Macromolecular thermodynamics studied by titration calorimetry.

This thesis is centred on the use of an isothermal titration microcalori-meter which can measure heat changes of the order of a few microcalories. The calorimeter provides a rapid and convenient method, incorporating a thermodynamic analysis, for estimating enthalpic pairwise interaction parameters, hjj, for solute-j in aqueous solution. Estimates of hjj for three solutes; urea, monoethylurea and hexamethylenetetramine are reported. Comparisons with published estimates of hjj show good agreement, and confirm the importance of the new technology in calorimetry. The critical micellar concentrations (cmc's) of a series of l-alkyl-4-alkylpyridinium halide and bisquaternary ammonium bromide surfactants are reported, using q, heat change on injection, as the reporter. The standard enthalpy of micelle formation is also obtained, directly, from titration experiments. An equation is derived for the standard Gibbs energy of micelle formation, for these compounds. Thus, the standard entropy is calculated and the driving force (enthalpy or entropy) for micellisation, of these surfactants, is identified. Injection of small aliquots of fusogenic agents (the dianions of dipicolinic acid and sodium sulphate) into aqueous solutions of dioctadecyldimethyl-ammonium bromide (DOAB) vesicles is endothermic at 50 Celsius. For solutions containing greater than equimolar ratios of DOAB and fusogenic agent, the injection process is effectively athermal. The patterns of enthalpy change are attributed to vesicle-dianion interaction (exothermic) and headgroup dehydration (endothermic). Other effects such as the presence of buffer and the interaction of DOAB with halide monoanions are also discussed. Binding of the substrate, chloramphenicol (CM), to the enzyme, chloramphenicol acetyltransferase (CAT), is exothermic. Calorimetric measurements of a series of injections of CM into CAT, and subsequent analysis, yield the binding constant, number of binding sites per macromolecule (enzyme) and the standard enthalpy of binding for the interaction. These three parameters are presented, together with the standard entropy and standard Gibbs energy of binding.