David F. Sangster

A pulse radiolysis investigation of the reactions of myeloperoxidase with superoxide and hydrogen peroxide.

Using pulse radiolysis, the rate constant for the reaction of ferric myeloperoxidase with O2- to give compound III was measured at pH 7.8, and values of 2.1.10(6) M-1.s-1 for equine ferric myeloperoxidase and 1.1.10(6) M-1.s-1 for human ferric myeloperoxidase were obtained. Under the same conditions, the rate constant for the reaction of human ferric myeloperoxidase with H2O2 to give compound I was 3.1.10(7) M-1.s-1. Our results indicate that although the reaction of ferric myeloperoxidase with O2- is an order of magnitude slower than with H2O2, the former reaction is sufficiently rapid to influence myeloperoxidase-dependent production of hypochlorous acid by stimulated neutrophils.

A comparison of some aqueous chemical dosimeters for absorbed doses of less than 1000 rads

Abstract Several aqueous dosimeter solutions that have been used for the measurement of absorbed doses of less than 1000 rads were compared under identical irradiation conditions. The solutions used were: 1. (1) 2 × 10 −3 M ferrous sulphate/0.4 M sulphuric acid 2. (2) 1 × 10 −3 M benzoic acid 3. (3) 1 × 10 −4 M terephthalic acid/4 × 10 −4 M sodium hydroxide 4. (4) 1 × 10 −4 M ferrous sulphate/2 × 10 −4 M xylenol orange/5 × 10 −3 M benzoic acid/0.025 M sulphuric acid (FBX dosimeter). Particular attention was paid to spontaneous changes in solutions before and after irradiation. Small errors in the absorbance measurement of the unirradiated solution caused large errors in dose estimated from (1) for doses less than about 300 rads; however, careful calibration gave precision of better than 2% for all doses above 100 rads. Solution (2) is not as sensitive as (3) and suffers from post-irradiation changes. Solutions (3) and (4) can be used to measure doses as low as 4 rads, but both need to be calibrated using radiation sources of known dose rate. The measurement of fluorescence in solution (3) is not as convenient as absorbance measurement, but excellent linearity with dose was obtained from 3 to 800 rads with a precision of about 5% in the dose range 3–100 rads. The response of solution (4) to dose is not linear; corrections have to be made for changes in absorbance with time in both unirradiated and irradiated solutions.

Insolubilisation of biologically active materials with novel radiation graft copolymers

Abstract The use of radiation grafting to immobilise a typical enzyme, trypsin, is reported. The technique involves radiation grafting to a backbone polymer a monomer containing an appropriate functional group to which the enzyme is bonded. In the present work, p-nitrostyrene has been grafted to representative trunk polymers, polypropylene and PVC, the nitro group in the resulting copolymer converted to the isothiocyanato derivative to which trypsin is attached. Of importance to this insolubilisation process, especially for radiation sensitive backbone polymers, is the inclusion of additives which enhance grafting. A new class of additives which increase the grafting yields is reported using as representative backbone polymers, naturally occurring cellulose and synthetic low density polyethylene. The new additives are specific metal salts such as LiClO4. The reactivity of these salts in grafting enhancement has been compared with that of mineral acid which has previously been used as an additive to increase grafting yields in both preirradiation and simultaneous techniques. A new model for grafting enhancement in the presence of the metal salts as well as acids is proposed whereby increased grafting yields are attributed to increased partitioning of monomer into the graft region in the presence of ionic solutes. The value of these additives in preparing copolymers suitable for general reagent insolubilisation reactions is discussed.

The entry of free radicals into polystyrene latex particles

Mechanistic understanding of the processes governing the kinetics of emulsion polymerization has both scientific and technical interest. One component of this process that is poorly understood at present is that of free radical entry into latex particles. Measurements were made of the entry rate coefficient as a function of temperature for free radicals entering polystyrene latex particles in seeded emulsion polymerizations initiated by γ-rays. The activation energy for entry was found to be less than 24±3 kJ mol-1, consistent with entry being controlled by a physical (e.g., diffusional ) rather than a chemical process. Measurement of the entry rate coefficient as a function of the γ-ray dose rate suggested that the factors that determine the entry rate when the primary free radicals are uncharged are similar to those that determine the entry rate for charged free radicals derived from chemical initiation by peroxydisulfate. This result was consistent with measurements of the entry rate coefficient of charged free radicals derived from peroxydisulfate; these data were found to be virtually independent of both the extent of the latex surface coverage by the anionic surfactant sodium dodecyl sulfate and the ionic strength of the continuous phase. The data refute several proposals given in the literature for the rate-determining step for entry, being inconsistent with control by (1) collision of free radicals with the latex particles, (2) surfactant desorption , and (3) an electrostatic barrier arising from the colloidal nature of the entering free radical. The origin of the activation energy for entry remains obscure.

Electron solvation processes in alcohol-hydrocarbon mixtures

The solvated electron has been investigated in mixtures of polar and non- polar liquids (methanol, ethanol, propanol, and isopropyl alcohol respectively in hexane) by pulse-radiolysis. The respective yields and transition energies of the electron in the mixtures do not show the same dielectric dependence as has been established for pure liquids. The relative number of electrons escaping geminate recombination increases with an increase in the alcohol concentration, but the mechanisms of optical transition and electron decay remain unchanged. Infrared spectroscopic studies have shown that electron stabilization is related to the existence of quasi-stable associated alcohol complexes. For ethanol, an observed decrease in the solvated free electron yield with an increase in temperature is in accord with a shift in associative equilibrium. (auth)

Reactivity of semiquinone radicals and its relation to the biochemical role of superoxide

Semiquinone radicals derived from 9, 10-anthraquinone-2-sodium sulphonate (AQS), menadione (MD), duroquine (DQ) and 2,5-dimethylbenzoquinone (DMBQ) by pulse radiolysis of aqueous solutions containing propan-2-ol and quinone are shown to reduce methaemoglobin and cytochrome c at rates which greatly exceed those of the corresponding reductions by superoxide. Reasons for this are suggested. Rate constants are reported for these and related reactions, including dismutation of the semiquinones.From studies of the γ radiolysis of similar quinone–haem solutions containing oxygen, it is shown that the equilibrium O–2+ Q ⇄ O2+ Q–(4), is maintained in these systems, and in similar ones in which superoxide is generated by enzymes, thereby converting superoxide to the more reactive radical, semiquinone. Addition of superoxide dismutase to these systems suppresses reactions caused by semiquinone, a conclusion of current biochemical significance. Measurements of the equilibrium constant K4, and hence of redox properties of the semiquinones, have been obtained from studies of this suppression, and agree with previous measurements by other methods.

Effects of chain-transfer agents on the kinetics of the seeded emulsion polymerization of styrene

The kinetics of the seeded emulsion polymerization of styrene have been studied in the presence and absence of the chain-transfer agents carbon tetrachloride and carbon tetrabromide. Initiation was achieved by both a chemical initiator (potassium peroxydisulphate) and irradiation with γ-rays. The latter permitted relaxation studies to be performed.A combination of γ-ray initiation, relaxation and particle size distribution studies allowed the fate of the exited free radicals generated in the presence of carbon tetrabromide to be determined. Cross-termination in the aqueous phase was found to be operative in γ-ray initiated systems when the free radical concentration in the aqueous phase was relatively high. In contrast, re-entry of the exited free radicals into the latex particles was found to be important in relaxation studies when the free radical concentration in the aqueous phase was comparatively low. These results show that the exited free radical fate parameter can vary between –1 and +1.The exit rate coefficient was found from relaxation measurements to increase linearly with increasing concentration of chain-transfer agent; this result is consistent with a diffusion/transfer mechanism for exit. The increase in the exit rate coefficient paralleled the increase in the chain-transfer constant for the additives: CBr4 > CCl4 > styrene. On the other hand, the efficiency of exit from the latex particles of free radicals formed by chain transfer follows the inverse order: CBr4 < CCl4 < styrene. This order may well reflect the relative reactivities with monomer of the low-molecular-weight free radicals formed by atom abstraction.As expected from the increase in exit rate coefficient, the presence of carbon tetrabromide reduced the rate of polymerization of chemically initiated systems. At high initiator concentrations, for which the average number of free radicals per particle text-decoration:overlinen was ca. 0.5, the rate reduction was small but increased monotonically with increasing concentration of carbon tetrabromide. This shows that any effect of carbon tetrabromide on the propagation rate constant was small in these studies. At lower initiator concentrations, however, a much larger reduction in rate was observed, as expected theoretically for values of text-decoration:overlinen < 0.5. The rate in this case did not decrease monotonically with increasing concentration of carbon tetrabromide but passed through a minimum. This minimum was caused by the enhanced rate of entry of free radicals into the latex particles counterbalancing the rate reduction arising from the increased exit rate. The increase in the entry rate in the presence of carbon tetrabromide was explained by the production of hydrophobic free radicals by chain transfer in the aqueous phase and/or a colloidal contribution to the measured entry rate.

Pulse radiolysis and voltammetry of bis(diamine)dioxorhenium(V) complexes

Abstract The Re(V) complexes [Re(L) 2 O 2 ] + [L = en (1,2-diaminoethane) or pn (1,3-diaminopropane)] are reduced irreversibly near −1.5 V (SCE) in a neutral or basic aqueous solution, but protonation causes a significant anodic shift of ≈ + 1 V. No oxidation is observed in aqueous solution before the solvent limit (≈ + 1 V) at solid electrodes but oxidation of [Re(en) 2 O 2 ] + with OH in pulse radiolysis experiments generates a Re(VI) intermediate which decays by first-order kinetics ( k d 2.5 × 10 4 s −1 ), apparently by rate-determining ligand loss. Reduction with e − aq likewise forms transient d 3 -Re(IV) complexes which show first-order decay (6 × 10 3 s −1 for en, 9 × 10 3 s − for pn), ultimately to ReO 2 . Transient intermediates were characterized by their electronic spectra, which for d 3 -Re(IV) complexes are qualitatively similar to Cr(III) and known octahedral Re(IV) complexes with different donor ligands.

Pulsed radiolysis of perrhenate in neutral aqueous solution: Comparison with pertechnetate

Abstract Reaction of perrhenate with the aquated electron in neutral aqueous solution yields ReO 4 2− ( k f 1.3 × 10 10 M −1 s −1 ), with an absorption maximum at 290 nm (e 1700). This decays by a second-order path ( k d 1.5 × 10 9 M −1 s −1 ) at a rate ∼ 100-fold faster than the decay of TcO 4 2− under similar conditions.

Characterization, Redox Properties and Pulse-Radiolysis Study of Dichloro-(Tetraazacyclotetradecane)Manganese(III) Complexes, and X-Ray Crystal-Structure of the meso-5,7,7,12,14,14-Hexamethyl-1,4,8,11-Tetraazacyclotetradecane Complex

The complexes [Mn ( mac )Cl2]+ [ mac = 1,4,8,1l-tetraazacyclotetradecane (cyclam), meso and rac-5,7,7,12,14,14-hexamethylcyclam ( teta and tetb respectively)] have been prepared. The trans-[ Mn ( teta )Cl2] Cl.3H2O complex crystallizes in the monoclinic space group P21 /c, Z 2, a 8.4006(8), b 12.465(2), c 11 .659(2) A, β 103.20(1)o. The geometry about the manganese(III) is close to octahedral with the macrocycle coordinated to four equatorial sites ( Mn -N 2.043(2), 2.074(2) A) and chlorides in trans axial sites (Mn -Cl 2.549(1) A. Since electronic spectra of the cyclam and teta complexes are similar, the ions should exist in similar geometries; however, the tet b analogue is spectroscopically unique, and the complex may adopt a different geometry. All complexes in water exhibit quasi-reversible one-electron couples near O V at a mercury electrode; irreversible reductions of manganese(II) are also observed before the solvent limit near -1.5 V for the teta and tet b complexes. No metal-centred oxidation waves are seen before the solvent limit (c.+ 1 V) at a platinum electrode. However, pulse radiolytic oxidation (with OH) of all complexes is observed with formation kf > 109 dm3mol-1 s-1; apparently d3 manganese(IV) monomers with electronic spectra qualitatively similar to d3 chromium(III) complexes are formed. Decay of these intermediates is first-order, with all kd ≥ 5×103 s-1, but varying with the macrocyclic ligand present. Alkyl substitution on the cyclam stabilizes the manganese(IV) oxidation state, the intermediate cyclam complex decaying 25 times faster than the teta analogue. The final decay product appears to be MnO2, and rate-determining ligand loss is presumed.