Biomedical subjects

I Francis Cheng

Publications and source records attributed to I Francis Cheng.

7 recordsLinked to original sources

PubMed2005

EDTA degradation induced by oxygen activation in a zerovalent iron/air/water system.

A method for the removal of ethylenediaminetetraacetic acid (EDTA) at room temperature and 1 atm is demonstrated. EDTA (1 mM, 50 mL) containing 2.5 g of granular zerovalent iron (ZVI) (20-40 mesh) was degraded in 2.5 h. Using a recently developed form of O2 activation, reactive oxygen species are generated in situ, resulting in the degradation of EDTA when complexed with FeII. ESI-MS measurements indicate that degradation of EDTA yields low-molecular carboxylic acids. The presence of oxygen is crucial: the observed pseudo-first-order rate constants for EDTA removal are kobs = 1.02 h(-1) (kSA = 1.85 +/- 0.046 L h(-1) m(-2)) and kobs = 0.04 h(-1) (kSA = 0.00724 +/- 0.002 L h(-1) m(-2)) under air and under N2 purge, respectively. kSA represents surface area normalized rate constants. Large excesses of EDTA in the reaction mixture slowthe rate of degradation. Increasing the concentration of EDTA from 1.0 to 10.0 mM while holding all other parameters constant gave observed rates of kobs = 1.02 +/- 0.26 h(-1) (kSA = 1.85 +/- 0.046 L h(-1) m(-2)) and kobs = 0.044 +/- 0.01 h(-1) (kSA = 0.00796 +/- 0.002 L h(-1) m(-2)), respectively. The rate-limiting step is determined to be homogeneous oxygen activation.

Air↗

PubMed2005

Stability of ferric complexes with 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3',4'-dihydroxyflavone.

The acid dissociation and ferric stability constants for complexation by the flavonoids 3-hydroxyflavone (flavonol), 5,7-dihydroxyflavone (chrysin), and 3',4'-dihydroxyflavone in 50:50 (v/v) ethanol/water are determined by pH potentiometric and spectrophotometric titrations and the linear least-squares curve-fitting program Hyperquad. Over the entire range of pH and reagent concentrations spanning the titration experiments, the stoichiometry for iron-flavonoid complex formation was 1:1 for all three flavonoids examined. The three flavonoids were chosen for their hydroxy substitution pattern, with each possessing one of the three most commonly suggested sites for metal binding by the flavonoids. On the basis of the calculated stability constants, the intraflavonoid-binding site competition is illustrated as a function of pH via speciation curves. The curves indicate that the binding site comprised of the 3',4'-hydroxy substitutions, the catecholic site, is most influential for ferric complexation at the physiological pH of 7.4. The possibility for antioxidant activity by flavonoid chelation of ferric iron in the presence of other competitive physiological complexing agents is demonstrated through additional speciation calculations.

Drug Stability↗

PubMed2005

Voltammetric studies of Zn and Fe complexes of EDTA: evidence for the push mechanism.

The 'push' hypothesis for the antioxidant action of Zn2+ is based on its displacement of iron from a low molecular weight pro-oxidant complex. In this study, the chemical plausibility of that proposed function is investigated by cyclic voltammetry. As a model for a pro-oxidative low molecular weight iron complex the Fe(II/III)/EDTA couple was examined. This complex was selected for its well-defined electrochemical, iron stability constants, and similarity to other low molecular weight chelates in physiological fluids in terms of logical binding sites, i.e. amino, and carboxylate groups. Also investigated were iron complexes of nitrilotriacetic acid and DL-glutamic acid. Results demonstrate that approximately 90% of the cyclic voltammetric peak current for Fe(III)EDTA reduction and the EC' current for the mediated reduction of H2O2 by Fe(II/III)EDTA (Fenton Reaction) are lost when Zn2+ is introduced to a 1:1 molar ratio relative to iron. All experiments were conducted in HEPES buffered solutions at pH 7.4. Iron (II/III) complexes of nitrilotriacetic acid and DL-glutamic acid followed the same trends. Cyclic voltammetric experiments indicate that Zn2+ displaces Fe(III) from EDTA despite the much larger stability constant for the iron complex (10(25.1)) versus zinc (10(16.50)). The hydrolysis aided displacement of Fe(III) from EDTA by Zn2+ is considered by the equilibria modeling program, HySS. With Fe(III) hydrolysis products included, Zn2+ is able to achieve 90% displacement of iron from EDTA, a result consistent with cyclic voltammetric observations.

Catalysis↗

PubMed2004

A hypothesis for the basis of the pro-oxidant nature of calcium ions.

A new hypothesis describing the role of the redox inactive Ca2+ ion in the expression of physiological oxidative damage is described. The hypothesis is based on the optimization of the chelation characteristics of iron complexes for pro-oxidant activity. In a previous investigation it was found that an excess of ligand kinetically hindered the Fenton reaction activity of the FeII/III EDTA complex (Bobier et al. 2003). EDTA, citrate, NTA, and glutamate were selected as models for the coordination sites likely encountered by mobile iron, i.e. proteins. The optimal [EDTA]:[FeIII] ratio for Fenton reaction activity as measured by electrocatalytic voltammetry in a solution was found to be 1:1. An excess of EDTA in the amount of 10:1 [ligand]: [metal] suppresses the Fenton reaction activity to nearly the control. It is expected that the physiological coordination characteristics of mobile Fe would have a very large excess of [ligand]:[metal] and thus not be optimized for the Fenton reaction. Introduction of Ca2+ in to a ratio of 10:10:1 [Ca2+]:[EDTA]:[FeIII] to the system reinvigorated the Fenton reaction activity to nearly the value of the optimal 1:1 [EDTA] :[FeIII] complex. The pH distribution diagrams of Ca2+ in the presence of EDTA and FeII/III indicate that Ca2+ has the ability to uptake excess EDTA without displacing either FeII of FeIII from their respective complexed forms. The similarity in the presence for hard ligand sites albeit with a lower binding constant for Ca2+ accounts for this action.

Calcium↗

PubMed2003

Variability of the Fenton reaction characteristics of the EDTA, DTPA, and citrate complexes of iron.

The common metal chelation agents, DTPA and EDTA are often used as models for physiological low-molecular weight iron complexes in biochemical studies, or for common biochemical protocols. In the biochemical literature there are apparent conflicts as to whether EDTA and DTPA are pro-oxidant or antioxidant additives. This apparent conflict is puzzling since in chemical systems FeIIEDTA and FeIIDTPA are well known Fenton reaction reagents. In this investigation we examined the voltammetric characteristics of the iron complexes of EDTA, DTPA, and citrate and the effect of the ligand:metal ratio (L:M) on the electrocatalytic (EC') waves that result from reduction of H2O2 by this complex. At a ratio of 1:1, the cyclic voltammetric waves of the complexes indicate the presence of a reversible species corresponding to the Fe(II/III)L couple, along with a second irreversible reduction peak. The second irreversible voltammetric peak decreases at higher L:M ratios for EDTA and citrate. The 1:1 iron complexes of EDTA, DTPA, and citrate clearly induce the catalytic reduction of H2O2. In the presence of a greater than 100 fold excess of H2O2 relative to iron, higher L:M ratios greatly reduced the catalytic EC' wave compared to the 1:1 ratios. At H2O2:Fe ratios less than 50, the L:M ratio has very little effect of the EC' current. These observations may explain the apparent discrepancies in the biochemical literature. Addition of EDTA or DTPA may enhance oxidative processes if the L:M is low (less than unity), whereas rates of on-going oxidative processes may decrease if that ratio, along with the relative amount of H2O2, are both high (excess ligand). The impact of this study is of particular importance given the widespread use of these ligands in biochemical studies.

Catalysis↗

PubMed2002

Supercritical fluid extraction and high-performance liquid chromatography-diode array-electrochemical detection of signature redox compounds from sand and soil samples.

A supercritical fluid extraction procedure and a chromatographic separation/detection method were developed for the detection of Earth-based microorganisms. After microbes in a sand or a soil sample were hydrolyzed in a diluted NH(4)OH/acetone solution, several redox compounds from bacteria could be effectively extracted with trimethylamine-modified supercritical CO(2) at 35 degrees C and 300 atm. These signature redox-active compounds were separated by a reversed-phase HPLC column in an ion-pair mode and then monitored with a diode array detector and an electrochemical detector. The analytical results demonstrated the feasibility of using the reported techniques to detect the chemical signature of life in barren desert sand samples.

Bacteria↗

PubMed2002

Dechlorination of chlorinated phenols by catalyzed and uncatalyzed Fe(0) and Mg(0) particles.

Uncatalyzed, and palladium-catalyzed Fe(0) and Mg(0) systems were examined for their efficiencies of dechlorination of 2.86 mM 4-chlorophenol (4-CP), 2.52 mM 2,6-dichlorophenol (2,6-DCP), 3.03 mM 2,4,6-trichlorophenol (2,4,6-TCP), and 2.48 mM pentachlorophenol (PCP) in 50/50 (v/v) 2-propanol/water under room temperature and pressure conditions. Previous investigators have found that PCP is extremely recalcitrant under these conditions. In this investigation, complete dechlorination of 5.0 ml of 2.48 mM PCP was observed for 1.0 g of 2659 ppm Pd/Mg (20 mesh) after 48 h. The only detectable products were cyclohexanol and cyclohexanone at 25% yield. No other chlorinated or otherwise products were observed by mass spectral analysis. It is hypothesized that volatile low molecular weight species were formed from the Pd/Mg dechlorination of PCP. Under conditions of equal surface area (0.0786 m2), the approximate order of PCP dechlorination power of these systems followed as 2659 ppm Pd/Mg>319 ppm Pd/Mg>Mg approximately 4856 ppm Pd/Fe>Fe. Degradation of the other chlorinated phenols by all metallic systems was more facile than PCP.

Catalysis↗