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PubMed2002

Surface treatment agent for dental metals using a thiirane monomer and a phosphoric acid monomer.

To develop a new surface treatment agent which improves the bond strength of adhesive resin to both non-precious and precious metals, experimental treatment agents containing both an adhesive bonding promoter for precious metals and one for non-precious metals were prepared by dissolving epithioalkyl methacrylate (EP3MA or EP8MA) and 10-methacryloyloxydecyl dihydrogen phosphate (MDP) in acetone. The surfaces of dental metals were treated by the treatment agents and metal specimens were butt-jointed together with MMA-PMMA resins. After 2,000 thermal cyclings in water at temperatures of 4 and 60 degrees C, tensile bond strengths were measured. The effectiveness of surface treatments was evaluated by tensile bond strengths and microscopic failure mode analysis after the tensile test. The combined treatment of EP3MA-MDP or EP8MA-MDP was used effectively for non-precious metals as well as precious metals, and was shown to be extremely effective compared with the single treatment of EP3MA, EP8MA, or MDP.

Adhesiveness↗
PubMed2005

Synthesis of a stable compound with fivefold bonding between two chromium(I) centers.

Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(mu-Cl)Ar'}2 [where Ar' indicates C6H3-2,6(C6H3-2,6-Pri2)2 and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr-Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar'CrCrAr' were isolated with greater than 40% yield. X-ray diffraction revealed a Cr-Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d5 chromium(I) ions.

Journal Article↗
PubMed1990

Hexavalent chrome: threshold concept for carcinogenicity.

Certain hexavalent chromium (Cr6+) compounds when administered via inhalation at high doses have the potential to induce lung tumors in humans and experimental animals. Trivalent chromium (Cr3+) is an essential human and animal nutrient at levels of 50 to 200 micrograms/day. Recent data have shown that the human body is able to reduce Cr6+ to Cr3+. This reduction occurs in bodily fluids such as gastric juice, epithelial lining fluid of the respiratory tract, blood, and other fluids. Secondary reduction occurs at the cellular level by the cytosol, mitochondria, and microsomes. Thus, at low levels of exposure hexavalent chromium ions are reduced before the 6+ ions can interact with DNA unless the dose is sufficient to overwhelm the body's reduction capacity. This paper summarizes the available data concerning the reducing ability of the body and formulates the steps in the mechanism of cancer induction. These steps include: (1) only certain Cr6+ compounds have the capacity to interact with cellular components; (2) Cr6+ is reduced by body fluids and excess Cr6+ enters the cell (Cr3+ is poorly absorbed across membranes); (3) cellular organelles and the cytoplasm reduce Cr6+ to Cr3+; (4) excess Cr6+ can enter the nucleus; (5) Cr6+ reduction through 5+ and 4+ to 3+ has a potential to interact with the DNA molecule; and (6) if unrepaired, this DNA damage can lead to cancer induction. On the basis of current evidence Cr6+ has a threshold for carcinogenic potential in humans that is greater than the current TLV.

Carcinogens↗
PubMed2006

Oxidation of alcohol by lipopathic Cr(VI): a mechanistic study.

The oxidation kinetics of various aliphatic primary and secondary alcohols having varied hydrocarbon chain length were studied using cetyltrimethylammonium dichromate (CTADC) in dichloromethane (DCM) in the presence of acetic acid and in the presence of a cationic surfactant. The rate of the reaction is highly sensitive to the change in [CTADC], [alcohol], [acid], [surfactant], polarity of the solvents, and reaction temperature. A Michaelis-Menten type kinetics was observed with respect to substrate. The chemical nature of the intermediate and the reaction mechanism were proposed on the basis of (i) observed rate constant dependencies on the reactants, that is, fractional order with respect to alcohol and acid and a negative order with respect to oxidant, (ii) high negative entropy change, (iii) inverse solvent kinetic isotope effect, k(H2O)/k(D2O) = 0.76, (iv) low primary kinetic isotope effect, kH/kD = 2.81, and (v) the k(obs) dependencies on solvent polarity parameters. The observed experimental data suggested the self-aggregation of CTADC giving rise to a reverse micellar system akin to an enzymatic environment, and the proposed mechanism involves the following: (i) formation of a complex between alcohol and the protonated dichromate in a rapid equilibrium, equilibrium constant K = 5.13 (+/-0.07) dm(3) mol(-1), and (ii) rate determining decomposition (k(2) = (7.6 +/- 0.7) x 10(-3) s(-1)) of the ester intermediate to the corresponding carbonyl compound. The effect of [surfactant] on the rate constant and the correlation of solvent parameters with the rate constants support the contribution of hydrophobic environment to the reaction mechanism.

Alcohols↗
PubMed2006

Valence-to-core X-ray emission spectroscopy identification of carbide compounds in nanocrystalline Cr coatings deposited from Cr(III) electrolytes containing organic substances.

Valence-to-core X-ray emission spectroscopy was applied to study a composition of chromium coatings electrodeposited from Cr(III) sulfate electrolytes with the addition of formic or oxalic acid. It was shown that the obtained crystallographically amorphous deposits contain chromium carbide compounds. These results indicate that nanodimensional Cr crystallites formed during the electrodeposition process are characterized by very high electrocatalytic activity.

Journal Article↗
PubMed1977

Metals as mutagens.

A number of metals are mutagenic in bacteria or phage. These include compounds of arsenic, chromium, copper, iron, manganese, molybdenum, platinum, and selenium. Compounds containing alumium, antimony, arsenic, cadmium, copper, lead, mercury, nickel, and tellium have been shown to induce chromosomal aberrations or abnormal cell divisions in animal or plant cells. Genetic evidence suggests that arsenic, chromium, and molybdenum compounds may influence the accuracy of DNA repair processes in microorganisms.

Animals↗
PubMed2006

Hexavalent chromium removal from wastewater using aniline formaldehyde condensate coated silica gel.

A resinous polymer, aniline formaldehyde condensate (AFC) coated on silica gel was used as an adsorbent in batch system for removal of hexavalent chromium from aqueous solution by considering the effects of various parameters like reaction pH, dose of AFC coated silica gel, initial Cr(VI) concentration and aniline to formaldehyde ratio in AFC synthesis. The optimum pH for total chromium [Cr(VI) and Cr(III)] adsorption was observed as 3. Total chromium adsorption was second order and equilibrium was achieved within 90-120 min. Aniline to formaldehyde ratio of 1.6:1 during AFC synthesis was ideal for chromium removal. Total chromium adsorption followed Freundlich's isotherm with adsorption capacity of 65 mg/g at initial Cr(VI) 200mg/L. Total chromium removal was explained as combinations of electrostatic attraction of acid chromate ion by protonated AFC, reduction of Cr(VI) to Cr(III) and bond formation of Cr(III) with nitrogen atom of AFC. Almost 40-84% of adsorbed chromium was recovered during desorption by NaOH, EDTA and mineral acids. AFC coated silica gel can be effectively used for treatment of chromium containing wastewaters as an alternative.

Adsorption↗
PubMed1998

Differential sensitivity of chromium-mediated DNA interstrand crosslinks and DNA-protein crosslinks to disruption by alkali and EDTA.

Some compounds of hexavalent chromium are well-established carcinogens. Chromium enters mammalian cells in the hexavalent form and is reduced to chromium (III). Treatment of purified DNA with chromium (III) produces DNA-DNA interstrand crosslinks (DDC) which obstruct the progression of DNA polymerases in vitro. DDC were also detected in chromate-treated cultured normal human lung cells using the renaturing agarose gel electrophoresis (RAGE) assay and correlated with base-specific inhibition of DNA replication. Curiously, DDC have gone undetected in studies of cultured cells using the alkaline elution (AE) technique, whereas chromium-mediated DNA-protein crosslinks (DPC) were readily detected by AE. We tested the hypothesis that AE conditions [60 mM tetraethyl ammonium hydroxide (TEA), 20 mM EDTA, pH 12.6, for 16 h at room temperature] dissociate DDC but not DPC using chromium(III)-treated plasmid DNA and the RAGE assay. Dose-dependent chromium-induced DDC were unaffected by TEA (pH 11.8) alone or by more rigorous alkaline denaturation conditions (200 mM NaOH, pH 13.5, for 16 h). DDC were, however, completely disrupted by EDTA (pH 12.6) alone or the combination of TEA and EDTA (pH 12.6). In contrast, DPC remained largely intact under these conditions. Therefore, past AE-based studies which have failed to detect chromium-induced DDC do not prove the absence of this lesion. AE may not be suitable for detecting DDC induced by EDTA-chelatable agents such as metals.

Caustics↗