiolene group, the presence or absence of a nucleotide appended to the phosphate of the molybdopterin AND the variation in the coordination chemistry at the metal render the term "THE molybdenum cofactor" meaningless and confusing. Rather, there is a series of such cofactors, related by the common denominators of a single molybdenum atom bound to the dithiolene group of the molybdopterin and, at some stage in the catalytic cycle, at least one terminal oxo group. This Mo(O)(molybdopterin) moiety is considered to be the metal-centred functional unit (McFU) of the oxomolybdoenzymes. Variations in the coordination chemistry and, therefore, the properties of the metal centre occur with the binding of other ligands, which can include: a terminal oxo or sulfido group, OH– and/or H2O group(s), a second pterin, and/or a serine, a cysteine or selenocysteine group from the polypeptide backbone of the protein. The role of molybdopterin is considered with particular reference to its potential involvement in the various redox processes necessary for the operation of the catalytic cycles of these enzymes; special attention is given to the possible cooperativity between formally metal-based and pterin-based redox processes.

John H. Enemark

Papers

The first mononuclear molybdenum(V) complex with a terminal sulfido ligand: dichloro[hydrotris(3,5-dimethyl-1-pyrazolyl)borato]sulfidomolybdenum

Investigation of the coordination structures of the molybdenum(v) sites of sulfite oxidizing enzymes by pulsed EPR spectroscopy.

The active sites of molybdenum- and tungsten-containing enzymes

Pulsed ELDOR spectroscopy of the Mo(V)/Fe(III) state of sulfite oxidase prepared by one-electron reduction with Ti(III) citrate

The coordination chemistry and function of the molybdenum centres of the oxomolybdoenzymes