Role of (single/double chain surfactant) micelles on the protein aggregation

Highlights

• Role of micelles in interaction of BSA with cationic monomeric/dimeric surfactants.

• Lifetime studies show that after the CMC the aggregation is more in surfactants.

• Aggregation is found more in presence of 16-6-16 compare to CTAB.

• AFM shows the structural change above/below the CMC of corrosponding surfactants.

• CMC plays a role in controlling of aggregation: Alter the binding interactions.

Abstract

To investigate the interaction between the bovine serum albumin (BSA) and cationic surfactants (monomeric, cetyltrimethylammonium bromide, CTAB) and dimeric/gemini, 1, 6 bis (N, N-hexadecyl dimethyl ammonium bromide, 16-6-16) and to find out the role of micelles in the aggregation of the protein using spectroscopic (UV–visible, fluorescence, fluorescence lifetime measurements, circular dichroism (CD), etc.) and microscopic (atomic force microscope (AFM)) techniques. The different surfactant has an effect on the polarity of the microenvironment of the protein shows in all the spectroscopic technique at below and above the critical micelle concentration (CMC). The far-UV CD spectra show that BSA is more disrupted by the dimeric surfactant compared to the monomeric CTAB above the CMC. The binding of the surfactant induce changes in the microenvironment around the aromatic amino acids residues and disulfide bond of the BSA at different pHs. The binding constant values were found to be 20.278 × 10³ M and 8.443 M for the BSA-CTAB complex and BSA-16-6-16 complex, respectively. Atomic force microscope indicates the aggregation is more in case of dimeric (16-6-16) surfactant compared to the monomeric (CTAB) surfactant at the higher concentration (above their CMCs). Below and above the CMC, all changes are noticeable.

Introduction

Proteins are essential for a living organism and participating in almost all biological processes [[1], [2], [3], [4], [5], [6]]. Surfactant added to protein solution can modify the adsorption layer properties at liquid/fluid interfaces significantly [7]. Protein-surfactant systems widely used in modern technologies, such as food processing, cosmetics, and pharmaceutical industries [[8], [9], [10], [11]]. Proteins can interact with surfactant molecules in bulk amount and at the surface in many ways, which results in the complexes of the different surface activity. The interaction between surfactant and protein can be electrostatic/hydrophobic and change the surface activity and protein conformation in bulk [[12], [13], [14], [15]]. Due to hydrophobic and hydrophilic properties of amino acids, express the dual nature of the protein and make able to small amphiphilic molecules to interact [[16], [17], [18], [19]]. In case of membrane protein structure might form the alternating sequence in which there is a hydrophilic residue at every third and fourth position in a hydrophobic stretch of the amino acids and this kind of sequences for helices with hydrophobic surfaces, but edges possess the hydrophilic surface [[20], [21], [22]].

Proteins are used to binding with many surfactants cooperatively and form protein-surfactant complexes where surfactant's hydrophobic moieties caused the folding, unfolding and aggregation of the protein by interacting with nonpolar amino acids residues [23,24]. Anionic surfactants use to determine the molecular weight of the polymer or protein like SDS-PAGE [25]. The non-ionic surfactant is often added to a protein solution to prevent the aggregation and unwanted adsorption during the purification, freezing-drying, filtration, and storage [26]. The cationic surfactant can interact with the proteins by electrostatic and hydrophobic interaction, attributed the excellent deodorization and antimicrobial properties or cationic surfactant usually used as a bactericide in the various system by the protein denaturation [14,27].

Each surfactant tends to form micelle and the micelle form at a concentration which is called as critical micelle concentration, i.e., CMC. The protein and pH of the medium will affect the CMC of the surfactants [28]. Monomeric surfactant, like CTAB, has higher CMC value compare to dimeric surfactant/16-6-16. Dimeric surfactant contains two hydrophobic chains and two polar head groups connected by the spacer at or near the head groups [[29], [30], [31]]. They have received attention due to their unique properties, in particular, lower CMC correspondence to the singled chain surfactant [23,[32], [33], [34]].

Levinthel [35] explained that protein folding is not an instant process and it has fellow curtain interconversion to get the stable conformational space and change into fully folded three dimensional. C.M. Dobson [36] used the term misfolding to describe the processes that result in a protein acquiring the sufficient number of persistent non-active interactions to affects its overall conformation and its biological properties. The aggregation is a process which can lead to peptide or protein to form fibrils and these fibrils described as “misfolding,” as interactions determine the structure and properties of such fibrils be distinct from those that define the structure and properties of biologically active compounds.

The aggregation process of protein is strongly affected by the temperature, pH, ionic strength, metal ions, and concentration of the additive [[37], [38], [39]]. These variables cause the different conformations, and these conformations may cause the aggregation. The surfactant used to modulate the protein's structure and properties [40]. In this process, the molecule has been reported to form face to face stacked complex with protein molecules and favouring their aggregation [41]. Protein aggregation is an essential biological process in which misfolded proteins aggregate intra- or extracellular [42]. Protein aggregation is a significant problem in diverse areas such as efficient to form soluble proteins biopharmaceutical, production of recombinant protein, and studies of folding, unfolding, and stability of the protein. Many diseases caused by protein aggregation like sickle cell anemia, Alzheimer's disease, Parkinson's disease, Prion and Huntington's diseases [41,[43], [44], [45], [46], [47]].

In previous work, we have worked on BSA with anionic surfactant (SDS), and results were intersecting [28]. There are some papers where researchers studied the effect of below cmc and above cmc concentration of surfactants. However, to the best of our knowledge, no systematic details study was done on it. Therefore, in the present work, we are focusing on the interaction studies between BSA with monomeric (CTAB) and dimeric (16-6-16) surfactants by using the different spectroscopic techniques like UV–visible, fluorescence, time-resolved measurement, and circular dichroism and investigating the role of micelles (CMC) during the protein-surfactant interactions. The change before and after the CMC are appreciable. The results of the work could be useful for fundamental basic research and industrial applications.