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Amoxicillin is a common antibiotic drug used for preventing bacterial colonization of urinary and skin infections. The binding of amoxicillin (AX) to human serum albumin (HSA) in sodium phosphate buffer solution at pH 7.4 has been investigated by UV-visible spectroscopy, fluorescence spectroscopy, Förster resonance energy transfer, isothermal titration calorimetry (ITC), circular dichroism (CD) and FTIR spectroscopy. The binding studies using ITC revealed that the interaction is entropy driven rather than enthalpy as the change in enthalpy (DH°) and change in entropy (TDS°) both are positive, indicating the endothermic reaction with low affinity and hydrophobic interaction. UV-visible absorption spectra of HSA are increased upon addition of AX which shows complex formation of HSA-AX. The fluorescence spectra reveal that the AX has an ability to quench the intrinsic fluorescence of HSA tryptophan through a static quenching procedure. The molecular distance r between donor (HSA) and acceptor (AX) was estimated according to Förster theory of nonradiative energy transfer. The CD spectra showed that the a-helix of HSA increases with increasing the amount of AX. The thermal denaturation has been studied by far-UV CD, and it is found that HSA stability is decreased by the complex formation of HSA and AX. The remarkable change in amide I peak position in infrared spectrum after interaction with AX indicated that secondary structure of HSA has been changed.

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