Interpreting the adsorption of serum albumin and lactoglobulin onto ZnS nanopaticles: Effect of conformational rigidity of the proteins Jiban Saikia a , Bedabrata Saha b , Gopal Das a,b,⇑ a Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India b Centre for the Environment, Indian Institute of Technology Guwahati, Assam 781039, India article info Article history: Received 24 August 2013 Accepted 24 October 2013 Available online 9 November 2013 Keywords: ZnS nanoparticle BSA b-Lactoglobulin Protein adsorption Hard and soft proteins abstract The work we have undertaken is to investigate the adsorption of two different proteins (BSA and BLG) having near same IEP and differing in their conformational flexibility, onto the surface of ZnS nanoparti- cles (ZnS NPs). BSA and BLG both have an IEP value around pH  5. BSA is more prone to conformational deformation and considered ‘‘soft’’ while BLG holds the conformational rigidity and considered as ‘‘hard’’ protein. To ascertain the differences in surface coverage and conformation of the protein onto ZnS surface (PZC  3.7), we have evaluated the adsorption profile at pH 7, where the entire surface behaves nega- tively. An integrated approach was taken by incorporating zeta (f) potential, fluorescence and CD for ana- lyzing the adsorption process. In both systems, an increase in protein surface coverage was observed with the increase in free protein concentration in the solution and f values approaching that of native protein at high surface coverage. An alteration in the tertiary structure was observed for both BSA and BLG. The CD spectra analysis reveals that the secondary structure of the BSA was more deviated from the native protein structure, accommodating the increased adsorption value. For BLG no such prominent structural alteration was observed. These findings help us to understand better, how adjustment of the protein adsorption amount can be achieved onto the surface of nanoparticles having like charges. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Protein adsorption is the prima facie credibility in the construct of bio–nano-composite for implication in various fields. It is a com- plex process involving various factors such as van der Waals, hydrophobic and electrostatic interactions, and hydrogen bonding [1]. On the other hand, different proteins act differently with the introduced surface in the adsorption process depending on their physio-chemical and structural stability. Structural reorientation (both secondary and tertiary) is accompanied by the adsorption for most of the protein while some appears to resist significant conformational change [2,3]. This sort of conformational flexibility and conformation rigidity of proteins classified them into class of ‘‘soft’’ and ‘‘hard’’ proteins. Proteins that have a tendency to under- go major surface-induced conformational change are termed as ‘‘soft’’ and those less vulnerable to conformational change are coined as ‘‘hard’’ protein [4,5]. Based on this property of protein, different proteins (‘‘soft’’ and ‘‘hard’’) are likely to show different adsorption behavior after adsorption onto same surface. A lot of studies have been made to understand how ‘‘soft’’ and ‘‘hard’’ proteins interact with different surface. Norde and Haynes investigated the structural perturbation of lysozyme (hard) and a-lactalbumin (soft) on negatively charged polystyrene latex (PS  ) and variably charged hematite (a-Fe 2 O 3 ) surfaces [6]. Norde and Haynes also studied the change in secondary structural com- ponent of BSA (soft) and lysozyme (hard) upon adsorption onto finely dispersed silica particles [7]. Turci et al. elaborated the con- sequence of adsorption of four proteins:BSA (soft), lysozyme (hard), bovine pancreatic ribonuclease (hard) and bovine lactoper- oxidase (soft), onto two amorphous pyrogenic silica nanoparticles [8]. Further, Wertz et al. substantiated that lysozyme, despite its recognized ‘‘hardness’’ on hydrophilic surfaces, compromises its structural integrity upon adsorption onto hydrophobic surfaces [9]. Cole et al. found the well-structured molecule of collagen undergoes structural perturbations when interacting with the hydrophilic SiO 2 surface, but it weakly interacts with a hydropho- bic derivative of the same surface, which may be explained by its rigid molecular structure [10]. From the above observation it could be concluded that the consequences of adsorption is a very com- plex phenomenon and is very difficult to predict how a ‘‘soft’’ or ‘‘hard’’ protein would behave to a given surface. To quench our inquisitiveness of knowing the adsorption pro- cess better, we have taken two different protein having near same isoelectric point (IEP) but different structural stability for the adsorption interaction study. Two globular, highly abundant proteins, Bovine serum albumin (BSA) and b-lactoglobulin (BLG) 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.10.053 ⇑ Corresponding author at: Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039, India. Fax: +91 361 258 2349. E-mail address: gdas@iitg.ernet.in (G. Das). Journal of Colloid and Interface Science 416 (2014) 235–242 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis