Scientia Pharmaceutica Article FT-IR Spectroscopy for the Identification of Binding Sites and Measurements of the Binding Interactions of Important Metal Ions with Bovine Serum Albumin Hassan A. Alhazmi Department of Pharmaceutical Chemistry, College of Pharmacy, Jazan University, P.O. Box 114, Jazan 45142, Saudi Arabia; [email protected]; Tel.: +966-541433344 Received: 15 January 2019; Accepted: 13 February 2019; Published: 20 February 2019 Abstract: Proteins play crucial roles in the transportation and distribution of therapeutic substances, including metal ions in living systems. Some metal ions can strongly associate, while others show low affinity towards proteins. Consequently, in the present work, the binding behaviors of Ca2+, Ba2+, Ag+, Ru3+, Cu2+ and Co2+ with bovine serum albumin (BSA) were screened. BSA and the metal ions were allowed to interact at physiological pH and their binding interactions were screened by using FT-IR spectroscopy. Spectra were collected by using hydrated films over a range of 4000–400 cm−1. The interaction was demonstrated by a significant reduction in the spectral intensities of the amide I (C=O stretching) and amide II bands (C–N stretching coupled to NH bending) of the protein after complexation with metal ions. The binding interaction was further revealed by spectral shifting of the amide I band from 1651 cm−1 (free BSA) to 1653, 1654, 1649, 1655, 1655, and 1654 cm−1 for BSA–Ca2+, BSA–Ba2+, BSA–Ag+, BSA–Ru3+, BSA–Cu2+ and BSA–Co2+ complexes, respectively. The shifting of the amide I band was due to the interactions of metal ions with the O and N atoms of the ligand protein. Estimation of the secondary protein structure showed alteration in the protein conformation, characterized by a marked decrease (12.9–40.3%) in the α-helix accompanied by increased β-sheet and β-turn after interaction with the metal ions. The interaction results of this study were comparable with those reported in our previous investigation of metal ion–BSA interactions using affinity capillary electrophoresis (ACE), which has proven the accuracy of the FT-IR technique in the measurement of interactions between proteins and metal ions. Keywords: bovine serum albumin; FT-IR; metal ions; secondary structure; binding interaction 1. Introduction Metal ions play crucial roles in maintaining physiological functions inside the human body and are used for diagnostic as well as therapeutic purposes. Some metals are essential for life and their deficiency in the biological system may lead to a variety of ailments. For instance, insufficient zinc in the diet leads to growth retardation and copper deficiency results in cardiovascular complications [1]. Considering their detoxifying capability, certain metal ions are utilized as chelating agents for the treatment of different types of diseases. Recently, metal-based complexes have been extensively investigated mainly for the development of anticancer agents and many of them have shown encouraging results. Cisplatin, a platinum (IV)-based drug, is the best example of such metal ion complexes and is currently in clinical applications for the treatment of various types of cancers [2]. The association of metal ions with proteins serves several important functions in the body, such as the transportation of biologically important metal ions to their specific target binding sites as one of the most crucial tasks. In a protein, the amino acid residues provide negatively charged functional groups as potential ligands for cationic metal ions. As a consequence, there exists an attraction between the Sci. Pharm. 2019, 87, 5; doi:10.3390/scipharm87010005 www.mdpi.com/journal/scipharm Sci. Pharm. 2019, 87, 5 2 of 13 Sci. Pharm. 2019, 87, x FOR PEER REVIEW 2 of 13 betweenelectron-rich the electron functional-rich groups functional in proteins groups and in proteins electron-deficient and electron (cationic)-deficient metal (cationic ions,) leadingmetal ions, to a leadingbinding to interaction a binding betweeninteraction the between two species. the two Furthermore, species. Furthermore, the interaction the interaction of metal ions of tometal suitable ions tobiological suitable targetbiological binding target sites bindi alsong depends sites also on depends the valency on the and valency charge-accepting and charge capacity-accepting of thecapacity metal ofions the [3 metal]. Metal ions ion [3] interactions. Metal ion with interactions proteins maywith influenceproteins may the protein influence structure the protein and affect structure structural and affectstability. structural The impact stability. of metalThe impact ions binding of metal to ions proteins binding can to beproteins seen in can the be secondary, seen in the tertiary secondary, and tertiaryquaternary and structuresquaternary of structures the proteins. of the Albumin proteins. is theAlbumin major is plasma the major protein plasma in vertebrates, protein in vertebrates, comprising comprisingapproximately approximately 60% of the total60% of plasma the total content, plasma and content participates, and participate in the transportations in the transportation of a diverse of arange diverse of molecules, range of mo includinglecules, nutrients,including metal nutrients, ions, drugsmetal andions, metabolites. drugs and Duemetabolites. to their extraordinaryDue to their extraordinarybinding ability tobinding a variety ability of ligands, to a albuminsvariety haveof ligands, several biochemical,albumins have pharmaceutical several biochemical, and clinical pharmaceuticalapplications [4, 5and]. Bovine clinical serum applications albumin (BSA)[4,5]. isBovine among serum the most albumin commonly (BSA) investigated is among the proteins most commonlyfor research investigated purposes due proteins to its close for structuralresearch purposes resemblance due to to human its close serum structural albumin resemblance (HSA). to humanStructurally, serum albumin the BSA (HSA). molecule has a single chain consisting of 583 amino acids, which is cross-linkedStructurally, through the 17BSA cystine molecule residues, has a and single its molecularchain consisting mass is of about 583 amino 66.5 kDa. acids, The which BSA moleculeis cross- linkedis made through up of three 17 cystine structurally residues identical, and its domains molecular (I, II mass and III), is about which 66.5 are dividedkDa. The into BSA nine molecule loops (L1 is madeto L9) up through of three disulfide structurally bonds. identical Every domain domains consists (I, II and of twoIII), subdomainswhich are divided (A and into B) and nine the loops loops (L1 in toeach L9) domain through are disulfide composed bonds. of a Every sequence domain of large, consists small of andtwo largesubdomains loops making (A and a B) triplet. and the The loops BSA inmolecule each domain predominantly are composed consists of a ofsequence a helical of structure, large, small with and about large 74% loops helical making content. a triplet. It also The hasBSA a moleculefree thiol grouppredominantly attached toconsists the Cys34 of a residuehelical structure [6,7]. The, with three-dimensional about 74% helical structure content. of BSA It also is shown has a freein Figure thiol 1group. attached to the Cys34 residue [6,7]. The three-dimensional structure of BSA is shown in Figure 1. FigureFigure 1. 1. ThreeThree-dimensional-dimensional structure structure of of b bovineovine serum serum albumin albumin ( (BSA)BSA) [Source: [Source: protein protein data data bank; bank; ID: ID: 3V03V033 ( www.rcsb.org)www.rcsb.org)].]. TheThe s selectivityelectivity of of the interactions between between metal metal ions ions and and protein protein binding sites sites can can be best predicted as per the hard and soft acid base (HSAB) Lewis theory (Table (Table 11).). AccordingAccording toto thethe HSABHSAB LewisLewis theory, theory, metal metal ion ion ligands ligands bind bind preferentially preferentially to to the the protein protein binding binding sites sites of ofsimilar similar hardness hardness or softness.or softness. The The soft softmetal metal ligands ligands react react with with soft groups soft groups and hard and hardmetal metal ligands ligands interact interact with the with hard the bindinghard binding sites sitesof proteins of proteins [8,9] [.8 Generally,,9]. Generally, the theattraction attraction forces forces between between hard hard acid acid metals metals and and hard hard basicbasic protein sitessites areare ionic, ionic, whereas whereas covalent covalent interaction interaction exists exists between between soft soft acid acid metals metals and softand basicsoft basicprotein protein sites [sites8,10 ,11[8,10,11]]. The. interactionsThe interactions of metal of metal complexes complexes with with BSA BSA lead lead to the to disruptionthe disruption of the of thedisulfide disulfide bonds, bonds, altering altering the the secondary secondary structure structure of of proteins proteins and and result result inin unfoldingunfolding by significant significant lossloss of of the the αα-helix-helix conformation conformation [12] [12. ].The The metal metal complexes complexes–protein–protein interactions interactions may may also also modify modify the overallthe overall polarity polarity of the of theenvironment environment around around the the exposure exposure of of tryptophan tryptophan residues, residues, which which is is due due to molecularmolecular interactions interactions such as as molecular molecular re rearrangements,arrangements, excited excited state state reactions, reactions,