Elsevier

Chemical Physics Letters

Volume 700, 16 May 2018, Pages 15-21
Chemical Physics Letters

Research paper
Spectroscopic exploration of interaction between PEG-functionalized Ag2S nanoparticles with bovine serum albumin

https://doi.org/10.1016/j.cplett.2018.04.004Get rights and content

Highlights

  • The interactions of BSA with PEG capped Ag2S nanoparticles were investigated using spectroscopic techniques.

  • Quenching of BSA fluorescence is occurred after complexation with Ag2S nanoparticles.

  • The overall conformation of BSA is unaltered after complexation with PEG-Ag2S nanoparticles.

Abstract

The introduction of nanoparticles into biological fluids often leads to the formation of biocorona over the surface of nanoparticles. For the effective use of nanoparticles in biological applications it is very essential to understand their interactions with proteins. Herein, we investigated the interactions of Poly ethylene glycol capped Ag2S nanoparticles with Bovine Serum Albumin by spectroscopic techniques. By the addition of Ag2S nanoparticles, a ground state complex is formed. The CD spectroscopy reveals that the secondary structure of BSA is altered by complexation with PEG-Ag2S nanoparticles, while the overall tertiary structure remains closer to that of native BSA.

Introduction

Exploration of the complex interactions of biomolecules with nanoparticles is necessary for their safe and effective use in pharmaceutical and biomedical applications. Once the nanoparticles enter the body, they interact and adsorb proteins to form a protein “corona” and the biological fate of the nanoparticles are decided by the corona [1], [2]. The corona formed on the surface of the nanoparticles ultimately determines the cell-nanoparticles interactions [3], [4]. Serum albumins form the first layer of the long lived hard corona on the surface of the nanoparticles. The interactions of proteins with nanoparticles may induce some physiological changes in proteins and perturb its normal function leading to unexpected biological reactions and toxicity [5]. The in vitro investigations on the interactions of nanoparticles with proteins provide the nature of interacting forces, binding energy, binding sites etc.

Transition metal chalcogenides such as sulfides, selenides and tellurides offer tremendous interest due to their attractive properties. They are potential candidates for many applications including, bio imaging, optoelectronics, photo catalysis, pigments, and superconductors [6], [7], [8]. Silver chalcogenide (I–VI) semiconductor nanomaterials are most promising sulfide nanocrystals, and have extensive applications in optoelectronics, biosensing and catalysis [7], [8], [9], [10], [11]. Recently, Ag2S nanoparticles are widely used for the fluorescence label in biology and medicine because of its near infrared (NIR) photoluminescence and low toxicity [12], [13]. Also Ag2S semiconductor nanoparticles have narrow bandgap (1.19 eV) with good chemical stability, high absorption coefficient and excellent optical properties. Ag2S nanocrystals can take three forms: monoclinic α-Ag2S, body-centered cubic β-Ag2S and face-centered cubic γ-Ag2S [14]. The low solubility product of Ag2S (Ksp = 6.3 × 10−50) ensures the minimum release of Ag ion into the biological surroundings [15]. Considering these properties Ag2S nanoparticles are one of the best candidates for biological applications. Therefore it is necessary to understand the molecular level interactions of Ag2S nanoparticles with bio macromolecules for their effective use in diagnostics.

In the present study, we selected Bovine Serum Albumin (BSA) as a model protein for the interactions with PEG capped Ag2S nanoparticles. BSA is a remarkable protein for the investigation of the interactions with various molecules because it is capable of binding numerous ligands with high binding affinity [16], [17]. BSA possesses specific fluorescence properties due to the presence of tryptophan residues on its surface and in the hydrophobic pocket. The tertiary structure of BSA consists of three domains (I, II & III) and each domain is divided into two subdomains (A & B) and are connected by 17 disulfide bonds. The secondary structure of BSA contains 67% of α-helices and remaining contents consist of β-sheets, turns and random coils. The intrinsic fluorophores of BSA are tryptophan, tyrosine and phenylalanine and the major contribution from the tryptophan residues, Trp-212 and 134 located in the hydrophobic environment in the subdomain IIA and the hydrophilic environment in the subdomain IB near the surface respectively [18].

An extensive study regarding the interaction of various nanoparticles with BSA had been carried out. Based on the recent reports the interactions of MnO2 and Au nanoparticle with BSA have been studied and found insignificant changes in the conformation of BSA [19]. Along with they also demonstrated the role of Van der Waals and hydrogen bonding interactions in binding specificity [20]. Apart from this, the interactions of different biopolymers coated mesoporous silica nanoparticles with BSA were elucidated and investigated the role of different biopolymers on the protein corona formation and the destiny of the nanoparticles [21]. Based on the background of these studies, we have also selected BSA for investigating the interactions of Poly ethylene glycol capped Ag2S nanoparticles (PEG-Ag2S NPs) with BSA to the biological system by spectroscopic methods. However to our knowledge the binding mechanism of PEG capped Ag2S nanoparticles with BSA are not investigated so far. Ag2S nanoparticles were synthesized using PEG as capping agent. For the specific interactions of nanoparticles with biomolecules it is necessary to modify the surface of the nanoparticles with suitable chemical species [22]. Polymers are excellent candidates for the functionalization of nanoparticles. Among the various polymeric materials PEG plays an important role because it reduces the degree of opsonisation and provides excellent long term stability [23], [24]. The objectives of the present study are to investigate the binding of Ag2S nanoparticles with BSA, the impact of binding on the BSA structure, determine the binding parameters and binding mechanism. This study will strengthen our understanding of the proper use of Ag2S nanoparticles for biological applications.

Section snippets

Materials

Poly ethylene glycol 4000 (H (OCH2CH2)n OH), Silver nitrate and Sodium sulfide were purchased from Merck and Bovine Serum Albumin (BSA) was purchased from Sigma Aldrich. All of the reagents were of analytical grade and used without further purification.

Instrumentation

The physicochemical characterization of the nanoparticles was carried out using transmission electron microscopy (JEOL JEM 2100 with LaB6 filament with an operating voltage of 200 kV), XRD (PANalytical X-ray diffractometer (CuKα radiation

Characterization of PEG capped Ag2S nanoparticles

Fig. 1(a) shows the X- ray diffraction spectrum of the PEG-Ag2S NPs. The major peaks in the diffraction pattern were indexed to the monoclinic structure of Ag2S nanoparticles (JCPDS 893840). The average particle size estimated from Scherer formula is ∼30 nm [25].

To identify the presence of PEG on the surface of Ag2S nanoparticles FTIR spectrum was recorded (Fig. 1(b)). The peak at 2909 cm−1 corresponds to the C-H stretching vibration. The stretching vibrations of Cdouble bondO, Csingle bondC and Csingle bondO are observed at

Conclusions

The adsorption of proteins on the surface of nanoparticle upon exposure to biological fluids needs to be embraced and exploited. In this work, we have explored the interactions of BSA with PEG capped Ag2S nanoparticles using spectroscopic techniques. The adsorption of BSA on the surface of PEG capped Ag2S nanoparticles leads to the fluorescence quenching of BSA and the mechanism of quenching is static, confirmed from fluorescence decay measurements. The increase in the absorbance of BSA upon

Acknowledgements

The authors SP and DRR would like to acknowledge UGC for BSR-RFSMS fellowship.

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