Colloids and Surfaces B: Biointerfaces 146 (2016) 514–522 Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb Lysozyme binding ability toward psychoactive stimulant drugs: Modulatory effect of colloidal metal nanoparticles Vikash K. Sonu, Mullah Muhaiminul Islam, Mostofa Ataur Rohman, Sivaprasad Mitra ∗ Centre for Advanced Studies, Department of Chemistry, North-Eastern Hill University, Shillong, 793 022, India a r t i c l e i n f o Article history: Received 8 March 2016 Received in revised form 27 May 2016 Accepted 28 June 2016 Available online 29 June 2016 Keywords: Lysozyme Theophylline Theobromine Fluorescence quenching Colloidal nanoparticles Hydrophobic & hydrogen bonding interaction Molecular docking a b s t r a c t The interaction and binding behavior of the well-known psychoactive stimulant drugs theophylline (THP) and theobromine (THB) with lysozyme (LYS) was monitored by in-vitro fluorescence titration and molecu- lar docking calculations under physiological condition. The quenching of protein fluorescence on addition of the drugs is due to the formation of protein-drug complex in the ground state in both the cases. How- ever, the binding interaction is almost three orders of magnitude stronger in THP, which involves mostly hydrogen bonding interaction in comparison with THB where hydrophobic binding plays the predomi- nant role. The mechanism of fluorescence quenching (static type) remains same also in presence of gold and silver nanoparticles (NPs); however, the binding capacity of LYS with the drugs changes drastically in comparison with that in aqueous buffer medium. While the binding affinity of LYS to THB increases ca. 100 times in presence of both the NPs, it is seen to decrease drastically (by almost 1000 fold) for THP. This significant modulation in binding behavior indicates that the drug transportation capacity of LYS can be controlled significantly with the formation protein-NP noncovalent assembly system as an efficient delivery channel. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Lysozyme (LYS) is widely distributed in various biological flu- ids and tissues including avian egg and animal secretions, human milk, tears, saliva, airway secretions, and secreted by polymor- phonuclear leukocytes [1,2]. It shows a variety of physiological properties and pharmaceutical importance like antibacterial, anti- inflammatory, antiviral, immune modulatory, anti-histaminic and anti-tumor activities [1–6]. LYS is a small globular protein with molecular weight of 14.6 kDa and consisting of 129 amino acid residues with four disulfide bonds including -helix, -sheet, turns and disorder. It contains six tryptophan (trp) and three tyrosine (tyr) residues in its structure along with four disulfide bonds [7–9]. Three of the trp residues are located at the substrate binding sites, two in the hydrophobic matrix box, while one is separated from the others [10]. Among the amino acid residues, trp62 and trp108 are the most dominant fluorophores, both being located at the substrate binding sites [11]. The importance of LYS relies on its extensive use as a model system to understand the underlying prin- ciples of protein structure, function, dynamics and folding through ∗ Corresponding author. E-mail addresses: smitra@nehu.ac.in, smitranehu@gmail.com (S. Mitra). theoretical and experimental studies. High natural abundance is also considered as one of the primary reasons for choosing LYS as a model protein for studying protein-ligand interaction. Both theobromine (THB) and theophylline (THP) are members of the xanthine family and possess structural as well as pharma- cological similarity [12]. THP is used in therapy for respiratory diseases such as chronic, obstructive pulmonary disease, increasing heart rate (positive chronotropic) [13] and also helpful in improv- ing the sense of smell (anosmia) [14]. On the other hand, THB is considered as drugs for vasodilator heart stimulant [14]; and because of its diuretic effect, it is also used for the treatment of high blood pressure [15]. Reports are also available to demonstrate that arteriosclerosis, certain vascular diseases, angina pectoris, and hypertension etc. can also be cured by this drug [16]. Nowadays, nanoparticles (NPs) and other nanomaterials have entered in our everyday lives. In the biomedical field, it is used as nano-vaccines [17], nano-drugs [18] and diagnostic imaging tools [19]. A number of nanoparticles and nano-emulsions like titanium dioxide (TiO 2 ), zinc oxide (ZnO), alumina, silver, silicon dioxide, calcium fluoride and copper etc. are used as cosmetics [20] and also found to be present in nature. Metal nanoparticles like gold, silver, copper, zinc, iron, etc. with activated surfaces by amines, epoxides, and aldehydes are increasingly common in the design of biological materials. On the other hand, many researchers have http://dx.doi.org/10.1016/j.colsurfb.2016.06.061 0927-7765/© 2016 Elsevier B.V. All rights reserved.