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Query: UMLS:C0847097 (acidity)
 15,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Acid-base physiology is concerned with sources, extent, and control of hydrogen ion donation in the body, at the organ-physiological as well as the molecular level of study. With the introduction of Van Slyke's methods for quantitative carbon dioxide measurements in biological fluids, one important source of hydrogen ion donation became identifiable; and these and derived methods have permitted of fairly precise quantitative descriptions of transport and pulmonary elimination of carbon dioxide. However, the inevitable operational concept of non-carbonic (non-volatile) contributions to the titratable acidity of the body fluids has been a cause of considerable methodological and conceptual difficulties; and whereas it is now possible by means of the micro-equilibration technique to make accurate assessments of the concentration of non-volatile titratable acid (base) in blood, the question of the physiological relevance of the concept of 'base excess' remains open. In particular, the concept of non-carbonic acid does not possess a specific relevance with respect to the acid-base physiology of kidney, bone, and gastro-intestinal tract comparable to the 'substrate-specificity of carbon dioxide with respect to the lung. Our studies indicate that a subdivision of the titratable non-carbonic acid of any biological medium in two subcomponents will provide an improvement of specificity, adequate for a system physiological approach at the organ level. Thus, a distinction should be made between (1) processes of hydrogen ion donation, reversible by endogenous metabolic means (quantitated in terms of the component MA = metabolizable non-carbonic acid) and (2) processes of hydrogen ion donation associated with gastro-intestinal, skeletal, and renal transport, storage, and control of non-metabolizable non-carbonic acid (NA). Some implications of this distinction for acid-base physiology and acid-base diagnostics are discussed.
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PMID:Physiological viewpoints on clinical acid-base diagnostics. 1 79

Calculus may be considered as an aggregate of calcified deposits or deposits that are going to calcify in the oral cavity. From a topographical point of view calculus is classified in supragingival and subgingival calculus. Calculus is composed by inorganic (70-80%) and organic (20-30%) components. Calculus results from calcification of plaque and epithelial cells exfoliated from oral mucosae. Calcification phenomena (carbonic acid theory) are caused by a rapid fall down of salivary acidity when saliva springs out gland ducts. In fact in the mouth there is a lower pressure of carbon dioxide than in the gland ducts. From this fact results that calcium bicarbonate (dissolvable) becomes calcium carbonate (undissolvable) that forms, starting from nucleating particles, calcium carbonate crystals. Then calculus is one of the most important cofactors in the etiology and pathogenesis of periodontal disease: a) it favors plaque growth and stabilizes it to dental and periodontal tissues; b) it favors retention of food debris and hinders dental cleaning; c) it has endotoxins and lets them free slowly; d) it hinders periodontal recovery. From this study it results that scaling and root planing are one of the first steps in periodontal therapy.
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PMID:[Tartar and periodontal disease--a cofactor in etiopathogenesis]. 186 21

The apparent first dissociation constant of carbonic acid has been defined in different ways in the literature. Harned and co-workers (8-10) have defined it in terms of molalities of the participating species, including H ions: Ks = mHmHCO3/mCO2. In contrast, Hastings and Sendroy have defined an apparent constant in which acidity is expressed as H ion activity: K'1 = aHmHCO3/mCO2. These constants differ by a factor gamma H, the activity coefficient of H ions at the prevailing ionic strength. Therefore, pK'1 is greater than pKs by an amount equal to -log gamma H, which, at mu = 0.16 M, is approximately 0.1. It is important that the correct value for the apparent dissociation constant or its logarithmic form be entered in the mass action expression or in the Henderson-Hasselbalch equation in order to prevent significant errors in the computation by means of these equations of quantities that cannot be directly measured. Specifically, for the derivation of bicarbonate concentration from PCO2 and pH (-log aH), pK'1 is to be used and not an uncorrected pKs.
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PMID:Which value for the first dissociation constant of carbonic acid should be used in biological work? 190 96

31P MRS studies have shown that the intracellular compartment ot tumours is kept near neutrality, whereas the interstitial fluid is acidic (pH 6.5-6.8). Why is this compartment acidic? Balance studies confirm that tumours produce excessive lactic acid, although less than usually supposed, but this cannot be the whole story, since Tannock and co-workers have shown interstitial acidity in glycolysis-deficient tumours. Another major acid load is caused by hydration of CO2 molecules to carbonic acid, catalysed by carbonic anhydrase. The distance that H+ must diffuse from cancer cells to capillaries is further than in normal tissue and this will increase acidification near the cells. We show that previous quantitative models based on simple H+ diffusion are unsatisfactory. This is because most H+ ions cross the interstitial space bound to buffers such as inorganic phosphate. Although these protonated buffers (i.e. conjugate acids) diffuse much more slowly than H+ ions they carry most of the protons, so the pH predicted by this model is closer to neutrality for a given proton production rate than that predicted by the dissolved H+ model. We have developed a mathematical model of this carrier-mediated system that predicts pHe values as low as those observed in some tumours.
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PMID:Why are cancers acidic? A carrier-mediated diffusion model for H+ transport in the interstitial fluid. 1172 36

Fermentation of lactose in whey permeate directly into ethanol has had only limited commercial success, as the yields and alcohol tolerances of the organisms capable of directly fermenting lactose are low. This study proposes an alternative strategy: treat the permeate with acid to liberate monomeric sugars that are readily fermented into ethanol. We identified optimum hydrolysis conditions that yield mostly monomeric sugars and limit formation of fermentation inhibitors such as hydroxymethyl furfural by caramelization reactions. Both lactose solutions and commercial whey permeates were hydrolyzed using inorganic acids and carbonic acid. In all cases, more glucose was consumed by secondary reactions than galactose. Galactose was recovered in approximately stoichiometric proportions. Whey permeate has substantial buffering capacity-even at high partial pressures (>5500 kPa[g]), carbon dioxide had little effect on the pH in whey permeate solutions. The elevated temperatures required for hydrolysis with CO2-generated inhibitory compounds through caramelization reactions. For these reasons, carbon dioxide was not a feasible acidulant. With mineral acids reversion reactions dominated, resulting in a stable amount of glucose released. However, the Maillard browning reactions also appeared to be involved. By applying Hammet's acidity function, kinetic data from all experiments were described by a single line. With concentrated inorganic acids, low reaction temperatures allowed lactose hydrolysis with minimal by-product formation and generated a hexose-rich solution amenable to fermentation.
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PMID:Hydrolysis of lactose in whey permeate for subsequent fermentation to ethanol. 1545 74

Generally recognised as save compounds (G.R.A.S) are attractive substitutes to synthetic chemicals in postharvest control diseases. They meet safety requirements, are cheap and able to be integrated with other disease control technologies. Among G.R.A.S compounds, carbonic acid salts have been investigated on carrots, bell pepper, melons, sweet cherries and their efficacy was also evaluated when combined with biological control agents. Moreover, the possibility to use sodium carbonate and sodium bicarbonate to prevent P. digitatum an P. italicum spread on Citrus fruit was studied since the begin of the 20th century. We explored the possibility to extend the use of carbonate-bicarbonate salts on loquat fruit in order to control the pathogens and to extend postharvest life. Loquat is a very perishable fruit, susceptible to decay, mechanical damage, moisture and nutritional losses during its postharvest life. We tested the combined effect of temperature and sodium or potassium carbonate-bicarbonate and ammonium carbonate. The fruit was dipped in the salt solutions at variable concentrations (0.5, 1 and 2% w/v) at 25 or 45 degrees C for two minutes and than stored under shelf life conditions (25 degrees C and 70% RH). Decay, weight loss, pH, titrable acidity and sugar content were detected after twelve days. Preliminary data show that the combined treatments were effective in decay control depending on salts. Best results were obtained with 2% potassium and sodium carbonate solution at 25 degrees C. Weight losses were related to treatment temperature and salts concentrations whereas, no differences were detected in the chemical parameters compared to the control.
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PMID:Carbonic acid salts at 25 or 45 degrees C to control loquat decay under shelf life conditions. 1663 1

We conducted an experimental investigation into the kinetics and mechanism of tetrahydrofuran synthesis from 1,4-butanediol via dehydration in high-temperature liquid water (HTW) without added catalyst at 200-350 degrees C. The reaction was reversible, with tetrahydrofuran being produced at an equilibrium yield of 84% (at 200 degrees C) to 94% (at 350 degrees C). The addition of CO2 to the reaction mixture increased the reaction rate by a factor of 1.9-2.9, because of the increase in acidity resulting from the formation and dissociation of carbonic acid. This increase was much less than that expected (factor of 37-60) from a previously suggested acid-catalyzed mechanism. This disagreement prompted experiments with added acid (HCl) and base (NaOH) to investigate the influence of pH on the reaction rate. These experiments revealed three distinct regions of pH dependence. At high and low pH, the dehydration rate increased with increasing acidity. At near-neutral pH, however, the rate was essentially insensitive to changes in pH. This behavior is consistent with a mechanism where H2O, in addition to H+, serves as a proton donor. This work indicates that the relatively high native concentration of + (large KW), which has commonly been thought to lead to the occurrence of acid-catalyzed reactions in HTW without added catalyst, does not explain the dehydration of 1,4-butanediol in HTW without catalyst. Rather, H2O serves directly as the proton donor for the reaction.
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PMID:Kinetics and mechanism of tetrahydrofuran synthesis via 1,4-butanediol dehydration in high-temperature water. 1687 9

Water autoionization reaction 2H2O --> H3O- + OH- is a textbook process of basic importance, resulting in pH = 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H3O+ and OH- ions, density functional studies of (H2O)(48)H+ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O+ does, but OH- does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.
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PMID:Water surface is acidic. 1745 50

The determinations are based on potentiometric titration data (volume of reagent, potential). To avoid any systematic error, the acidity constants are refined together with every other significant parameter (initial protolyte concentration, concentration of impurities, electrode chain characteristics ...) with the program MUPROT. Comparison of the multiparametrically adjusted results with those obtained by the usual methods reveals noticeable discrepancies. Particular attention is paid to carbonate as impurity; to take it into account, modified expressions for the titrant volume are established. The current of nitrogen used in the classical methods shifts the liquid-gas equilibrium of carbonic acid; in that case, only the approximate equations are to be considered. Working with a closed system is proposed as a more rigorous procedure.
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PMID:[Not Available]. 1896 39

Acid-catalyzed decarboxylation reactions of carboxylic acids should avoid formation of protonated carbon dioxide, a very high energy species. A potential alternative route parallels ester hydrolysis, with addition of water to the carboxyl group followed by protonation of the unsaturated leaving group and formation of protonated carbonic acid, a species that had been predicted to be a viable reaction intermediate. The hydrolytic mechanism for the decarboxylation of pyrrole-2-carboxylic acid is consistent with observed (12)C/(13)C kinetic isotope effects (1.010 +/- 0.001 at H(0) = -0.01 and 1.043 +/- 0.001 at H(0) = -2.6), solvent kinetic isotope effects (k(H(2))(O)/k(D(2))(O) = 2 at H(0) = 0.9; k(H(2))(O)/k(D(2))(O) = 1 at H(0) = -2.9), and activation parameters [DeltaH() = 23.5 kcal.mol(-1) and DeltaS() = 5.5 cal.deg(-1).mol(-1) at H(0) = -2.9]. Thus, the specific route for a decarboxylation process is a consequence of the nature of the potential carbanion (or its conjugate acid), the acidity of the medium and avoidance of formation of protonated carbon dioxide.
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PMID:Hydrolytic decarboxylation of carboxylic acids and the formation of protonated carbonic acid. 2012 Nov 87


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