Water thermodynamic activity

In the next example, the effect of water availability (measured as water thermodynamic activity aw) on CALB enantioselectivity was studied in a solid/gas reactor, using the same reaction model as that mentioned above esterification of 2-penta-nol with methyl propanoate [32]. In this case, experimental data showed a pronounced effect of aw on enantioselectivity E value was multiplied by 3 for aw... 

The yield of GOS can also be increased by using highly concentrated starting lactose solutions, decreasing the water thermodynamic activity and modifying the enzyme. Table 19.5 compiles some works using different downstream techniques for the purification of oligosaccharides. 

A useful diagnostic tool for investigating possible hydration of cations of bases for which pA is greater than about one is the measurement of their ultraviolet spectra in aqueous acid solutions and also in an anhydrous acidic solvent such as dichloroacetic acid (for which the Hammett acidity function, Hq, is — 0.9, and in which hydration of the cation cannot occur). This technique has been used with quinazoline to obtain spectra approximating those of the hydrated and anhydrous cations, respectively. For weaker bases, spectral measurements in sulfuric acid-water mixtures of increasing acid content may be used to reveal a progressive conversion of hydrated into anhydrous species as the thermodynamic activity of the water decreases. 

Putting ionic NaCl in the gravy increases the number of ions in solution, each of which can then interact with the water and the solute, which decreases the perceived concentration of solute. In fact, we can now go further and say the thermodynamic activity a represents the concentration of a solute in the presence of interactions. 

Slade, L. and Levine, H. 1985. Intermediate moisture systems concentrated and supersaturated solutions pastes and dispersions water as plasticizer the mystique of bound water thermodynamics versus kinetics (Number 24). Presented at Faraday Division, Royal Society of Chemistry Discussion Conference - Water Activity A Credible Measure of Technological Performance and Physiological Viability Cambridge, July 1-3. 

The amount of water in the reaction mixture can be quantified in different ways. The most common way is to nse the water concentration (in mol/1 or % by volume). However, the water concentration does not give much information on the key parameter enzyme hydration. In order to have a parameter which is better correlated with enzyme hydration, researchers have started to nse the water activity to quantify the amount of water in non-conventional reaction media (Hailing, 1984 Bell et al, 1995). For a detailed description of the term activity (thermodynamic activity), please look in a textbook in physical chemistiy. Activities are often very nselul when studying chemical equilibria and chemical reactions of all kinds, but since they are often difficult to measure they are not used as mnch as concentrations. Normally, the water activity is defined so that it is 1.0 in pure water and 0.0 in a completely dry system. Thus, dilute aqueous solutions have water activities close to 1 while non-conventional media are found in the whole range of water activities between 0 and 1. There is a good correlation between the water activity and enzyme hydration and thns enzyme activity. An advantage with the activity parameter is that the activity of a component is the same in all phases at eqnihbrium. The water activity is most conveniently measnred in the gas phase with a special sensor. The water activity in a liqnid phase can thns be measured in the gas phase above the liquid after equilibration. 

Ideally, thermodynamic activities of the reactants should be used in the equation, but since concentrations are normally easier to measure these are often used instead. The use of the activity of water (which can be measured fairly easily) and the concentrations of the other reactants has been recommended for studies of enzyme catalyzed reactions in organic media (Hailing, 1984). In order to increase the synthesis of the ester, the water concentration (or activity) should be reduced. This can be achieved by replacing part of the water with a water miscible solvent. 

In Equations 2, 4, and 6, ax represents thermodynamic activities based on molar concentrations Cj of the species indicated, y represents mean ionic activity coefficients, i/ha is the activity coefficient of HA(S) molecules, and the activity of water is chosen to be one in all solvents. Consequently values of K, AG°, and AS° are based on these choices regarding standard states. 

With the use of thermodynamic relations and numerical procedure, the activity coefficients of the solutes in a ternary system are expressed as a function of binary data and the water activity of the ternary system. The isopiestic method was used to obtain water activity data. The systems KCl-H20-PEG-200 and KBr-H20-PEG-200 were measured. The activity coefficient of potassium chloride is higher in the mixed solvent than in pure water. The activity coefficient of potassium bromide is smaller and changes very little with the increasing nonelectrolyte concentration. PEG-200 is salted out from the system with KCl, but it is salted in in the system with KBr within a certain concentration range. 

Boyd, B.J., Porter, C.J., and Charman, WN, Using the polymer partitioning method to probe the thermodynamic activity of poorly water-soluble drugs solubilized in model lipid digestion products,... 

From the finding that the 14C-butylparaben flux decreased at increasing lipid content, it was concluded that only 14C-butylparaben dissolved in the water phase contributed to the percutaneous penetration. However, another mechanism may also play a role. An increase in lipid content reduces the thermodynamic activity of butylparaben in the bilayers, which might lead to a decrease in the driving force from the lipid phase to either the water phase or the SC. This may also contribute to a reduced penetration through the skin. 

When water molecules are associated with interfaces such as those provided by membranes or colloidal particles, they have less tendency either to react chemically in the bulk solution or to escape into a surrounding vapor phase. Interfaces thus lower the thermodynamic activity of the water ( w), especially near their surfaces. Solutes also lower the water activity (Eq. 2.10). As a useful first approximation, we can consider that these two effects lowering water activity are additive in a... 

Lipases have been extensively used for the kinetic resolution of racemic alcohols or carboxylic acids in organic solvents. Chiral alcohols are usually reacted with achiral activated esters (such as vinyl, isopropenyh and trichloroethyl esters) for shifting the equilibrium to the desired products and avoiding problems of reversibility. For the same reasons, chiral acids are often resolved by using acidolysis of esters. In both cases, the overall stereoselectivity is affected by the thermodynamic activity of water of water favors hydrolytic reactions leading to a decrease in the optical purity of the desired ester. Direct esterifications are therefore difficult to apply since water formed during the reaction may increase the o of the system, favors reversibiUty, and diminishes the overall stereoselectivity. 

By increasing the concentration of sulfuric acid in water, the thermodynamic activity of the water can be strongly decreased. If the acidity of a solution of a predominantly (or partly) hydrated cation is. progressively increased, the amount of free water remaining in the medium for hydration of the organic molecule is correspondingly decreased, and the spectrum thus becomes more similar to that of the anhydrous cation. In quinazoline, for example, bands present in the 271 and 305 mp r on at pH 7, which are associated with the predominantly anhydrous neutral species, disappear at pH 1 (because the hydrated cation is formed) but reappear on increasing the acidity to Hq — 3.5 (because the anhydrous monocation is formed). A similar effect is observed with quinazoline 3-oxide (see Fig. 4) and 3-nitro- and 8-... 

Lipase-mediated esterification of glycerol and fatty acids is also an equilibrium reaction. The extent of esterification depends on the water content of the medium. The accumulation of water during esterification is a concern because it enhances the hydrolysis of the resultant esters. The water, which is produced during this reaction, may be continuously removed and this is usually accomplished by carrying out the reaction in the presence of molecular sieves (17). A small amount of water, however, is needed in the reaction medium to maintain the activity of the enzyme. The ester concentration at equilibrium is dependent on various medium properties, i.e., the water activity of the reaction system. Usually, a low water activity is necessary to obtain a high ester concentration. Water activity, however, is not the only parameter that determines the equilibrium position. The thermodynamic activities of other reaction variables also are important. 

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