The results of the kinetics of binding of the surfactant cetyltrimethylammonium bromide (CTAB) with bovine serum albumin (BSA) under varied conditions of CTAB concentration, pH, ionic strength and temperature are presented. The kinetic process has been found to be comprised of either two or three stages; the rate constants of the first, second, and third stages follow the general order of k1 >> k2 > k3. The three-stage kinetic process has been observed at higher CTAB concentrations, higher temperature and in presence of the protein denaturant urea. The activation parameters for this three-stage kinetic process have been determined. Most entropies of activation have been found to be negative (a very few are positive) whereas the activation enthalpies and energies are both positive and negative. The enthalpy and entropy for all the kinetic stages in different environmental conditions have been found to compensate each other, whereas the free energy of activation has remained unaffected. A model for the three-stage kinetic process has been proposed.
The binding of CTAB with the proteins, gelatin, hemoglobin, beta-lactoglobulin and lysozyme follow first order kinetics and occurs either in two or three distinct stages. The number of stages depends on the overall configuration of the biopolymers. The denatured protein, gelatin has shown three-stage kinetics under all conditions, whereas the native proteins, hemoglobin, beta-lactoglobulin and lysozyme have exhibited two stage kinetics. Heat treated lysozyme in 8 mol dm-3 urea medium has also shown a two-stage kinetics. On the basis of non interacting binding sites on the proteins and independent sequential binding, the rates of reaction have been observed to increase with temperature and follow the trend k1 >> k2 > k3. The interaction of CTA+ with the proteins is both electrostatic and hydrophobic. Hemoglobin has shown maximum reaction rate whereas, beta-lactoglobulin has shown a minimum. The activation parameters for the kinetic process have exhibited almost non-variant delta G++ and delta H++ < T delta S++. The formation of activation complex in the Eyring model is entropy controlled so also the overall kinetics. An isokinetic entropy-enthalpy compensation phenomenon has been observed for the respective kinetic stages.
The kinetics of binding of the cationic surfactant cetyltrimethyl ammonium bromide with the Na salt of carboxymethyl cellulose was studied by the electrometric method using cetyltrimethyl ammonium+ (CTA +) ion-selective polyvinyl chloride membrane electrode. The binding process followed thejirst-order kinetics and occurred in three stages. Its ajinity increased with increasing CTA bromide concentration and decreased with ionic strength. The activation process comprised moderate E$ and AH' and negative ASz for all three stages with a AH' < TAS' trend proving it to be entropy controlled. The AG' values followed the trend AG: < AG: < AG: (in accordance with k, > k2 > 4). The enthalpies (AH') and entropies (AS') of activation followed a systematic and interdependent trend. The multiple-stage binding kinetics is grossly comparable with the kinetics of binding ofproteins to solid surfaces. 0 I995 John Wiley & Sons, Inc.
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