Pulse radiolytic study of α-tocopherol radical mechanisms in ethanolic solution

Pulse radiolytic studies of α-tocopherol (αTH) oxidation-reduction processes were carried out with low doses (5 Gy) of high-energy electrons in O₂₋, N₂₋, and air-saturated ethanolic solutions. Depending on the concentration of oxygen in solution, two different radicals, A· and B·, were observed. The first, A·, was obtained under N₂ and results from aTH reaction with solvated electron (k_aTH+csolv⁻ = 3.4 × 10⁸ mol⁻¹ liter s⁻¹) and with H₃C-ĊH-OH, (R·) (k_{aTH + R·} = 5 × 10⁵ mol⁻¹ liter s⁻¹). B·, observed under O₂, is produced by αTH reaction with RO₂ peroxyl radicals (k_aTH + RO₂^. = 9.5 × 10⁴ mol⁻¹ liter s⁻¹).     

Solvent Effects in the Spin Trapping Of Lipid Oxyl Radicals

Studies documenting spin trapping of lipid radicals in defined model systems have shown some surprising solvent effects with the spin trap DMPO. In aqueous reactions comparing the reduction of H₂O₂ and methyl linoleate hydroperoxide (MLOOH) by Fe^z+, hydroxyl (HO·) and lipid alkoxyl (LO·) radicals produce identical four-line spectra with line intensities 1:2:2:1. Both types of radicals react with commonly-used HO· scavengers, e.g. with ethanol to produce ·C(CH₃)HOH and with dirnethylsulfoxide (DMSO)togive ·CH₃. However, DMSO radicals (either ·CH₃or ·OOCH₃) react further with lipids, and when radicals are trapped in these MLOOH systems, multiple adducts are evident. When acetonitrile is added to the aqueous reaction systems in increasing concentrations, ·CH₂CN radicals resulting from HO· attack on acetonitrile are evident, even with trace quantities of that solvent. In contrast, little, if any, reaction of LO· with acetonitrile occurs, even in 100% acetonitrile. A single four-line signal persists in the lipid systems as long as any water is present, although the relative intensity of the two center lines decreases as solvent-induced changes gradually dissociate the nitrogen and β-hydrogen splitting constants. Extraction of the aqueous-phase adducts into ethyl acetate shows clearly that the identical four-line spectra in the H₂0₂ and MLOOH systems arise from different radical species in this study, but the lack of stability of the adducts to phase transfer may limit the use of this technique for routine adduct identification in more complex systems. These results indicate that the four-line 1:2:2:1. a_N = a_H = 14.9G spectrum from DMPO cannot automatically be assigned to the HO· adduct in reaction systems where lipid is present, even when the expected spin adducts from ethanol or DMSO appear confirmatory for HO-. Conclusive distinction between HO· and LO· ultimately will require use of ¹³C-labelled DMPO or HPLC-MS separation and specific identification of adducts when DMPO is used as the spin trap.     

Kinetic studies on partial oxidation of methane over samarium oxides

Kinetic studies on the oxidative coupling of methane over Sm₂O₃ have been carried out. The experimental rate equation observed could be well explained in terms of the reaction mechanism proposed. The reaction is initiated by abstracting hydrogen atom from the methane adsorbed by the diatomic oxygen on the surface. The coupling of two CH₃· radicals leads to C₂H₆. Deep oxidation of CH₃· produces CO and CO₂. The large activation energy (149 kJ mol⁻¹) needed for the formation of CH₃· explains the sharp increase in the selectivity to C₂-compounds (C₂H₆ + C₂H₄) as raising temperatures. The oxygen species responsible for initiating the reaction was suggested to be O₂²⁻ or O₂⁻ on the surface.     

The kinetics and mechanisms of the reactions of chromium(III) with pentane-2,4-dione in aqueous solution

The equilibrium, kinetics and mechanism of the reaction of chromium(III) with pentane-2,4-dione (Hpd) have been investigated in aqueous solution at 55°C and ionic strength 0.5 mol dm⁻³ NaClO₄. The equilibrium constant (log β₁) is 10.08(±0.01) while the pK of Hpd is 8.69(±0.01). The kinetics are consistent with a mechanism in which [Cr(H₂0)₆]³⁺ and [Cr(H₂0)₅(OH)]²⁺ react with the enol tautomer of Hpd with rate constants of 1.05(±0.26) × 10⁻² and 2.78(±0.08) × 10⁻¹ dm³ mol⁻¹ s⁻¹ respectively. These rate constants are considerably more rapid than those predicted by the Eigen-Wilkins mechanism. These data are compared with literature values.     

THE KINETICS OF THE REACTION WHICH TAKES PLACE BETWEEN IODOACETIC ACID AND GLYCINE

1. The kinetics of the reaction which takes place between glycine and iodoacetic acid was studied by means of the polarographic method. 2. On the basis of kinetic equations, evidence was obtained that (a) The reaction proceeds in two steps in which the hydrogens of the amino group are consecutively replaced by the acetyl radicals, the velocity constants being in the ratio 2:1. (b) Only the anionic form of glycine is able to react since the velocity constants at any pH are proportional to the concentration of glycine anion. (c) The reaction is of the ionic type, showing a positive salt catalysis, which, according to Brönsted''s hypothesis, involves the primary and the secondary salt effects. 3. The fact that only the glycine anion is able to react was explained as being due to the existence of an unbonded pair of electrons on the nitrogen in the NH₂ group. The NH₃ ⁺ group, however, in which these electrons are shared by H⁺, must, therefore, be inactive.     

Isoorientin-6″-O-glucoside,a water-soluble antioxidant isolated from Gentiana arisanensis

The antioxidant activities of isoorientin-6″-O-glucoside were studied using various models. Isoorientin-6″-O-glucoside was more potent than Trolox, probucol and butylated hydroxytoluene (BHT) in reducing the stable free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH). It also scavenged superoxide anion, peroxyl and hydroxyl radicals that were generated by xanthine/xanthine oxidase, 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) and Fe³⁺–ascorbate–EDTA–H₂O₂ system, respectively. The IC₅₀ value, stoichiometry factor and second-order rate constant were 9.0 ± 0.8 μM, 1.8 ± 0.1 and 2.6 × 10¹⁰ M⁻¹ s⁻¹ for superoxide generation, peroxyl and hydroxyl radicals. However, isoorientin-6″-O-glucoside did not inhibit xanthine oxidase activity or scavenge hydrogen peroxide (H₂O₂), carbon radical or 2,2′-azobis(2,4-dimethylvaleronitrile) (AMVN)-derived peroxyl radical in hexane. Isoorientin-6″-O-glucoside inhibited Cu²⁺-induced oxidation of human low-density lipoprotein (LDL) as measured by fluorescence intensity, thiobarbituric acid-reactive substance formation and electrophoretic mobility. Since isoorientin-6″-O-glucoside did not possess pro-oxidant activity, it may be an effective water-soluble antioxidant that can prevent LDL against oxidation.     

An EPR Investigation of Human Methaemoglobin Oxidation by Hydrogen Peroxide: Methods to Quantify all Paramagnetic Species Observed in the Reaction

The method of Electron Paramagnetic Resonance (EPR) spectroscopy was used to study the reaction of human methaemoglabin (metHb) with hydrogen peroxide. The samples for EPR measurements were rapidly frozen in liquid nitrogen at different times after H₂O₂ was added at 3- and 10-fold molar excess to 100 μM metHb in 50 mM phosphate buffer, pH 7.4, 37°C. Precautions were taken to remove all catalase from the haemoglobin preparation and no molecular oxygen evolution was detected during the reaction. On addition of H₂O₂ the EPR signals (- 196°C) of both high spin and low spin metHb rapidly decreased and free radicals were formed. The low temperature (- 196°C) EPR spectrum of the free radicals formed in the reaction has been deconvoluted into two individual EPR signals, one being an anisotropic signal (g° = 2.035 and g° = 2.0053), and the other an isotropic singlet (g = 2.0042, AH = 20 G). The former signal was assigned to peroxyl radicals. As the kinetic Pehaviour of both peroxyl (ROO^*) and nonperoxyl (P^*) free radicals were similar, we concluded that ROO^* radicals are not formed from P^* radicals by addition of O₂. The time courses for both radicals showed a steady state during the time required for H₂O₂ to decompose. Once all peroxide was consumed, the radical decayed with a first order rate constant of 1.42 ± 10^-3 s^-1 (1:3 molar ratio). The level of the steady state was higher and its duration shorter at lower initial concentration of H₂O₂. The formation of the rhombic Fe(III) non-haemcentres with g = 4.35 was found. Their yield was proportional to the H₂O₂ concentration used and the centers were ascribed to haem degradation products. The reaction was also monitored by EPR spectroscopy at room temperature. The kinetics of the free radicals measured in the reaction mixture at room temperature was similar to that observed when the fast freezing method and EPR measurement at —196°C were used.     

Phenol derivatives as enhancers and inhibitors of luminol–H2O2–horseradish peroxidase chemiluminescence

Systematic studies on phenol derivatives facilitates an explanation of the enhancement or inhibition of the luminol–H₂O₂–horseradish peroxidase system chemiluminescence. Factors that govern the enhancement are the one-electron reduction potentials of the phenoxy radicals (PhO^•/PhOH) vs. luminol radicals (L^•/LH⁻) and the reaction rates of the phenol derivatives with the compounds of horseradish peroxidase (HRP-I and HRP-II). Only compounds with radicals with a similar or greater reduction potential than luminol at pH 8.5 (0.8 V) can act as enhancers. Radicals with reduction potentials lower than luminol behave in a different way, because they destroy luminol radicals and inhibit chemiluminescence. The relations between the reduction potential, reaction rates and the Hammett constant of the substituent in a phenol suggest that 4-substituted phenols with Hammett constants (σ) for their substituents similar or greater than 0.20 are enhancers of the luminol–H₂O₂–horseradish peroxidase chemiluminescence. In contrast, those phenols substituted in position 4 for substituents with Hammett constants (σ) lower than 0.20 are inhibitors of chemiluminescence. On the basis of these studies, the structure of possible new enhancers was predicted. © 1998 John Wiley & Sons, Ltd.     

Pre-steady-state kinetic study on the formation of Compound I and II of ligninase

The reaction between ligninase and hydrogen peroxide yielding Compound I has been investigated using a stopped-flow rapid-scan spectrophotometer. The optical absorption spectrum of Compound I appears different to that reported by Andrawis, A. et al. (1987) and Renganathan, V. and Gold, M.H. (1986), in that the Soret-maximum is at 401 nm rather than 408 nm. The second-order rate constant (4.2·10⁵ M⁻¹·s⁻¹) for the formation of Compound I was independent of pH (pH 3.0–6.0). In the absence of external electron donors, Compound I decayed to Compound II with a half-life of 5–10 s at pH 3.1. The rate of this reaction was not affected by the H₂O₂ concentration used. In the presence of either veratryl alcohol or ferrocyanide, Compound II was rapidly generated. With ferrocyanide, the second-order rate constant increased from 1.9·10⁴ M⁻¹·s⁻¹ to 6.8·10⁶ M⁻¹·s⁻¹ when the pH was lowered from 6.0 to 3.1. With veratryl alcohol as an electron donor, the second-order rate constant for the formation of Compound II increased from 7.0·10³ M⁻¹·s⁻¹ at pH 6.0 to 1.0·10⁵ M⁻¹·s⁻¹ at pH 4.5. At lower pH values the rate of Compound II formation no longer followed an exponential relationship and the steady-state spectral properties differed to those recorded in the presence of ferrocyanide. Our data support a model of enzyme catalysis in which veratryl alcohol is oxidized in one-electron steps and strengthen the view that veratryl alcohol oxidation involves a substrate-modified Compound II intermediate which is rapidly reduced to the native enzyme.     

Esr Spectra of Radicals of Single-Stranded and Double-Stranded DNA in Aqueous Solution. Implications for Oh-Induced Strand Breakage

In situ photolysis at 20^oC (argon plasma light source, $, $ 200 mm) of oxygen-free solutions containing 2mM H₂0₂ and heat-denatured, single-stranded (sS)DNA from calf-thymus resulted in the ESR spectra of the 6-hydroxy-5,6-dihydro-thymin-5-yl {1} and 5-methyleneuracil {3} radicals linked to the sugar-phosphate backbone. They were generated by reaction of OH radicals with DNA. By comparison of the decay characteristics of the ESR signals with rate constants from pulse-conductivity measurements [E. Bothe, G.A. Qureshi and D. Schulte-Frohlinde, Z. Naturforsch. 38c 1030, (1983)] the thymine-derived radicals {1} and {3} can be excluded as precursors of the fast, dominating component of strand breakage of ssDNA. In the absence of H₂0₂ from native, doubie-stranded (ds)DNA an ESR signal was obtained (singlet, g ～ 2.004, $1/2 ～ 0.8 mT) which was assigned to the deprotonated guanine radical cation, {G'(-H)} of a DNA subunit. It is assumed that by the UV irradiation the guanine radical cation, {G⁺}, is generated, either by monophotonic photoionisation or by electron transfer to pyrimidine bases. By rapid transfer of the bridging proton from {G⁺} to the hydrogen bonded cytosine {G'(-H)} is formed. When photolysis of dsDNA was carried out in the presence of H₂0₂, reaction of photolytically generated OH resulted in peroxyl radicals and purine radicals. The oxygen for formation of the peroxyl radicals is probably produced by reaction of {G' (-H)} with H₂0₂. Photolysis of N₂0-saturated solutions containing dsDNA or ssDNA provided another possibility of generation of OH radicals. Under those conditions the OH-induced radicals {1} and {3} were obtained not only from ssDNA but also from dsDNA.     

Anti-hemolytic and Peroxyl Radical Scavenging Activity of Organoselenium Compounds: An In Vitro Study

Selenium-containing amino acids, selenocystine (CysSeSeCys), methylselenocysteine (MeSeCys), and selenomethionine (SeMet) have been examined for anti-hemolytic and peroxyl radical scavenging ability. Effect of these compounds on membrane lipid peroxidation, release of hemoglobin, and loss of intracellular K⁺ ion as a consequence of peroxyl radicals-induced oxidation of human red blood cells were used to evaluate their anti-hemolytic ability. The peroxyl radicals were generated from thermal degradation of 2,2′-azobis(2-methylpropionamidine) dihydrochloride. Significant delay (t _eff) was observed in oxidative damage in the presence of the selenium compounds. From the IC₅₀ values for the inhibition of hemolysis, lipid peroxidation, and K⁺ ion leakage, the relative anti-hemolytic ability of the compounds were found to be in the order of CysSeSeCys > MeSeCys > SeMet. The anti-hemolytic abilities of the compounds, when compared with sodium selenite (Na₂SeO₃) under identical experimental conditions, were found to be better than Na₂SeO₃. Relative rate constants estimated for the reaction of MeSeCys and SeMet with peroxyl radicals by competition kinetics using ABTS²⁻ as a reference confirmed that all the compounds are efficient peroxyl radical scavengers. Comparison of the GPx-like activity of these compounds, by NADPH–GSH reductase coupled assay, indicated that CysSeSeCys exhibits the highest activity. Based on these results, it is concluded that among the compounds examined, CysSeSeCys, possessing the ability to reduce peroxyl radicals and hydroperoxides showed efficient anti-hemolytic activity.     

Preparation and structural characterization of methylmercury(II) complexes of the minor tRNA-base 7-methylguanine

Methylmercury(II) complexes of 7-methylguanine (7mguaH) have been isolated from aqueous solution in the pH range 1-12 and structurally characterized. 1:1 complexes [(7mgua)HgCH₃]·2H₂O and [(7mguaH)HgCH₃][NO₃]· H₂O with respectively N1 - and N9-coordination (X-ray analyses) were obtained from solutions in the respective pH ranges 9–12 and 1–4. A 2:1 complex [(7mgua)(HgCH₃)₂][NO₃] with N1,N9-coordination (X-ray) may be prepared in the intermediate pH range 4–7. Two 3:1 complexes were isolated: [(7mgua)(HgCH₃)₃][NO₃]₂ from strongly acid solution (pH = 1–3), and [(7mguaH₋₁)(HgCH₃)₃][NO₃] in the pH range 7–9. Whereas an X-ray analysis establishes N1,N3,N9-coordination for the former species in the solid state, the ¹H NMR data suggest N2,N3,N9-coordination for the former and N2,N2,N9-coordination for the latter species in d₆-DMSO solution.     

The synthesis and reaction kinetics of some Co(IIl) and Cr(III) chloro complexes of 1,4,8-Triazaoctane (2,3-tri) and the isolation of chiral trans-dichloro [CoCl2(NH3)(2,3-tri)] ClO4

The reaction of 2,3-tri with CrCl₃·6H₂O₁, dehydrated in boiling DMF, results in the formation of mer-CrCl₃(2,3-tri) and anation of hydrolysed solutions of mer-MCl₃(2,3,-tri) (M=Co, Cr) with 6 M HCl containing HClO₄, forms trans-dichloro- mer-[MCl₂(2,3-tri)(OH₂)]ClO₄·H₂O (M=Cr, Co; I, II). trans-Dinitro-mer-[Co(NO₂)₂(NH₃)(2,3-tri)] ClO₄ crystallises from the reaction between mer-Co(NO₂)₃(2,3-tri) and aqueous 7 M ammonia, on addition of NaClO₄·H₂O, and trans-dichloro-mer-[CoCl₂(NH₃)(2,3-tri)]ClO₄ (III) can be isolated by treatment of the dinitro with 12 M HCl. Reaction of mer-CoCl₃(2,3-tri) with C₂O₄₂⁻, followed by addition of aqueous NH₃ and NaClO₄·H₂O results in the isolation of racemic mer-[Co(ox)(NH₃)(2,3-tri)]ClO₄· H₂O. This complex was resolved into its enantiomeric forms and treatment of these with SOCl₂/MeOH/ HClO₄ gave the chiral forms of trans-dichloro-mer- [CoCl₂(NH₃)(2,3-tri)]ClO₄ (R or S at the see-NH center). The rates of loss of the first chloro ligand from these dichloro complexes have been measured spectrophotometrically in 0.1 M HNO₃ over a 15 K temperature range to give the following kinetic parameters; (I) k_H(298)=7.25 × 10⁻⁵ s⁻¹, E_a=78.5 kJ mol⁻¹, δS₂₉₈^#=₋69 J K⁻¹ mol⁻¹; (II) k_H(298)=4.00 × 10⁻³ s⁻¹, E_a=89.9, δS₂₉₈^#= +87.5; (III) k_H(298)=3.09 × 10⁻⁴ s⁻¹, E_a=103, δS₂₉₈^#=+27. Treatment of the dichloro cations with Hg²⁺/HNO₃ results in the generation of mer- M(2,3-tri)(OH₂)₃³⁺ (M=Cr, Co; IV, V) and trans- diaqua-mer-Co(NH₃)(2,3-tri)(OH₂)₂³⁺ (VI). The Co(III) cations isomerise to the fac configuration with (V) K_isom(298) μ=1.0 M)=2.97 × 10⁻⁵ s⁻¹, E_a=115, δS₂₉₈^#=+46. (VI) K_isom(298) (μ=1.0 M)=4.13 × 10⁻⁵ s⁻¹, E_a=113, δS₂₉₈^#=+52.     

Evaluation of the Antioxidant Actions of Ferulic Acid and Catechins

We have evaluated the abilities of ferulic acid, (±) catechin, (+) catechin and (-) epicatechin to scavenge the reactive oxygen species hydroxyl radical (OH±), hypochlorous acid (HOCl) and peroxyl radicals (RO₂).

Ferulic acid tested at concentrations up to 5 mM inhibited the peroxidation of phospholipid liposomes. Both (±) and (+) catechin and (-) epicatechin were much more effective. All the compounds tested reacted with trichloromethyl peroxyl radical (CCl₃O₂) with rate constants > 1 × 10⁶M^?1s^?1.

A mixture of FeCl₃-EDTA, hydrogen peroxide (H₂O₂) and ascorbic acid at pH 7.4, has often been used to generate hydroxyl radicals (OH.) which are detected by their ability to cause damage to the sugar deoxyribose. Ferulic acid, (+) and (±) catechin and (-) epicatechin inhibited deoxyribose damage by reacting with OH. with rate constants of 4.5 × 10⁹M^?1s^?1, 3.65 × 10⁹M^?1s^?1, 2.36 × 10⁹M^?1s^?1 and 2.84 × 10⁹M^?1s^?1 respectively. (-) Epicatechin, ferulic acid and the (+) and (±) catechins exerted pro-oxidant action, accelerating damage to DNA in the presence of a bleomycin-iron complex. On a molar basis, ferulic acid was less effective in causing damage to DNA compared with the catechins.

A mixture of hypoxanthine and xanthine oxidase generates O₂ which reduces cytochrome c to ferrocytochrome c. (+) Catechin and (-) epicatechin inhibited the reduction of cytochrome c in a concentration dependent manner. Ferulic acid and (±) catechin had only weak effects.

All the compounds tested were able to scavenge hypochlorous acid at a rate sufficient to protect alpha-1-antiproteinase against inactivation. Our results show that catechins and ferulic acid possess antioxidant properties. This may become important given the current search for “natural” replacements for synthetic antioxidant food additives.     

Peroxyl radical is produced upon the interaction of hypochlorite with tert-butyl hydroperoxide

As we reported previously, hypochlorite interacting with organic hydroperoxides causes their decomposition ((1995) Biochemistry (Moscow), 60, 1079-1086). This interaction was supposed to be a free-radical process and serve as a source of free radicals initiating lipid peroxidation (LP). The present study is the first attempt to detect and identify free radicals produced in the reaction of hypochlorite with tert-butyl hydroperoxide, (CH₃)₃COOH, which we have used as an example of organic hydroperoxides. We have used a direct method for free radical detection, EPR of spin trapping, and the following spin traps: N-tert-butyl--phenylnitrone (PBN) and -(4-pyridyl-1-oxyl)-N-tert-butylnitrone (4-POBN). When hypochlorite was added to (CH₃)₃COOH in the presence of a spin trap, an EPR spectrum appeared representing a superposition of two signals. One of them belonged to a spin adduct formed as a result of direct interaction of hypochlorite with the spin trap (hyperfine splitting constants were: _H H = 0.148 mT; a_N = 1.537 mT; and H_PP = 0.042 mT for 4-POBN and _H = 0.190 mT; a_N = 1.558 mT; and H_PP = 0.074 mT for PBN). The other signal was produced by hypochlorite interactions with (CH3)3COOH itself (hyperfine splitting constants were: _H = 0.233 mT; aN = 1.484 mT; H_PP = 0.063 mT and _H = 0.360 mT; a_N = 1.547 mT; H_PP = 0.063 mT for 4-POBN and PBN, respectively). Comparison of spectral characteristics of this spin adduct with those of tert-butoxyl or tert-butyl peroxyl radicals produced in known reactions of (CH₃)₃COOH with Fe²⁺ and Ce⁴⁺, respectively, showed that the radical (CH₃)₃COO. is produced from the interaction of hypochlorite with (CH₃)₃COOH^. Like Ce⁴⁺ but not Fe²⁺, hypochlorite addition to (CH₃)₃COOH was accompanied by a bright flash of chemiluminescence characteristic of the reactions in which peroxyl radicals are produced. Thus, all these results suggest peroxyl radical production in the reaction of hypochlorite with hydroperoxide. This reaction is one of the most possible ways for the initiation of free-radical LP that occurs in vivo, when hypochlorite interacts with unsaturated lipids comprising natural protein–lipid complexes, such as lipoproteins and biological membranes.     

Hydroxyl Radical-Induced Reactions in Polyadenylic Acid as Studied by Pulse Radiolysis: Part I. Transformation Reactions of Two Isomeric OH-Adducts

The absorption spectra of polyadenylic acid (polyA) radicals in N₂0 saturated aqueous solution have been measured as a function of time (up to 15 s) following an 0.4μS electron pulse. The spectra and their changes were analysed by comparison with those from monomeric adenine derivatives (nucleosides and nucleotides) which had been studied by Steenken.¹

The reaction of OH^· radicals with the adenine moiety in poly A results in the formation of two hvdroxvl adducts at the positions C-4 [polyA40H^·] and C-8 [polyA80H^·]. Each OH-adduct undergoes a unimol-ecular transformation reaction before any bimolecular or other unimolecular decay occurs. These reactions are characterized by different rate constants and _pH dependencies. The polyA40H^· adduct undergoes a dehydration reaction to yield a neutral N⁶ centered radical (rate constant K_deh= 1.4 × 10⁴s^-1 at ^pH7.3). This reaction is strongly inhibited by H⁺. In comparison with the analogous reactions in adenosine phosphates, the kinetic _pK value for its inhibition is two ^pH units higher. This shift is the result of the counter ion condensation or double-strand formation. The polyA80H^· adduct undergoes an imidazole ring opening reaction to yield an enol type of formamidopyrimidine radical with the resulting base damage (k^r.o. = 3.5 × 10⁴ s ^-1 at pH7.3). This reaction in contrast is strongly catalysed by H⁺and OH^-, similar as for adenosine but different compared to the nucleotides.     

Ideal enantiomeric resolution by recrystallization of a racemic compound (part 4): Relationship between enantiomeric resolution phenomenon and crystal properties

A new example of a racemate showing unusual enantiomeric resolution phenomenon, in which simple recrystallization of the racemate leads to remarkable enantiomeric enrichment of either enantiomer up to 100% ee in the mother liquor, has been found. This compound is (±)-[2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]-ethyl]dimethylsulfonium p-nitrobenzenesulfonate [EtOCH₂CH(OH)CH₂OC₆H₄NHCOCH₂CH₂SMe₂⁺O₂NC₆H₄SO₃⁻] [(±)-SN]. By repeating recrystallization of (±)-SN and the resulting deposited crystals successively and collecting the resulting enantiomerically enriched mother liquors with the same chirality sense, highly efficient enantiomeric resolution of the racemate into its separate enantiomers has been accomplished. The relationship between the occurrence of this enantiomeric resolution phenomenon and the crystal properties has been investigated with respect to SN and its aryl- and alkylsulfonate derivatives. The mode of enantiomeric resolution of (±)-SN was similar to that of para-substituted benzenesulfonate derivatives (±)-ST (4-MeC₆H₄SO₃⁻) and (±)-SC (4-ClC₆H₄SO₃⁻) previously reported, whereas the unsubstituted derivative (±)-SB (C₆H₅SO₃⁻) and alkysulfonate derivatives (±)-SO (n-C₈H₁₇SO₃⁻) and (±)-SM (CH₃SO₃⁻) did not show such an enantiomeric resolution phenomenon. The crystalline form of the former racemates that underwent the enantiomeric resolution was racemic compounds, while the latter were mixed crystals (solid solutions) composed of the respective optical antipodes. Chirality 9:220–224, 1997. © 1997 Wiley-Liss, Inc.     

Characterisation of metal binding sites for 8-azaadenine. Formation and X-ray structural analysis of methylmercury(II) complexes

The compounds [(CH₃Hg)AAdH]NO₃ (1) and [(CH₃Hg)AAd]·4H₂O (2) have been isolated from aqueous 1:1 solutions of CH₃HgOH and 8-azaadenine (AAdH) at respective pH values of 2 and 5. Their structures have been established by X-ray structural analysis. N9 is the metal binding site in both complexes. Alteration of the metal to ligand ratio to 2:1 at a pH of 5 allows the preparation of [(CH₃Hg)₂AAd]NO₃·H₂O (3) in which the base is coordinated at both N3 and N9. The compound [(CH₃Hg)₃AAdH₋₁]NO₃ (4), in which N1, N6 and N9 function as binding sites for the CH₃Hg⁺ cation, is formed in a 3:1 solution at a pH of 6.5. X-ray structural analyses have been performed on 3 and 4. N8 takes part in weak intermolecular secondary bonds to symmetry related Hg9 atoms in all four complexes. The relevance of the structures to an understanding of the basicities of the nitrogen atoms in 8-azaadenine and their alteration upon metal coordination of N9 and N6 is discussed.     

Growth of Methanobacterium thermoautotrophicum on H2CO2: High CH4 productivities in continuous culture

The thermophilic methanogenic bacterium, Methanobacterium thermoautotrophicum, was grown on H₂CO₂. In continuous culture, high CH₄ productivities were obtained (288 litres litre⁻¹ day⁻¹) with 96% CH₄ in the effluent gas, i.e. the productivity was twice as high as that obtained previously by other authors, with pure or mixed cultures; the biomass was 3·6 g dry wt litre⁻¹.     

Thermodynamics of hexokinase-catalyzed reactions.

The enthalpies of the hexokinase-catalyzed phosphorylation or glucose, mannose, and fructose by ATP to the respective hexose 6-phosphates have been measured calorimetrically in TRIS/TRIS HCl buffer at 25.0, 28.5, and 32.0°C. The effects on the measured enthalpy of the glucose/hexokinase reaction due to variation of pH (over the range 6.7 to 9.0) and ionic strength (over the range 0.02 to 0.25) have been examined. Correction for enthalpy of buffer protonation leads to δH^o and δC_p^o values for the processes: eq-D-hexose + ATP⁴⁻ = eq-D-hexose 6-phosphate²⁻ + ADP³⁻+ H⁺. Results are δH^o = −23.8 ± 0.7 kJ · mol⁻¹ and δC_p^o = −156 ± 280 J·mol⁻¹·K⁻¹ for glucose. δH^o = −21.9 ± 0.7 kJ·mol⁻¹ and δC_p^o = 10 ± 140 J·mol⁻¹·K⁻¹ for mannose, and δH^o = −15.0 ± 0.9 kJ·mol⁻¹ and δC_p^o = −41 ± 160 J·mol⁻¹·K⁻¹ for fructose. Combination of these measured enthalpies with Gibbs energy data for hydrolysis of ATP⁴⁻ and that for the hexose 6-phosphates lead to δS^o values for the above hexokinase-catalyzed reactions.