Water mimics

The pattern of temperature effects is the same as in aqueous media. The initial rate of reaction increases with temperature, in the usual Arrhenius fashion. However, the stability of the enzyme will decline with temperature, and at high enough values catalytic activity will be lost rapidly before significant conversions are reached. Hence, for given conditions, there will be an optimum temperature to maximize product yield after a given time. This is rarely a real fixed optimum for the enzyme, and for example will usually become higher if the reaction time is reduced. Progressive enzyme inactivation will have less effect over a shorter reaction time. 

One important feature that can be exploited in low-water media is an increase in stability to temperature. Hence reactions may be carried out at temperatures higher than would be possible in aqueous media, often by many tens of degrees. It is fairly clear that the most important factor here is the amount of water in the molecular environment of the enzyme molecules, as determined primarily by the water activity of the system. The presence or nature of a solvent has little additional effect. Thus, beware of statements that enzymes become more thermostable in organic solvents . It is the reduction in hydration that increases stability. If anything, the presence of an organic solvent will be destabilizing (in a comparison at equal water activity). In an organic solvent at water activity close to 1 (i.e. water saturated), the stability will be no better than in water. If, however, water activity is reduced to substantially below 1, a very valuable increase in stability may be achieved. 

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Because of their low intrinsic electronegativities, neutral late transition metals (bearing an abundance of lone pairs) can serve as good donors in nM— ctah interactions of the form (5.69a). Furthermore, transition-metal-hydride bonds (Section 4.4.1) often display sufficient covalency or polar-covalency (particularly in transition-metal cations) to serve as good acceptors in ns— ctmh interactions of the form (5.69b). In the present section we shall briefly examine the simple example of platinum dihydride (PtH2) as a water-mimic in binary H-bonded complexes with H20,... 

Addition of water and water mimics Addition of 4% H20 Thermolysin tert-Amyl alcohol 3 500 31... 

Water mimics are effective in increasing the incorporation of the desired fatty acids in PC by PLA2-mediated acidolysis. By employing glycerol as a water mimic, 35% and 34% incorporation of EPA and DHA into soy PC were obtained, respectively. 

H. Kitaguchi and A.M. Klibanov Enzymatic peptide synthesis via segment condensation in the presence of water mimics. Journal of the American Chemical Society 111 (1989) 9272-9273. 

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