Salt Activation

N-acetyl-L-phenylalanine ethyl ester N-acetyl-L-phenylalanine propyl ester 

1% (w/w) SC, and 1% phosphate buffer (b) 99% (w/w) SC and 1% phosphate buffer. These initial rates were plotted against the percentage of active enzyme in the lyophilized sample (the fraction of active enzyme is an adequate measure of enzyme concentration in these systems, because the total weight of enzyme per weight of catalyst is constant for each biocatalyst preparation). 

This study employed conventional diffusion-reaction theory, showing that with diffusion-limited reactions the internal effectiveness factor of a heterogeneous catalyst is inversely related to the Thiele modulus. Using a standard definition of the Thiele modulus [100], the observed reaction rate of an immobilized-enzyme reaction will vary with the square root of the immobilized-enzyme concentration in a diffusion-limited system. In this case, a plot of the reaction rate versus the enzyme loading in the catalyst formulation will be nonlinear. 

Figu re 3.9 The catalytic efficiency, kca,/Km (IVT s-1), of subtilisin Carlsberg activated by various sodium (O) and potassium ( ) salts. Values represent the average of four trials from lyophilized enzyme samples prepared independently, and the 

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Michael condensations are catalyzed by alkaU alkoxides, tertiary amines, and quaternary bases and salts. Active methylene compounds and aUphatic nitro compounds add to form P-substituted propionates. These addition reactions are frequendy reversible at high temperatures. Exceptions are the tertiary nitro adducts which are converted to olefins at elevated temperatures (24). 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

SORBIC ACID K SALT Activity mainly between pH 2-6... 

A mixture of Pt(ll) and metallic Pt in an aqueous medium was shown to oxidize ethane to yield acetic and glycolic acids. A series of deuterium-exchange processes enabled a complex mechanism to be elucidated metallic platinum catalyzes the oxidation of intermediate alcohols to acid products, whereas the Pt(ll) salt activates the initial alkene (Scheme 7X29... 

Glycosylations with Cp2ZrCl2/Silver Salt Activators... 

An extensive study of the effect of salts on the pH-activity curves of PM was made on alfalfa49 and on orange21 and tomatoes.44 In general, the effect of salts is to lower the pH at which maximum activity is attained and to extend the activity into lower pH regions. At the higher pH values (7-8), salts have practically no activating effect. The main usefulness of salt activation of PM seems to lie in counteracting adverse pH conditions. 

Salt activation falls into two classes with respect to the valency of the cation component. With divalent cations and at pH 6, maximum activation is produced at about 0.03 M concentration. At higher concentrations with the same pH, suppression of activity occurs. Monovalent cations in general produce maximum activation at pH 6 in 0.10 M concentration and do. not suppress activity below molarities of 1.0. Maximum activity is obtained at a lower pH and lower concentration of divalent cations than monovalent. The maximum activity obtained at the optimum salt concentration for a given pH value is, within experi-... 

Various pancreatic enzymes hydrolyze lipids, including lipase with its auxiliary protein colipase (see p. 270), phospholipase A2, and sterol esterase. Bile salts activate the lipidcleaving enzymes through micelle formation (see below). 

Could this salt activation phenomenon be a result of relaxed diffusional limitations in a concentrated salt/enzyme formulation as compared to the salt-free preparation To answer this question, Bedell et al. [99] measured the initial rates of subtilisin Carlsberg-catalyzed transesterification of APEE with n-PrOH in hexane (Scheme 3.4) with two different enzyme preparations (Figure 3.8) (a) 98% (w/w)... 

These results were then correlated to the Jones-Dole coefficient to investigate the dependence of enzyme activation on the kosmotropicity of the salt in a solvent such as hexane. Specifically, plotting enzyme activity as a function of the difference in J DB coefficients of the cations and anions of the salts, resulted in a clear trend towards increased enzyme activity when the difference between the kosmotropicity of the anion and the chaotropicity of the cation was increased (Figure 3.10) [46]. These results were consistent with those of Ru et al. [33], in that enzyme activity in salt-activated preparations in hexane positively correlates with increased kosmotropicity on the anion. As a result of the elucidation of the influence of the kosmotropic/chaotropic properties of salts on enzyme function, the role of water... 

Solvent polarity is known to affect catalytic activity, yet consistent correlations between activity and solvent dielectric (e) have not been observed [12,102]. However, a striking correlation was found between the catalytic efficiency of salt-activated subtilisin Carlsberg and the mobility of water molecules (as determined using NMR relaxation techniques) associated with the enzyme in solvents of varying polarities (Figure 3.11) [103]. As the solvent polarity increased, the water mobility of the enzyme increased, yet the catalytic activity of the enzyme decreased. This is consistent with previous EPR and molecular dynamics (MD) studies, which indicated that enzyme flexibility increases with increasing solvent dielectric [104]. 

Figure 3.11 Catalytic efficiency, (fcca,/K ,)app ( ), of salt-activated subtilisin Carlsberg in hexane, THF, and acetone in comparison with T2 (transverse relaxation constant) (O) of mobile deuterons as a function of dielectric constant of solvent [103].

Eppler et al. [103] viewed these results as having a potential relationship to salt-activated enzyme preparations, particularly in relation to the mobility of enzyme-bound water. Specifically, the authors examined both water mobility [as measured by T2-derived correlation times, (tc)D20] and NaF-activated enzyme activity and observed a linear relationship. This suggests that the salt-activated enzymes contain a more mobile water population than salt-free enzymes, which facilitates a more aqueous-like local environment and dramatically increases enzyme activity through increased flexibility. Therefore, enzyme activation appears to correlate with the properties of enzyme-associated water. Once again, the physicochemical properties of water dictate enzyme structure, function, and dynamics. Hence, salt activation has proven to be a useful technique in activating enzymes for use in organic solvents and has provided a quantitative tool to better understand the role of water in enzymatic catalysis in dehydrated media. 

In addition to LPL, human milk contains a bile salts-activated lipase, which probably contributes to the metabolism of lipids by breast-fed babies who have limited pancreatic lipase activity. Bovine milk and milks from other dairy animals do not contain this enzyme. 

Wang, C. S. 1981. Human milk bile salt-activated lipase. J. Biol. Chem. 256, 10198-10203. 

By measuring the solubility, r, of the silver chloride in different concentration of added salt and extrapolating the solubilities to zero salt concentration, or better, to zero ionic strength, one obtains the solubility when v = 1. and from Eq. (29) K can be found. Then y can be calculated using this value of K and any measured solubility. Actually, this method is only applicable to sparingly soluble salts. Activity coefficients of ions and of electrolytes can be calculated from the Debye-HOckel equations. For a uni-univalent electrolyte, in water at 25 C, the equation for the activity coefficient of an electrolyte is... 

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