Thermodynamic Water Activity Research Articles

Use of salt hydrates for controlling activity of tyrosinase in organic solvents

Three procedures were employed to identify salt hydrates which were efficient water donors to dry tyrosinase suspended in a substrate solution in chloroform or toluene. Three salts (Na2SO4·10H2O, Na2HPO4·12H2O and Na2CO3·10H2O) were effective activators whereas eleven others were ineffective. It was concluded that the thermodynamic water activity (aw) of the hydrate was the major determinant controlling enzyme activity.

Effect of thermodynamic water activity on amino-acid ester synthesis catalyzed by agarose-chymotrypsin in 3-pentanone

Chymotrypsin linked to agarose beads by multi-point covalent attachment catalyzes synthesis of Ac-Trp-OEt in 3-pentanone even when the thermodynamic water activity ( a w) of the system is reduced to as low as 0.4. If fully hydrated catalyst is added to the reaction mixture before removal of water, product is formed linearly once a w has stabilized. The initial rate is reduced from that if a w is kept close to 1 (0.47 mmol s −1 (kg enzyme) −1), to 50% ( a w 0.9), 25% ( a w 0.4) and <1% ( a w 0.25). The large drop between a w of 1 and 0.9 probably reflects the effects of water removal on the agarose gel structure. Catalyst partly dried (even only to a w 0.86) before adding to the organic phase is inactive. At reduced a w, the equilibrium (when reached) is shifted in favor of the ester, as expected.

Effects of the Support Material and the Thermodynamic Water Activity on Enzymatic Synthesis in Organic Media a

Effects of the Support Material and the Thermodynamic Water Activity on Enzymatic Synthesis in Organic Media ^a

Effects of the Support Material and the Thermodynamic Water Activity on Enzymatic Synthesis in Organic Media

Effects of the Support Material and the Thermodynamic Water Activity on Enzymatic Synthesis in Organic Media

Lipase‐catalyzed transesterification of phosphatidylcholine at controlled water activity

The incorporation of a free fatty acid into thesn‐1 position of phosphatidylcholine by lipase‐catalyzed transesterification was investigated. The thermodynamic water activity of both the enzyme preparation and the substrate solution was adjusted to the same value prior to the reaction. The reaction rate increased with increasing water activity but the yield of modified phosphatidylcholine decreased due to hydrolysis. By using a large excess of the free fatty acid (heptadecanoic acid), the hydrolysis reaction was slowed down, so a higher yield was obtained at a given degree of incorporation. The best results were obtained withRhizopus arrhizus lipase immobilized by adsorption on a polypropylene support. With this preparation, a yield of 60% and nearly 50% incorporation of heptadecanoic acid (100% incorporation in thesn‐1 position) was obtained at a water activity of 0.064. The enzyme preparation had good operational stability and position specificity. Little incorporation (&lt;1%) was observed in thesn‐2 position, when almost all the fatty acid in thesn‐1 position was exchanged.

Lipases from different sources vary widely in dependence of catalytic activity on water activity

We have measured the rates of esterification in hexane catalysed by suspended immobilised lipases (triacylglycerol acylhydrolase, EC 3.1.1.3), with pre-equilibration to known thermodynamic water activity (a(w)). There were important differences between the enzymes from five different microbes in their retention of activity at low a(w). That from Rhizomucor miehei showed over 40% maximal activity at an a(w) of 0.12, and that from Rhizopus niveus was also fairly active at low a(w). Lipases from other sources required higher a(w) values to show good activity, increasing in the sequence Humicola sp., Candida rugosa and Pseudomonas cepacia. The behaviour was generally similar to two very different support materials, anion-exchange resin and macroporous polypropylene. Comparison of the sequences of the homologous enzymes from Rh. miehei, Rh. niveus and Humicola sp. suggests that changes in charged residues in the 'hinge and lid' region of the structure may be significant in low a(w) tolerance.

Influence of solvent and water activity on kinetically controlled peptide synthesis

α-Chymotrypsin deposited on Celite was used to catalyse peptide synthesis reactions between N-protected amino acid esters and leucine amide in organic media with low water content. The influence of the solvent and the thermodynamic water activity on the reaction kinetics was studied. The substrate specificity in the reactions was shown to be a combination of the substrate specificity of the enzyme in aqueous media and the influence of the solvents. The magnitude of the solvent effects differed greatly depending on the substrates used. In hydrophobic solvents high reaction rates were observed and the competing hydrolysis of the ester substrate occurred to only a minor extent. Reactions occurred at water activities as low as 0.11, but the rate constants increased with increasing water activity and were about two orders of magnitude higher at the highest water activity tested (0.97).

Reaction rate with suspended lipase catalyst shows similar dependence on water activity in different organic solvents

We have studied the effect of thermodynamic water activity (a W) on the initial rate of esterification catalysed by an immobilised lipase (Lipozyme) suspended in an organic reaction mixture. The catalyst and the organic phase were separately pre-equilibrated to the same aw value. The rate shows similar dependence on aw in reaction mixtures based on five different organic solvents ranging in polarity from pentan-3-one to hexane, and in a liquid reactant mixture. There is a maximum at aw about 0.5, with a decline to 30-70% at aw of either 0.9 or less than 0.01. When the rates are presented in terms of water concentration in the organic phase (or total water content of the system), the maxima for the various solvents come at very different positions, reflecting the widely varying solubilities of water in the organic phase.

Salt hydrates buffer water activity during chymotrypsin-catalysed peptide synthesis

Chymotrypsin (EC 3.4.21.1) powder suspended in hexane in the presence of Na2CO3.10H2O is a good catalyst for peptide synthesis. The salt hydrate releases water to fix the thermodynamic water activity of the system in accord with its dissociation pressure. Salt hydrates can be useful to buffer water activity in mainly organic enzyme reaction mixtures at a value permitting activity of the catalyst while minimising hydrolytic side reactions.

High-affinity binding of water by proteins is similar in air and in organic solvents

Published data for water adsorption by proteins suspended in organic solvents (of interest as enzyme reaction mixtures) have been converted to a basis of thermodynamic water activity ( a w). The resulting adsorption isotherms have been compared with those known for proteins equilibrated with water from a gas phase. This comparison can show any effects of the solvent on the interaction between the protein and water at the molecular level. At lower water contents ( a w less than about 0.4), similar adsorption isotherms are found in each solvent and in the gas phase; differences are probably less than the likely errors. Hence, it may be concluded that the presence of an organic solvent has little effect on the interaction between proteins and tightly bound water; on a molecular scale there is probably little penetration of the primary hydration layer by solvent molecules, even fairly polar ones such as EtOH. At higher a w values, there are differences between the isotherms which probably are significant. Nonpolar solvents increase the amount of water bound by the enzyme (at fixed a w), while polar solvents (mainly EtOH) may reduce the amount of water bound by the enzyme, presumably by occupying part of the secondary hydration layers in place of water.

Measurement and control of water activity with an aluminium oxide sensor in organic two-phase reaction mixtures for enzymic catalysis

An aluminium oxide film sensor (Endress & Hauser type DY20B) gives correct readings for the partial pressure of water dissolved in hexane or in the presence of various organic solvent vapors. This value, together with temperature, may be used to calculate thermodynamic water activity. The sensor must, however, be used with care, due to limitations of stability, sensitivity, and measurement range. Response time and lifetime are adequate. The sensor has been used for continuous monitoring and control of water activity during lipase-catalysed esterification in a hexane medium: water formed as a reaction product was removed by controlled recirculation of the headspace gases through a drying column. The rate of water production measured with the sensor agreed with the rate of ester synthesis determined by chemical analysis.

Lipase‐Catalysed Modification of Oils and Fats in Organic Two‐Phase Systems

AbstractA wide variety of lipases remain catalytically active in systems containing high proportions of water‐immiscible organic liquids; such reaction mixtures offer several advantages for the industrial application of biocatalysis. The system splits into two or more phases, but in some cases the amount of water is so low that even the polar phase around the enzyme catalyst is far from a dilute aqueous solution. Lipases in such systems are efficient catalysts of synthetic reactions, because two factors can contribute to shifting the position of hydrolytic equilibria: relative solvation of reactants and products, and reduction in the thermodynamic activity of water (aw). Reaction rates can be comparable to those in higher water systems, though may be lower with some forms of catalyst, and depend on the precise water content and hydration history. Where all reactants and products can be transferred via the organic phase, as in the selective transesterification of triacylglycerols in oils and fats, then attractive packed bed reactors may be operated with water only present adsorbed in the catalyst bed. If access to both phases is needed, as in enzymic lipolysis, then a continuous membrane reactor may be useful.

Thermodynamic activity of some aqueous-organic solutions

It has been found that the concentration dependences of the thermodynamic activity of water in aqueous-dimethylacetamide and aqueous-dimethylformamide solutions have breaks in the concentration regions of organic component of 40, 60, and 85% by wt.

Activity of water in some aqueous-salt and aoueous-organic solutions

Values for the thermodynamic activity of water in aqueous salt solutions corresponding in composition to the polycomponent precipitation baths in the viscose method of preparing fibres, and also in aqueous solutions of N-methylmorpholine-N-oxide and diethylene glycol have been obtained experimentally.

Effect of thermodynamic water activity on thermolysin-catalysed peptide synthesis in organic two-phase systems

The equilibrium position for thermolysin-catalysed peptide synthesis in organic two-phase systems of varied water content is controlled by the thermodynamic water activity (a w ). Water levels studied were (1) a bulk aqueous phase; (2) all water adsorbed to the silica-thermolysin catalyst, but with a w close to 1; and (3) the same catalyst not fully hydrated, so a w is significantly below 1. Compared with (1), system (3) shows a shift of the equilibrium position in favor of synthesis, but system (2) does not. Silica-thermolysin retains substantial catalytic activity at a w of 0.5; reaction rates depend on the hydration history of the catalyst as well as its current level.

Rates Of Enzymic Reactions In Predominantly Organic, Low Water Systems

This article makes a quantitative assessment of the activity of enzymes in predominantly organic reaction mixtures of very low water content (and thermodynamic water activity significantly less than 1). This is done by attempting to relate reaction rates to those in systems of higher water content, although several factors make fair comparison difficult. However, it seems that rates with lipases in low water systems can sometimes be at least as high as when the same amount of enzyme catalyses the same reaction in a more traditional system. Rates are usually higher when the catalyst is dispersed on a support, rather than with simple dried particles of enzymes. Rates per unit weight of catalyst are rather similar, even when the amount of active enzyme varies widely, suggesting that physical factors may be limiting. Little data is available for enzymes other than lipases.

Effects of water on equilibria catalysed by hydrolytic enzymes in biphasic reaction systems

In the literature on hydrolase-catalysed synthetic reactions in aqueous-organic biphasic systems, it has been stated that low water concentrations contribute to favourable shifts in equilibrium. It is argued here that this is not a sufficient condition for mass action effects of water. A simple method of treating such equilibria is suggested, using the thermodynamic activity of water. Only when its activity is significantly reduced below 1 can water contribute to a shift of equilibrium in favour of synthetic products. Such a reduction is not necessarily obtained by creating a biphasic system, however low the water content, but requires that the aqueous phase becomes a very concentrated solution/dispersion of hydrophilic species.

Influence of water activity on the rate of dihydrogen oxidation by metal complexes in aqueous solutions of electrolytes

The rate of hydrogen oxidation by complexes of PdII, HgII, AgI and MnO 4 − has been established to be an exponential function of the thermodynamic activity of water in aqueous solutions of electrolytes, including mineral acids and salts.

Counter-ion intradiffusion in sulphonic ion-exchange systems

The intradiffusion data of the sodium counter-ion in different sulphonic ion-exchange systems in the sodium form (from linear polystyrene to commercial membranes of graft copolymers of styrene) are analysed as a function of q, the number of water molecules per ionic group. These data represent experimental results from a considerable number of workers during the last three decades. A general comprehensive picture is obtained, which shows how individual characteristics (water content, exchange capacity, crosslinking and inert material added) affect in a gradual and congruent way the value of the intradiffusion coefficient. The comparison with simple electrolyte systems such as aqueous sodium chloride and sodium p-ethylbenzene sulphonate shows not only a clear similarity, which shows that the transport mechanism is of the same type in all these systems, but also a clear differentiation owing to the polymeric and polyelectrolytic character of the ion-exchange systems. The complex nature of the obstruction to ion movement (tortuosity) is evidenced. The overall behaviour observed leads to the view that it is realistic to consider ion-exchange systems as homogeneous hydrated gels, since intermolecular interactions between charged sites, counter-ions and water are very important. Consequently, it is more convenient to study the transport properties as functions of the swelling variable q than in terms of the commonly used stoichiometric volume fraction of polymer. For several systems a linear relation between ln D†Na+ and 1/q holds, within the range 2 ⩽q⩽ 70. The effect of cross-linking and of inert material on these relations is discussed. Furthermore, a similar analysis leads to the verification of a previously obtained linear equation between ln D†Na and a0, the thermodynamic activity of water.

Thermodynamic prediction of the water activity, NH 4NO 3 dissociation constant, density and refractive index for the NH 4NO 3-(NH 4) 2SO 4-H 2O system at 25°C

The thermodynamic properties, water activity, density and refractive index of NH 4NO 3-(NH 4) 2SO 4-H 2O aerosols are estimated from binary solution data and existing mixing rules. Particle growth is shown to be predictable from the particle composition, the NH 4NO 3-(NH 4) 2SO 4-H 2O phase diagram and the water activity calculation technique of C.L. Kusik and H.P. Meissner (1978, A.I.Ch. E. Symp. 173, 14–20). Good agreement between the theoretical predictions and the experimental measurements of I.N. Tang et al. (1981, Atmospheric Environment 15, 2463–2471), J. Thudium (1978,Pageoph. 116, 130–148) and H.H. Emons and W. Hahn (1970, Wiss Z. 12, 129–132) is shown. Also, the effect of (NH 4) 2SO 4 on the relative humidity dependence of the NH 4NO 3 dissociation constant is evaluated.