The mechanism of the interaction between bovine serum albumin (BSA) and ceftriaxone with and without zinc (II) (Zn2+) was studied employing fluorescence, ultraviolet (UV) absorption, circular dichroism (CD), and synchronous fluorescence spectral methods. The intrinsic fluorescence of BSA was quenched by ceftriaxone in a static quenching mode, which was authenticated by Stern-Volmer calculations. The binding constant, the number of binding sites, and the thermodynamic parameters were obtained, which indicated a spontaneous and hydrophobic interaction between BSA and ceftriaxone regardless of Zn2+. Changes in UV absorption, CD, and synchronous fluorescence spectral data are due to the microenvironment of amide moieties in BSA molecules. In the BSA-ceftriaxone-Zn2+ system, Zn2+ must first interact with ceftriaxone forming a complex, which inhibits BSA binding to ceftriaxone. The present work uses spectroscopy to elucidate the mechanism behind the interaction between BSA and ceftriaxone in the presence and absence of Zn2+. The BSA and ceftriaxone complex provides a model for studying drug-protein interactions and thus may further facilitate the study of drug metabolism and transportation. 1. Introduction The interaction between biomacromolecules, especially between plasma proteins and drugs, has been an interesting research field in life sciences, chemistry, and clinical medicine [1]. Drug-albumin complexes may be considered as models for gaining fundamental insights into drug-protein interactions. Serum albumin is a major soluble protein constituent of the circulatory system and has many physiological functions such as acting as a plasma carrier by nonspecifically binding to several hydrophobic steroid hormones and as a transport protein for hemin and fatty acids [2]. Albumins are characterized by a low content of tryptophan and methionine and a high content of cystine and charged amino acids [3–5]. Bovine serum albumin (BSA), an example of a mammalian albumin, has been studied extensively because of its stability, neutrality in many biochemical reactions, and low cost [6, 7]. Brown elucidated the 607 amino acid residue, primary structure of BSA in 1975, twenty one of which are tyrosine (Tyr) residues and two of which are tryptophan (Trp) residues located at positions 134 and 212, respectively [3, 6]. These two Trp residues cause BSA to have intrinsic fluorescence. Ceftriaxone is a third-generation cephalosporin antibiotic. Cephalosporins are semisynthetic antibiotics produced by fungi Cephalosporium and, like penicillins, are -lactam antibiotics, which
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