Rate Of Acid Formation Research Articles

Kinetics of chlorination of monochlorodimedone by myeloperoxidase.

The phagocyte-derived enzyme myeloperoxidase has been recently implicated in the pathogenesis of atherosclerosis, because it catalyzes the reaction of hydrogen peroxide with chloride ions to give the highly toxic oxidant hypochlorous acid. The aim of this study was to determine the dependence of this reaction on the concentration of hydrogen peroxide and of the enzyme by means of the photometric monochlorodimedone assay. The initial rate of hypochlorous acid formation increased less than proportionally with increasing myeloperoxidase concentrations. Variation of the concentration of hydrogen peroxide had a biphasic effect, with an optimal concentration of hydrogen peroxide. Above this concentration enzyme destruction is apparently predominant. The progress curves of hypochlorous acid formation showed two distinct maxima. It was concluded that hypochlorous acid not only reacts with monochlorodimedone but also with the amino groups of myeloperoxidase to form intermediary chloramines that may further chlorinate monochlorodimedone. This was supported by the kinetics in the presence of the amino compound glycine, a competitive substrate for chlorination by hypochlorous acid. In the presence of high concentrations of glycine the progress curve rises continuously, yielding a greatly increased concentration of chlorinating species, either hypochlorous acid or chloramines. We concluded that glycine protects myeloperoxidase against hypochlorous acid-induced self-destruction.

The influence of glucose concentration on citric acid production and morphology of Aspergillus niger in batch and culture

Studies in conventional batch culture confirmed that initial glucose concentration in the fermentation medium affected both the rate of citric acid fermentation by Aspergillus niger and the morphology of the producer micro-organism. A series of fed-batch (glucostat) experiments, in which the concentration of glucose was kept constant whereas all other conditions were allowed to change, were used to eliminate the effects of decreasing sugar concentration found during conventional batch experiments. The level of glucose had a marked effect on production rates; the specific rate of citric acid formation increased with increasing initial glucose concentration (batch culture) or glucose levels (glucostat culture), and in both culture methods, the specific growth rate increased with decreasing glucose concentration for the first 48 h of fermentation. The reduction observed in the mean length of filaments at low glucose levels could be explained by increased branching frequency as a result of the increasing specific growth rate at the early stages of fermentation. The size of the mycelial clumps was reduced at low glucose levels and their shape was also affected as the ratio between the perimeter of the clump and the perimeter of its core increased as glucose concentration decreased. Very low glucose levels during fermentation in glucostat culture had a striking effect on the metabolism of the micro-organism because the specific production rate decreased in favor of the specific growth rate and the morphology changed with a doubling of the size of clumps and pellet formation. The main morphological observations in both batch and glucostat cultures seemed to be related to the growth rate, which is restricted by the glucose concentration, and, thus, indirectly related to the concentration of glucose in the fermentation media. The glucose levels in the medium were shown to lead to a significant noncatalysed entry into the mycelium, which accounted for the citric acid productivity and growth rate changes.

Boric Acid Self-Lubrication of B2O3-Filled Polymer Composites

Utility of boric oxide particles in PTFE and epoxy composite materials, in sliding contact with stainless steel, is explored. Boric oxide filler can provide PTFE with a two-decade reduction in wear rate, to 10−5 mm3/N-m. With adequate ambient humidity reduced wear rate can be achieved without inducing counterface abrasion, and the friction of PTFE is further reduced slightly. In such environments, boric oxide fillers can also reduce friction coefficient of epoxy from μ>0.7 to as low as μ=0.07. This lubrication mechanism results from replenishment of lubricous boric acid lamellar solid provided to the sliding interface by reaction of boric oxide with ambient water. Maintenance of the lubricating effect depends upon a sufficient rate of boric acid formation, relative to subsequent removal by wear. It is demonstrated that this formation/removal balance is affected by relative humidity and volume fraction of boric oxide filler, as well as normal load and sliding speed. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998

Optimising glucose conversion in the production of reduced alcohol wine using glucose oxidase

The use of a glucose oxidase (GOX)-catalase enzyme system for the production of reduced-alcohol white wine is investigated. The process reduces alcohol potential by converting glucose to gluconic acid. Trials were conducted with grape juice and model solution to determine key factors affecting the activity of the GOX system, and to optimise the process for use with grape juice. Under our processing conditions, the low pH of grape juice was found to be a dominant limiting factor in the rate and extent of glucose conversion by GOX. An optimised process for glucose conversion (up to 87%) was developed after investigation of the effects of enzyme dosage, sparging, aeration and mixing rates, and temperature. Maximum GOX activity was observed during the first 4 to 6h of treatment, after which a significant decrease in the rate of gluconic acid formation and glucose degradation occurred. An expected increase in titratable acidity and concurrent decrease in pH during GOX treatment was also observed, and is attributable to an increase in the juice gluconic acid concentration of ca 73g/l.

Article

The complex interaction of softwood residual kraft lignin with oxygen at elevated temperatures, pressures, and pH's was investigated by isolating and systematically oxidizing the lignin, as a function of time and temperature. The isolation of the oxidized lignin was carried out using a multistep procedure developed specifically to recover all species. Detailed quantitative 31P NMR measurements provided, for the first time, three-dimensional plots describing the formation and (or) elimination of the various functional groups. Oxidative demethoxylation reactions were found to induce the formation of catechols, which are, most likely, reactive intermediates of the oxidation. The rate of carboxylic acid formation and guaiacyl phenol elimination was found to follow two distinct phases: a rapid initial phase followed by a slower phase. More specifically, the rate of carboxylic acid group formation was found to dramatically increase as the reaction temperature increased, analogous to the profiles obtained for the elimination of guaiacyl phenolic units. At reaction temperatures typical of conventional commercial oxygen delignification installations (80-100°C), only minor oxidation was found to occur within the lignin. At these temperatures, increasing the reaction time did not significantly aid the formation of carboxylic acid groups on lignin despite the idealized two-phase homogeneous conditions. The technological ramifications of these data imply that a three-phase oxygen delignification system, operating at temperatures below 100°C, causes only minor oxidative changes to the structure of the lignin in the fibre. It was also possible to isolate the effect of alkali and temperature on the structure of residual kraft lignin through experiments in which pressurized oxygen was replaced by nitrogen. These data revealed a concealed set of alkali-induced condensation and fragmentation reactions.Key words: bleaching, catechols, carboxylic acids, delignification, guaiacyl, kinetics, oxygen, phenols, phosphorus, nuclear magnetic resonance (nmr), residual kraft lignin.

Investigating the Photo-Oxidation of Wool Using FT-Raman and FT-IR Spectroscopies

Fourier transform infrared and Fourier transform Raman spectroscopies are used in combination to investigate the photo-oxidation of wool and the protective effect of the uv absorber Cibafast W. Extended light exposure decreases the intensity of the S-S and C-S bands due to cystine and also reduces the intensity of the C-C stretching band, which is mainly attributed to the α-helix. A "new" band at 977 cm-1 in the Raman spectrum increases with exposure time, but its nature is uncertain. Exposure to light also produces changes in the chemical environment of tyrosine residues. Cibafast W decreases the rate of disulphide bond fission and cysteic acid formation, being most effective at low levels of light exposure. This effect is confirmed by measurements of the rate of photoyellowing and phototendering of wool fabrics.

Kinetic Model for Methacrolein OxidationInfluence of Cesium and Vanadium on Heteropolyacid Catalysts CsxH3-x+y[PMo12-yVyO40

A kinetic model has been developed for the heterogeneously catalyzed methacrolein oxidation, the parameters of which reflect the redox and adsorptive properties of the heteropolyacid catalysts CsxH3-x+y[PMo12-yVyO40] (x = 0, y = 0, 1; x = 1, y = 0, 1, 2). The model based on the Mars−van Krevelen mechanism describes the reaction rates of methacrolein and oxygen consumption as well as the rate of methacrylic acid and byproduct formation as a function of the methacrolein and oxygen partial pressures. Substitution of Mo by V causes a strong decrease of the ratio of reaction rate constants of reoxidation and methacrolein oxidation. The substitution of Mo by V leads to a strong decrease of the rate of consecutive methacrylic acid oxidation. Salification of the free acid by cesium provides a decrease of byproduct formation via parallel and consecutive oxidation and also a reduction of the hindrance of catalyst reoxidation caused by strong adsorption of methacrolein.

Fundamentals of o×ygen delignification. Part II. Functional group formation/elimination in residual kraft lignin

The complex interaction of softwood residual kraft lignin with oxygen at elevated temperatures, pressures, and pH's was investigated by isolating and systematically oxidizing the lignin, as a function of time and temperature. The isolation of the oxidized lignin was carried out using a multistep procedure developed specifically to recover all species. Detailed quantitative 31 P NMR measurements provided, for the first time, three-dimensional plots describing the formation and (or) elimination of the various functional groups. Oxidative demethoxylation reactions were found to induce the formation of catechols, which are, most likely, reactive intermediates of the oxidation. The rate of carboxylic acid formation and guaiacyl phenol elimination was found to follow two distinct phases: a rapid initial phase followed by a slower phase. More specifically, the rate of carboxylic acid group formation was found to dramatically increase as the reaction temperature increased, analogous to the profiles obtained for the elimination of guaiacyl phenolic units. At reaction temperatures typical of conventional commercial oxygen delignification installations (80-100°C), only minor oxidation was found to occur within the lignin. At these temperatures, increasing the reaction time did not significantly aid the formation of carboxylic acid groups on lignin despite the idealized two- phase homogeneous conditions. The technological ramifications of these data imply that a three-phase oxygen delignification system, operating at temperatures below 100°C, causes only minor oxidative changes to the structure of the lignin in the fibre. It was also possible to isolate the effect of alkali and temperature on the structure of residual kraft lignin through experiments in which pressurized oxygen was replaced by nitrogen. These data revealed a concealed set of alkali-induced condensation and fragmentation reactions.

Separation of Fermentation Products by Membrane Techniques. II. Conversion of Lactate to Lactic Acid by Electrodialysis

ABSTRACT The yield of an electrodialysis unit operating for the conversion of lactate to lactic acid (LA) is depressed by low conductivity of the lactic acid solution. Experiments performed with a laboratory electrodialysis unit (ED) designed to establish the limiting current density ilim for the whole unit and for each of the membrane/solution elements revealed high voltage drops for membranes in contact with the lactic acid solution. By filling the LA compartment with a cation-exchange resin as the conducting turbulence promoter, it was possible to increase ilim from 2.7 ± 0.3 to 18.3 ± 2.0 mA/cm2. With a resin suspension the rate of lactic acid formation was 6.0 molâ2 â1, the current efficiency amounted to 92%, and the energy consumption was 3.3 kWgâ1. A hypothesis for the 7-fold increase of ilim by filling the LA compartment with a grained conducting resin is presented.

A Mathematical Model of the Influence of Salivary Urea on the pH of Fasted Dental Plaque and on the Changes Occurring during a Cariogenic Challenge

Urea diffusing from saliva into dental plaque is converted to ammonia and carbon dioxide by bacterial ureases. The influence of normal salivary urea levels on the pH of fasted plaque and on the depth and duration of a Stephan curve is uncertain. A numerical model which simulates a cariogenic challenge (a 10% sucrose rinse alone or one followed by use of chewing-gum with or without sugar) was modified to include salivary urea levels from 0 to 30 mmol/l. It incorporated: site-dependent exchange between bulk saliva and plaque surfaces via a salivary film; sugar and urea diffusion into plaque; pH-dependent rates of acid formation and urea breakdown; diffusion and dissociation of end-products and other buffers (acetate, lactate, phosphate, ammonia and carbonate); diffusion of protons and other ions; equilibration with fixed and mobile buffers; and charge-coupling between ionic flows. The K_m (2.12 mmol/l) and V_max (0.11 μmol urea/min/mg dry weight) values for urease activity and the pH dependence of V_max were taken from the literature. From the results, it is predicted that urea concentrations normally present in saliva (3–5 mmol/l) will increase the pH at the base of a 0.5-mm-thick fasted plaque by up to 1 pH unit, and raise the pH minimum after a sucrose rinse or sugar-containing chewing-gum by at least half a pH unit. The results suggest that plaque cariogenicity may be inversely related to salivary urea concentrations, not only when the latter are elevated because of disease, but even when they are in the normal range.

Growth of Boric Acid Crystallites on the Surface of Boron‐Doped Silicon Carbide Samples

White crystallites were visually observed on fractured or polished surfaces of SiC samples (grain sizes below ∼500 nm) during exposure to air at room temperature for several days. Characterization of the crystallites by scanning electron microscopy, secondary ion mass spectroscopy, and X‐ray diffraction identified B(OH)3 crystals with a strong (002) texture. The rate of boric acid formation was determined by a gravimetric experiment. The rate of weight gain increased significantly after an incubation period of 1 week. Nucleation is initially the rate‐limiting process. Subsequently small B(OH)3 crystals form on the surface, whose growth rate is determined by grain boundary diffusion of boron to the SiC surface. An estimated grain boundary diffusion coefficient of boron in SiC was many orders of magnitude higher than extrapolated literature values.

Biochemical Characterization of the Human Liver Cytochrome P450 Arachidonic Acid Epoxygenase Pathway

Human liver microsomal fractions metabolized arachidonic acid in the presence of NADPH yielding epoxyeicosatrienoic acids and their hydration products, dihydroxyeicosatrienoic acids, as the principal reaction products. Inhibition studies using polyclonal antibodies prepared against recombinant CYP2C8, an abundant human liver cytochrome P450 epoxygenase, demonstrated 85–90% inhibition of arachidonic acid epoxide formation. Both epoxyeicosatrienoic acids and dihydroxyeicosatrienoic acids were detected in large amounts in human liver using gas chromatography/mass spectrometry. Chiral analysis of the endogenous liver epoxides demonstrated a preference for the 14(R),15(S)-, 11(R),12(S)-, and 8(S),9(R)-epoxyeicosatrienoic acid enantiomers. Importantly, the chirality of liver epoxyeicosatrienoic acids matched that previously reported for recombinant CYP2C8 (Zeldinet al.(1995)Arch. Biochem. Biophys.322, 76–86). Incubations of both human liver cytosolic and microsomal fractions with synthetic epoxyeicosatrienoic acids revealed a 10-fold higher rate of dihydroxyeicosatrienoic acid formation with cytosolic relative to microsomal fractions. Recombinant human liver cytosolic epoxide hydrolase rapidly and regioselectively hydrated epoxyeicosatrienoic acids. We conclude, based on these data, that CYP2C8 is one of the primary, constitutive hepatic arachidonic acid epoxygenases responsible for formation of epoxyeicosatrienoic acids and that cytosolic epoxide hydrolase is the principal liver enzyme which forms the dihydroxyeicosatrienoic acids. We speculate that these biologically active eicosanoids may be important in maintaining homeostasis in the liver.

Effect of container smoking and incubation temperature on the microbiological and some biochemical qualities of fermenting Ergo, a traditional ethiopian sour milk

Raw milk was allowed to sour naturally at ambient temperature (25 °C) in smoked or unsmoked containers. Milk in smoked containers had a lower rate of pH drop and the fermented product had good flavour for a longer time after coagulation. The total count of non-lactic acid bacteria in milk in unsmoked containers reached a high count (> 108 cfu mL−1) within 12 h, whereas milk in smoked containers required more than 24 h to reach this level. Similarly, the growth of coliforms and lactic acid bacteria was slow in milk in smoked containers, thus assuring good and slow development of flavour components, safety of finished product and better keeping qualities. Lacto-bacilli dominated the flora of the fermented product in unsmoked containers, while lactococci were equally dominant in fermented milk in smoked containers. As the temperature of incubation increased (20 to 42 °C), the rate of pH drop also increased and the time of coagulation became shorter. At lower temperatures, the rate of acid formation was lower but the titratable acidity in the final products was higher. Milk incubated at lower temperatures had a better Ergo taste. The rate of growth of the various groups of bacteria increased with an increase in incubation temperature. At lower incubation temperatures, lactococci dominated the lactic flora, while lactobacilli dominated at higher incubation temperatures. Smoking of containers may help to produce a safer and tastier Ergo with better keeping qualities. In addition, lower incubation temperatures (around 20 °C) may favour the proliferation of lactic acid bacteria and thus guarantee a desirable fermentation.

Variable activation of lovastatin by hydrolytic enzymes in human plasma and liver

Lovastatin, widely used to lower cholesterol, is a pro-drug that requires metabolic activation through hydrolysis by carboxyesterases. There appear to be at least three distinct esterases in humans capable of catalysing this reaction, one in plasma and two in the liver. The rate of lovastatin hydroxy acid formation was measured as 15.8 pmol.ml-1.min-1 in plasma, 2.13 pmol.mg-1 protein.min-1 in hepatic microsomes and 0.92 pmol.mg-1 protein.min-1 in cytosol. The data suggest that on average the three esterases together are capable of activating about 220 nmol (90 micrograms) lovastatin per minute per person, to which the esterases of plasma, liver microsomes and liver cytosol contribute approximately 18, 15 and 67%, respectively. All three esterases showed evidence of inter-individual variability. In one of 17 livers, both cytosolic and microsomal esterase activity was completely missing, while two other liver specimens lacked one esterase. Such variability must be expected to influence the therapeutic efficacy of the drug, and they might be related to its occasional toxicity.

The Free Fatty Acid Content of Fish Oil, Part IV: Rates of Free Fatty Acid Formation from Phospholipids and Neutral Lipids in Anchovy (Engraulis capensis) Stored at Various Temperatures

AbstractAnchovy were stored at 17°C, 8°C, 0°C and −6°C. Samples were withdrawn periodically and the extracted lipids analysed for free fatty acids (FFA) and phosphorus (P) content. From the results of these analyses it was possible to calculate the rates of enzymatic hydrolysis of the phospholipids and the neutral lipids. Phospholipids hydrolysed much faster than neutral lipids at the four temperatures. An Arrhenius plot showed that at about –29°C the two rates were identical.

The effect of fermentation conditions on the nutrients and acceptability of tarhana

A laboratory method was evaluated for the production of tarhana, a popular Turkish fermented wheat-yogurt mixture. The changes in pH, the total titratable acidity and the vitamin content (thiamine, riboflavin and vitamin B12) of tarhana of different formulations (type of wheat flour, amount of yogurt and presence of salt) were monitored during fermentation. Chemical composition and acceptability of laboratory-produced tarhana were compared with those of authentic homemade and commercially produced tarhana. The pH and titratable acidity of tarhana samples did not change after the third day in the course of a 4-day fermentation. The final pH and acidity (expressed as percent lactic acid) of tarhana were found to be in the range of 4.3–4.8 and 1.8–2.3 (dry basis), respectively. The thiamine, riboflavin and vitamin B12 contents of tarhana did not change considerably during fermentation. The addition of salt to tarhana lowered the acid formation rate during fermentation, leading to a higher pH. The replacement of white wheat flour with wholemeal flour resulted in an expected increase in the protein and vitamin content of tarhana; however, a decrease in overall acceptability was observed.

Effect of glucose concentration on citric acid production by Yarrowia lipolytica

AbstractCitric acid production by Yarrowia lipolytica ATCC 20346 was studied in a laboratory‐scale fermenter and schematically subdivided into three different phases, appropriately characterised by unstructed kinetic models. As the sugar level in the growth medium was increased from 40 to 108 g dm−3; a negative effect on the kinetic growth rate constant of the trophophase was obtained, where both the yield coefficients for glucose and nitrogen were found to be approximately constant. By using a production medium devoid of nitrogen, a citrate lag phase was observed whilst the yeast reduced its intracellular nitrogen fraction from 7–8% (w/w) to a new equilibrium value as low as 2·3–4·4%. Finally, the idiophase was found to be a non‐growth associated process, the citric acid formation rate being dependent on the cellular nitrogen concentration only. The yield factors for citric, isocitric and fumaric acids appeared to be independent of the initial concentration of glucose in the production medium (Sop). However, the yield factors for α‐ketoglutaric, malic, pyruvic and cis‐aconitic acids exhibited a negative correlation with respect to Sop, being virtually equal to zero for Sop values greater than 160–175 g dm−3. The strain in question was capable of equalling the best mutant strains of Aspergillus niger used industrially in citric acid productivity (ca. 1·05 g dm−3 h−1), but not in selectivity, since citric acid represented only 85·5% of total citric and isocitric acid and 83·7% of the total acid.

Malate as Additional Substrate for Fatty Acid Synthesis in a C 4-Plant Type Developed by Salt Stress from a C 3-Piant Type Maize. A Screening for Malate as Substrate for Fatty Acid Synthesis in Chloroplasts

Summary By changing a Zea mays variety (cv. Pirat) from a C3-plant to a C4-plant type by salt stress with sodium chloride the capacity to form fatty acids from malate was induced with the appearance of the NADP+-malic enzyme. The reaction catalyzed by the chloroplast pyruvate dehydrogenase complex was the rate limiting step in the reaction sequence. This could be shown by comparing the rates of fatty acid formation from malate with those from pyruvate. However, the cytologically visible chloroplast dimorphism in mesophyll and bundle-sheath cells was already developed at the C3-plant stage and persisted in the C4-plant stage. On the other hand, the formation of fatty acids from malate in barley seedlings was very poor compared to that from acetate. Although the activity of the chloroplast pyruvate dehydrogenase complex in this species was comparatively high (Preiss et al., 1993; Hoppe et al., 1993), the activity of the NADP+-malic enzyme was negligible.

Unique Temperature Dependence of Acetic Acid Formation in CO2 Hydrogenation on Ag-promoted Rh/SiO2 Catalyst

Abstract The rate of acetic acid formation in CO2 hydrogenation on Ag-Rh/SiO2 catalyst took two maxima at 463 K and 553 K. At 463 K, the rate was much faster than that of C2-oxygenates formation in CO hydrogenation. It was suggested that, at lower temperature, acetic acid is formed through the direct incorporation of adsorbed CO2.

Activation and desensitization of phospholipase D in intact rat hepatocytes.

Activation of phospholipase D (PLD) by receptor-coupled stimuli (vasopressin, ATP), phorbol esters, and Ca2+ ionophores was studied in isolated rat hepatocytes, double labeled with [3H]arachidonate and [14C]stearate. Phosphatidylethanol (Peth) was formed when cells were stimulated in the presence of ethanol. The effect of combinations of agonists was not additive, indicating that the same PLD isozyme(s) were activated. With all agonists, the 3H- and 14C-specific radioactivity in Peth was higher than in any of the main phospholipid classes. The 3H/14C ratios of Peth and phosphatidylcholine (PC) were identical and differed from other phospholipid classes, indicating that the predominant PLD substrate was a PC pool labeled preferentially with radioactive fatty acids. Ethanol (50-300 mM) decreased the initial rate of phosphatidic acid (PA) formation, but did not affect total PLD activity. Agonist-induced changes in steady state accumulation of PA or 1,2-diacylglycerol were also unaffected. A slow degradation of Peth (apparent t1/2 > 60 min) occurred after ethanol removal from cells prestimulated with vasopressin. The rate of degradation was unaffected by agonists that stimulate PLD. Thus, Peth formation is a suitable cumulative indicator for PLD activation in intact hepatocytes. Peth accumulation declined over a period of 5-20 min, depending on the agonist. The decline was not due to increased Peth degradation, or limitations in substrate supply to PLD, or enzyme inhibition by accumulated Peth. Instead, a homologous desensitization of PLD occurs with all agonists. This desensitization may involve the action of selective protein kinase C isozymes.