Thermodynamic studies of interaction between cetylpyridinium bromide and lysozyme by means of calorimetric titrations

Abstract

In this study, we have investigated the micellization characteristics of n-cetylpyridinium bromide (CPB) and lysozyme–CPB system using isothermal titration calorimetry (ITC) technique. The ITC operates in a stepwise addition mode, providing an excellent method of determination of critical micelle concentration (CMC) and enthalpy of demicellization (and hence micellization). The micellization characteristics of CPB have been investigated by microcalorimetric technique at 25, 30, 35, and 40 °C in a buffer solution of Tris(hydroxymethyl)aminomethane and pH of 7.3. A scheme to describe lysozyme–CPB interaction has also been proposed on the basis of ITC technique in a buffer solution of Tris(hydroxymethyl)aminomethane at 30 °C, pH of 7.3, 0 mM NaCl, and 1 mM NaCl. The enthalpy changes associated with micelle dissociation were temperature and lysozyme concentration dependent, indicating the importance of hydrophobic interactions. The ΔG mic was found to be negative, implying that, micellization, as expected, occurs spontaneously once the CMC has been reached. The values of ΔG mic were found to become more negative with increasing temperature (in case of micellization of CPB) and with increasing the lysozyme concentration (in the case of lysozyme–CPB). The ΔS mic was also found to decrease with increasing temperature in both cases. The presence of NaCl (1 mM) in the solutions decreased the CMC of CPB.