Najdat B. Nazhat

A simple spectrophotometric determination of hydrogen peroxide at low concentrations in aqueous solution

Abstract The procedure described for the determination of hydrogen peroxide in aqueous solution, at concentrations in the rnage 1−120×10−6 mol l−1, is based on reduction of copper (II) ions by hydrogen peroxide in the presence of excess of 2,9-dimethyl-1,10-phenanthroline (DMP) to form the copper (I)-DMP complex. The copper (I)-DMP complex is determined directly by spectrophotometric measurement at 454 nm.

Adriamycin stimulates proliferation of human lymphoblastic leukaemic cells via a mechanism of hydrogen peroxide (H2O2) production.

It is becoming clear that adriamycin cytotoxicity may be mediated by semiquinone‐free radicals derived from the drug itself and reactive oxygen species (ROS). Recent evidence supports the concept that low concentrations of ROS are able to stimulate cell proliferation, and, based on the observation that subtoxic concentrations of adriamycin can also induce cell proliferation, we hypothesize that low concentrations of adriamycin stimulate cell proliferation by a ROS generation mechanism. We have employed spin‐trapping and electron spin resonance (ESR) spectroscopy to investigate the nature of the adriamycin‐generated ROS. The spin trap 3,5‐dibromo‐4‐nitrosobenzenesulphonate (DBNBS), which is oxidized in the presence of H2O2 and peroxidase enzymes, was used to produce a characteristic three‐line spectrum, and it was found that an identical spectrum was produced by human lymphoblastic leukaemic cells (CCRF‐CEM cells) after exposure to adriamycin. We tested our hypothesis further by exposing CCRF‐CEM cells to subtoxic concentrations of adriamycin (10−8, 10−9 and 10−10 M) and low concentrations of H2O2 (10−8, 10−9 and 10−10 M) and subsequently monitored cell proliferation. We found that low concentrations of both adriamycin and H2O2 significantly stimulate CCRF‐CEM cell proliferation. We therefore conclude that subtoxic concentrations of adriamycin are likely to induce cell proliferation via an H2O2 mediated mechanism.

Reaction of the aquacopper(I) ion with hydrogen peroxide. Evidence for a CuIII(cupryl) intermediate

Oxidation of CH3OH to HCHO occurs as a chain reaction on γ-irradiation of deaerated aqueous CH3OH + H2O2+ Cu2+aq solutions. The reaction of Cu+aq with H2O2 is a propagation step. The chain-length and the dependence of HCHO formation rate on radiation dose, CH3OH concentration and pH are not compatible with a mechanism that includes production of the free hydroxyl radical, OH ˙, by the Cu+aq+ H2O2 reaction. It is suggested that an intermediate is formed by this reaction that reacts with methanol with a rate constant not greater than 4 × 105 dm3 mol–1 s–1, and the main features of the results are explained on this basis. The reactivity of the proposed intermediate is shown to be similar to that of the CuIIIaq(cupryl) species, produced by reaction of OH ˙ with Cu2+aq.

Reaction of the aquocopper(I) ion with hydrogen peroxide: evidence against hydroxyl free radical formation

Kinetic evidence support the formation of a copper(III) species, rather than the hydroxyl free radical, when the aquocopper(I) ion reacts hydrogen peroxide in aqueous solution.

Reactions of the hydroxyl free radical with copper(II)–amino-acid complexes in aqueous solution

Reactions of the hydroxyl radical (OH˙) in amino acid (glycine and α-alanine)–copper(II) aqueous systems have been investigated by measurement of the γ-radiation yields of the products: ammonia, formaldehyde and glyoxylic acid (from glycine), acetaldehyde and pyruvic acid (from analine) and CuI. It is concluded that OH˙ reacts with the amino acid, whether free or coordinated with CuII, to give a radical that is oxidised by CuII, forming ligand decomposition products and CuI. Under conditions where the complex CuII(amino acid)2 is the main OH˙ scavenger, the reaction results in ligand decomposition for only ca. 65% of the OH˙ radicals. The results are interpreted in terms of a mechanism that assumes that OH˙ reacts with both the attached amino acid and at the metal centre. The latter reaction gives a CuIII species (possibly OH˙ bonded to CuII). The fate of the CuIII species depends on the prevailing CuI concentration. With and excess of CuI, CuIII is reduced to CuII by CuI, without ligand decomposition: the kinetics of the decay of CuIII, observed by pulse radiolysis, is consistent with this reaction, and gives a rate constant value k(CuIII+CuI)=(1.0±0.4)× 109 dm3 mol–1 s–1. If CuI is absent, or complexed with DMP, CuIII can react to oxidise the amino-acid ligand.

A Search for the Intermediate Radical, ONOO', in the Reaction Between Oxygen and Nitric Oxide in Solution

The most probable initial reaction between .NO and O2 is direct addition to give the peroxyl radical ONOO.. In view of the potential importance of this radical in biology, we have searched extensively for its formation, using EPR spectroscopy and rapid freezing techniques. At best, only extremely low concentrations were detected, and in most systems, no signals were detectable. We conclude that this radical is unlikely to be of major importance per se in biological systems, in contrast with its one electron adduct, the peroxynitrite anion.

Ligand decomposition in the photolysis of copper(II)–amino-acid complexes in aqueous solution

Ligand decomposition following charge-transfer excitation of bis(glycinato)cuprate(II) in aqueous solution gives NH3 and HCHO as the final products in yields equivalent to the reduction of the metal to CuI. An intermediate, observed by flash photolysis, is believed to be the Cu—C-bonded species, CuCH2NH2. The reaction mechanism is discussed for the process in deaerated solution and in O2-saturated solution, where H2O2 is formed in addition to the ligand-decomposition products. Cu(α-alanine)2 shows photolytic behaviour similar to that of the glycine complex.

Reactions of hydrogen atoms with Cu2+ and Cu+ in aqueous solution

The rate-constant ratio k(H˙+Cu2+)/k(H˙+methanol)= 34±6 was determined by measurement of the H2 yields from γ-irradiated Cu2++methanol solutions under conditions where the two solutes compete for H atoms.The value of the ratio k(H˙+Cu+)/k(H˙+Cu2+)= 200±100 was obtained from measurement of the steady-state concentration of CuI in the γ-radiolysis of the CuII+methanol system ([H+]= 0.01–0.1 mol dm–3). The H2 and HCHO yields (for [H+]= 0.1 mol dm–3) were consistent with the assumed mechanism. An observed dependence of the H2 yield on [H+] is interpreted in terms of a competition in which the intermediate, CuH+, formed by reaction of H˙ atoms with Cu+, can react with H+ or with Cu2+.