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D. K. Chattoraj ¹, P. Mahapatra ¹, S. C. Biswas ¹

Affiliations
1 Department of Food Technology and Biochemical Engineering, Jadavpur University, Calcutta 700 032, IN

Abstract

Using isopiestic vapour pressure technique, extents of binding of water to a globular protein, lysozyme, have been determined in the absence and presence of inorganic salts, sucrose and urea at a fixed temperature. The water vapour adsorption isotherm for lysozyme in the range of water activity varying between zero to-unity is similar to type III BET isotherm. Moles of water adsorbed per mole of lysozyme at unit water activity have been evaluated by extrapolation method and the results support monolayer model for water adsorption by lysozyme under ideal conditions. Using the Bull equation in the integrated form, standard free energies, ΔG⁰_w for water-protein binding interaction at two different temperatures have been evaluated. Based on Clausius-Clapeyron equation in integrated form, the integral enthalpy for water-protein interaction has also been evaluated. Using the isopiestic technique, excess binding of solutes T¹₂ per mole of lysozyme in the presence of different bulk mole-fractions (X₂) of the solutes (LiCl, NaCl, KCl, NaBr, NaI, KSCN, urea and sucrose) has been evaluated in each case from the expression for the Gibbs surface excess. In certain ranges of solute concentration, the plot of Γ²₁ vs X₁ becomes linear so that moles of water and solute bound per mole of lysozyme can be evaluated. X₁ stands for the mole-fraction of the solvent in the bulk phase. Also, using the integrated form of the Gibbs adsorption equation, standard free energy changes(ΔG⁰) for the solute-lysozyme and the solvent-lysozyme interactions for different systems have been computed and the values have been compared critically.

Keywords

Isopiestic Vapour Pressure Technique, Hydration, Lysozyme, Gibbs Surface Excess.

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S. C. Biswas ¹, D. K. Chattoraj ²

Affiliations
1 Department of Food Technology and Biochemical Engineering Jadavpur University, Calcutta-700 032, IN
2 Centre for Cellular and Molecular Biology, Uppal Road, Hyderabad - 500 007, IN

Abstract

Kinetics of adsorption of cationic sufactants, cetyltrimethyl ammonium bromide (CTAB), myristyltrimethyl ammonium bromide (MTAB) and dodecyltrimethyl ammonium bromide (DTAB) at the hydrophobic surface of charcoal have been studied at various values of bulk surfactant concentration (C^t₂), pH, ionic strength and temperature. The adsorption process has been found to follow two-step first order kinetic rate equation with two different rate constants k₁ and k₂. From the variation of k₁ and k₂ with temperature, values of energies of activation E_al and E_a2 for both the kinetic steps have been evaluated. The corresponding values of enthalpies of activation (ΔH^#₁, and ΔH^#₂) and entropies of activation (ΔS^#₁, and ΔS^#₂) have been evaluated usign Eyring's equation for absolute reaction rate. It has been found that the value of T_av ΔS^# decreases with increase in the value of concentration of CTAB below its cmc. At the concentration range 0.15 mM to 0.30 mM, ΔH^#₁ < T_avΔS^#₁, so that in this concentration range the first step is controlled by entropy. But as C^t₂ approaches the me of CTAB, the value of ΔH^#₁, becomes greater than T_avΔS^#₁ and T_avΔS^#₂ increases with increase of (C^t₂), when it approaches the cmc of CTAB and also below cmc, so that the second step in the activated state is mainly controlled by entropy. Above cmc of CTAB, when C^t₂, is altered from 1.5 to 6.0 mM, the values of T_avΔS^#₁ and T_avΔS^#₂ vary in an irregular manner. Both pH and ionic strength have significant effects on the values of ΔH^#₁ and ΔH^#₂ The free energies of activation ΔG^#₁ and ΔG^#₂ are found to vary between 62 to 85 kJ/mole and ΔH^#₁ and ΔH^#₂ vary linearly with T_avΔS^#₁ and T_avΔS^#₂. An entropy-enthalpy compensation effect in the CTAB-Charcoal interaction has also been noted.

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B. J. Sengupta ¹, S. K. Ghatak ¹, B. K. Nandi ¹, R. Singh ¹, P. C. Das ¹, S. C. Biswas ¹, L. K. Das ¹, B. P. Pal ¹

Affiliations
1 Geological Survey of India, Marine Wing, DK-6, Karunamayee, Sector-II, Salt lake, Kolkata - 700 091, IN

Abstract

No Abstract.

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