A calorimetric study of host-guest and protein-substrate interactions

This thesis describes a study of molecular recognition processes involving - and - cyclodextrins, and the enzymes chloramphenicol acetyltransferase and DNA gyrase.;An isothermal titration calorimeter of high sensitivity was used to investigate the binding of a number of relatively small guest molecules with much larger host molecules. The calorimetric technique allows the direct determination of the apparent binding enthalpy bindH0', the evaluation of the apparent association constant Ka' and hence evaluation of the apparent Gibbs energy and entropy of binding, bindG0' and TbindS0'.;The energetics of association for p-nitrophenol and 1-adamantane carboxylic acid in neutral and anionic forms, binding to - and - cyclodextrins were measured, and the results compared with those from previous studies. Binding of p-fluoronitrobenzene, p-nitrobenzyl alcohol and the antibiotic chloramphenicol to the cyclodextrins was also studied and the thermodynamic binding parameters identified. The results showed that where there is a close fit of the guest molecule inside the cyclodextrin cavity, the binding is enthalpy driven. Where there is a loose fit the binding is entropically favourable, but is still enthalpy dominated.;Binding of hydrophobic derivatives of the antibiotic chloramphenicol (CM), to the enzyme chloramphenicol acetyltransferase (CAT), was investigated. Six CM analogues were prepared with substitutions made at the 3-hydroxy group of CM, and enthalpies of binding to CAT determined by calorimetry. Comparison of calorimetric data with Gibbs energies of binding determined previously, showed that the binding is enthalpy driven.;Binding of coumarin drugs to the 43-kDa and 24-kDa protein fragments of the enzyme DNA gyrase was studied using titration calorimetry. The energetics of binding obtained from the calorimetric technique were compared with those obtained from rapid gel filtration (spin-column) studies. Titration calorimetry shows that the Ka' for coumarins binding to both proteins is of the order of 108 mol-1dm3 and that the binding is enthalpy driven.