Complexation of beta-lactam antibiotic drug carbenicillin to bovine serum albumin: energetics and conformational studies.

Binding of the antibiotic drug carbenicillin to bovine serum albumin (BSA) has been studied using isothermal titration calorimetry (ITC) in combination with fluorescence and circular dichroism (CD) spectroscopies. The thermodynamic parameters of binding have been evaluated as a function of temperature, ionic strength, and in the presence of anionic, cationic and nonionic surfactants, tetrabutylammonium bromide, and sucrose. The values of van't Hoff enthalpy do not agree with the calorimetric enthalpy indicating conformational changes in the protein upon drug binding. These observations are supported by the intrinsic fluorescence and CD spectroscopic measurements. A reduction in the binding affinity of carbenicillin to BSA is observed with increase in ionic strength of the solution, thereby suggesting, prevailing of electrostatic interactions in the binding process. The involvement of hydrophobic interactions in the binding of the drug to the protein is also indicated by a slight reduction in binding constant in the presence of tetrabutylammonium bromide. The experiments in the presence of sucrose suggest that hydrogen bonding is perhaps not dominant in the binding. The anionic surfactant sodium dodecyl sulphate (SDS) is observed to completely interfere in the ionic interactions in addition to its partial denaturing capacity. However, the presence of cationic surfactant hexadecyl trimethylammonium bromide (HTAB) and nonionic surfactant Triton-X 100 induce a slight reduction in the values of binding affinity. These calorimetric and spectroscopic results, provide quantitative information on the binding of carbenicillin to BSA and suggests that the binding is dominated by electrostatic interactions with contribution from hydrophobic interactions.