Journal
CHEMICAL PHYSICS
Volume 310, Issue 1-3, Pages 51-58Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.chemphys.2004.10.006
Keywords
human serum albumin; penicillin; isothermal titration calorimetry; DLVO theory; penicillin-protein complex
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The complexation process of the amphiphilic penicillins sodium cloxacillin and sodium dicloxacillin with the protein human serum albumin (HSA) in aqueous buffered solutions of pH 4.5 and 7.4 at 25 degreesC was investigated through isothermal titration calorimetry (ITC) and dynamic light scattering. ITC experiments were carried out in the very dilute regime and showed that although hydrophobic interactions are the leading forces for complexation, electrostatic interactions also play an important role. The possibility of the formation of hydrogen bonds is also deduced from experimental data. The thermodynamic quantities of the binding mechanism, i.e, the enthalpy, DeltaH(ITC)(i), entropy, DeltaS(ITC)(i), Gibbs energy, DeltaG(ITC)(i), binding constant, K-ITC(i) and the number of binding sites, n(i), were obtained. The binding was saturable and is characterised by Langmuir adsorption isotherms. From ITC data and following a theoretical model, the number of bound and free penicillin molecules was calculated. From Scatchard plots, K-ITC(i) and n(i) were obtained and compared with those from ITC data. The interaction potential between the HSA penicillin complexes and their stability were determined at pH 7.4 from the dependence of the diffusion coefficients on protein concentration by application of the DLVO colloidal stability theory. The results indicate decreasing stability of the colloidal dispersion of the drug-protein complexes with increase in the concentration of added drug. (C) 2004 Elsevier B.V. All rights reserved.
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