Dan Meyerstein

New Mechanistic Aspects of the Fenton Reaction

The kinetics of the Fenton reaction was studied in detail. A second reaction step in the presence of excess H2O2 is attributed to formation of the complex Fe(III)(-O2H)(aq). Therefore, the reaction of Fe(H2O)(6)(2+) with Fe(III)(-O2H)(aq) in the presence of Fe(II) to form Fe(III)(aq) (k=(7.7+/-1.5) x 10(5) M(-1) s(-1)) may contribute to the overall Fenton reaction, and could account for some of the debate in the literature concerning its detailed mechanism. If this is correct for LFe(III)(-O2H)(aq) also, then it might be of significant biological importance. The activation parameters DeltaH(not equal), DeltaS(not equal), and DeltaV(not equal) for the Fenton reaction were measured under various experimental conditions, and are used in the mechanistic interpretation.

ARE M-N BONDS INDEED INHERENTLY WEAKER WHEN N IS A TERTIARY RATHER THAN A PRIMARY OR SECONDARY NITROGEN ATOM ?

Abstract N -alkylation of amine ligands decreases the stability constants of their metal ion complexes, stabilizes low valent complexes, increases the acidity of the central cation and turns it into a harder acid. The major source of these effects is the decrease in the outer sphere solvation energy induced by the N -alkylation.

Charge-transfer complexes of iodine and inorganic anions in solution

In aqueous solution iodine forms complexes with several anions including halides, CNS/sup -/, and N/sub 3/. Their spectra and stability constants were determined. Energetic evidence is presented indicating that these are charge- transfer complexes and that their characteristic intense absorption bands are due to electron transfer from the anion to iodine. The structure of polyhalides and their photochemistry are discussed. (auth)

What is Unique About Superoxide Toxicity as Compared to Other Biological Reductants? — A Hypothesis

Usually the toxicity of superoxide is attributed to its ability to reduce metal ions and subsequently reoxidation of the metal by hydrogen peroxide yields deleterious oxidizing species. As many other nontoxic biological reductants reduce metal compounds, we suggest that part of the mechanism of superoxide toxicity results from its ability to oxidize metal ions bound to biological targets, which subsequently degrade the target via an intramolecular electron transfer reaction.

Mechanism of the catalytic hydrogen production by gold sols. Hydrogen/deuterium isotope effect studies

The H/D isotope effect on the catalytic production of hydrogen from water using either Cr/sup 2 +/ or (CH/sub 3/)/sub 2/COH radical as reductants was studied. The catalysts used were gold sols stabilized by either citrate or poly(vinyl sulfate). Similar isotope effects (in the range of 5 to 6.7) were measured for the various reductants and stabilizers. Comparison of this isotope effect to the one measured for the production of hydrogen on gold cathodes leads to the conclusion that similar mechanisms operate in all three systems. It was also observed that hydrogen abstraction from the relatively stable supports, when adsorbed on the colloid, occurs. This leads to the conclusion the hydrogen atoms adsorbed on the particles can either proceed to produced hydrogen molecules by the catalytic reaction or react via other competing pathways.

Reactions of Low Valent Transition Metal Complexes with Hydrogen Peroxide. Are they “Fenton-Like” or not? 4. The Case of Fe(II)L, L = Edta; Hedta and Tcma

The question whether hydroxyl free radicals are formed in the reactions of divalent iron complexes Fe(II)L; L = edta; hedta; tcma (tcma = 1-acetato-1,4,7-triazacyclononane) with hydrogen peroxide in neutral and slightly acidic solutions was studied by using the beta elimination reaction as an assay for the formation of hydroxyl free radicals, OH. The results show that at pH < 5.5 the iron(II)peroxide intermediate complex decomposes rapidly to yield free hydroxyl radicals for L = edta and hedta. This is in contrast to the mechanism of the corresponding Fe(II)nta peroxide complex, which probably decomposes to form Fe(IV)nta which then reacts with organic substrates to yield aliphatic free radicals. Thus, the non-participating ligand L has an appreciable effect on the mechanism of reaction of the metal center with hydrogen peroxide. Blank experiments using ionizing radiation as the source of .CH2CR(CH3)OH, R = H or CH3 radicals indicate that when L = tcma intermediates of the type LFeIII-CH2CR(CH3)OHaq are formed, but their major mode of decomposition is not the beta elimination reaction. Thus, the present assay for the formation of hydroxyl free radicals by the Fenton Reaction does not fit the latter system.

Acoustic cavitation in phacoemulsification: chemical effects, modes of action and cavitation index

High-intensity ultrasound (US) energy (HIUE) has been extensively used in the last 3 decades in a wide range of surgical procedures, including phacoemulsification. The generation of radicals and sonoluminescence (SL) by application of continuous-wave (CW) HIUE to an aqueous medium under conditions simulating cataract phacoemulsification surgery is demonstrated by electron paramagnetic resonance (EPR) spectroscopy and a sensitive photon-detecting system. The findings provide direct evidence for the generation of acoustic cavitation in the simulated intraocular environment, pointing out that generation of acoustic cavitation in clinical phacoemulsification and other surgical applications of US is possible. The findings imply that the effects of acoustic cavitation in aqueous medium may contribute to the endothelial damage observed clinically following phacoemulsification cataract surgery. Saturation of the irrigating solution with various gases modifies the acoustic cavitation. Saturation of the irrigating solution with CO2 practically eliminates acoustic cavitation, with the concomitant elimination of radicals and SonL. CO2 may be utilized clinically to suppress acoustic cavitation in phacoemulsification and other medical applications. A cavitation index (CI) is introduced for the purpose of standardizing phacoemulsification instrumentation and other medical US devices that employ HIUE.

Oxidation of first-row bivalent transition-metal complexes containing ethylenediaminetetra-acetate and nitrilotriacetate ligands by free radicals: a pulse-radiolysis study

The kinetics of reaction of OH radicals with the ligands (L) ethylenediaminetetra-acetate (edta) and nitrilotriacetate (nta), and their complexes [ML](M = ZnII, CuII, NiII, CoII, FeII, and MnII), have been studied. All these reactions have similar rate constants, although a hydrogen-abstraction reaction from the ligand is observed for the free ligands and their complexes with ZnII, MnII, and CoII. For [Co(edta)]2– a partial oxidation to [Co(edta)]– is observed. On the other hand, when M = NiII, CuII, or FeII the metal is oxidized by the OH radicals. The reasons for these observations are discussed in detail. The spectra, and kinetics of decomposition, of the unstable intermediates formed by the reactions of the OH radicals are reported and discussed. The reactions of nickel(III) complexes with I–, Br–, and O2 are reported. The kinetics of oxidation of the complexes (M = FeII, MnII, or CoII) by [O2]–·, ˙O2CH2CMe2OH, [Br2]–·, [(NCS)2]–·, and [I2]–·, the latter only with M = FeII, have also been studied. The kinetics indicate an inner-sphere mechanism for these reactions. The kinetics of hydrolysis of [MnIIIL(X)] and [CoIIIL(X)] thus formed are reported.

Protective effect of free-radical scavengers on corneal endothelial damage in phacoemulsification

PURPOSE: To examine the role of the water‐soluble antioxidants glutathione and ascorbic acid in the irrigating solution on corneal endothelial cells following exposure to high‐intensity ultrasound energy. SETTING: Goldschleger Eye Research Institute, Sheba Medical Center, Tel‐Aviv University, Tel Aviv, Israel. METHODS: Thirty‐two rabbit eyes were subjected to prolonged exposure to the phacoemulsification device in the anterior chamber. The eyes were divided into 4 groups that differed only in the composition of the irrigating solution applied to the eyes: balanced salt solution (BSS) BSS Plus BSS containing additional soluble components including glutathione, BSS with 10−3 M of oxidized glutathione (GSSG), and BSS with 10−2 M of ascorbic acid. Specular microscopy was performed preoperatively and 1 week after surgery. RESULTS: The BSS group exhibited the highest endothelial cell loss (19.3%), followed by the BSS Plus group (10.6%), the GSSG group (5.2%), and the ascorbic acid group (0.9%). An overall difference was found between the groups (F = 11.046, P<.0001), and all groups demonstrated a statistically significant difference from the control BSS group (P<.02, P = .001, and P<.0001, respectively). CONCLUSIONS: Damage to the cornea is largely due to the free radicals generated by high‐intensity ultrasound energy during phacoemulsification. Adding the antioxidants ascorbic acid and GSSG to the irrigation solution significantly reduced the endothelial corneal cell damage. Ascorbic acid in the concentration of 10−2 M had the highest protective effect; thus, it should be evaluated for clinical use.

Reactions of alkyl-radicals with gold and silver nanoparticles in aqueous solutions

Silver and gold nanoparticles are very efficient catalysts for the dimerization of methyl-radicals in aqueous solutions. The rate constants for the reaction of methyl-radicals with the gold and silver nanoparticles were measured and found to be 3.7 x 10(8) M(-1) s(-1) and 1.4 x 10(9) M(-1) s(-1), respectively. The results thus suggest that alkyl-radicals, also not reducing ones, are scavenged by these nanoparticles. This might explain the role, if such a role exists, of these nanoparticles in medical applications.