Info

Note: parameters are the same as in Table 1.

Note: parameters are the same as in Table 1.

hydration level, as shown in Fig. 5B, where the molar excess heat capacity (obtained from the raw data of Fig. 5A after baseline subtraction) is plotted as a function of the temperature at different water content in the presence of dodecane. As the fraction of water decreased, the transition shifted to higher and higher temperature, showing a remarkable increase of the protein stability.

From the integration of the calorimetric excess heat capacity of the DSC transition, the enthalpy change of unfolding, AH, is obtained. This parameter, as well as the middle point transition temperature, Tm, are listed in Table 3, which shows that the presence of celite itself in an aqueous environment (line 2) does not alter the thermal stability of the enzyme. Control experiments showed that the enzyme desorbed completely from the support in an aqueous solvent. Immobilization at low degree of hydration without any solvent brings about a stabilization effect, as both Tm and AH increase. This effect is even more enhanced in the presence of dodecane. The progressive decrease of the water content induces a shift of the Tm to higher and higher temperature, even above 100°C. A significant progressive and concurrent decrease of AH is also observed. These effects may be explained by considering that at low water content the protein molecule is more "rigid" than in pure aqueous solvent. At low hydration level, water molecules cannot act any longer as a molecular "lubricant," as indeed is the case when the protein is fully hydrated (24). As a result, the overall transition splits into two components, as shown in Fig. 5. This probably results from domain decoupling induced by the subtle interaction between protein macromolecule and support surface and/or by the accompanying uneven dehydration of the protein macromolecule (25). Decoupling of the protein domains as a function of hydration resembles what was observed in the case of covalently linked macromolecule (see Subheading 2.).

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