Materials and Methods
Section 4 Results and Discussion
4.6 Synthesis and characterization of Silica Nanoparticles for Recombinant PTEN Immobilization
Silica nanoparticles were synthesized by modification of the Stober’s process employing poly‐condensation of tetraethylorthosilicate (TEOS) under alkaline environment. The size and shape of the nanoparticles were examined by TEM and FESEM analysis. TEM analysis of the silica nanoparticles displayed synthesis of spherical particles with average diameter of 55 ±10 nm (Figure 4.16A). The size and morphology of nanoparticles was further confirmed by FESEM (Figure 4.16B), which validated the TEM results with average diameter of 60±10 nm. Average diameter was calculated from several images consisting of 100 particles using ImageJ software (Figure 4.16C and 4.16D). Once the synthesis of spherical silica nanoparticles were confirmed, the stabilization of the GST tagged PTEN onto the nanoparticles was evaluated.
Adsorption of GST‐PTEN on silica nanoparticles was determined by fluorescence spectroscopy, DLS measurements, ELISA and FTIR study. Binding was determined by probing the intrinsic fluorescence of protein in 25 mM Hepes pH 7.4 (Figure 4.17A).
Following binding, the supernatant was probed to determine the binding percentage. The maximum binding percentage was calculated to be 49 %, at a protein concentration of 12 nM
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83 Figure 4.16 Characterization of silica nanoparticles (A) TEM image of silica nanoparticles (Scale bar 50 nm, Average diameter 55 ± 10 nm) (B) FESEM image of silica nanoparticles (Scale bar 100 nm, Average diameter 60± 10 nm) (C) Particle size distribution of silica nanoparticles calculated from TEM images using ImageJ software (D) Particle size distribution of silica nanoparticles calculated from FESEM images using ImageJ software.
at silica nanoparticles concentration of 0.8 mg/ml (Figure 4.17B). Bead-based ELISA was performed using anti-PTEN primary antibody, HRP‐conjugated secondary antibody and o- phenylenediamine dihydrochloride (OPD) as substrate. The product measured at 450 nm showed substantial difference between GST-PTEN-SNP and SNP control, indicating successful interaction between the recombinant PTEN and the silica nanoparticles (Figure 4.17C). Protein‐nanoparticles interactions are controlled by a number of factors including charge of the moieties, pH, and surface functionalization among others which can be tuned based on the prospective application (Saptarshi et al., 2013). Based on electrostatic affinity, one would assume negatively charged protein to interact with positively charged nanoparticles. However, electrostatic interactions alone does not represent the major driving force regulating the silica–protein interactions (Clemments et al., 2015). The interaction of
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84 Figure 4.17 Immobilization of GST tagged PTEN onto silica nanoparticles (A) Binding of GST‐PTEN onto silica nanoparticles as determined by probing the intrinsic fluorescence of protein (B) Percentage binding of GST‐PTEN onto silica nanoparticles at varying concentration of proteins, maximum binding obtained was 49 % at a protein concentration of 12 nM (C) ELISA for analysis of binding of GST-PTEN to silica nanoparticles, where 1 is GST-PTEN immobilized on silica nanoparticles, 2 is only silica nanoparticles and 3 is only buffer control.
GST‐PTEN and silica nanoparticles may be predominantly based on large number of surface exposed hydroxyl (OH) groups (Marucco et al., 2014), which provided reactive sites to interact with the protein by several non‐covalent interactions, such as Vander Waal’s force, hydrogen bonding leading to adsorption of protein onto its surface with minimal structural disruption.
Once interaction was confirmed it was important to verify that the nanoparticles shape and structure is intact. TEM images of GST‐PTEN immobilized silica nanoparticles revealed no significant change in the average size and morphology of the nanoparticles after binding of the protein (Figure 4.18A).
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85 Figure 4.18 Characterization of GST‐PTEN‐SNPs (A) TEM image of GST-PTEN bound silica NPs (Scale bar 50 nm, Average diameter 58± 10 nm ) (B) Particle size distribution of GST‐PTEN‐SNPs calculated using ImageJ software.
Average diameter calculated from several images consisting of 100 particles using Image J software (Figure 4.18B) was in the range of 58 ±10 nm. The binding was further analyzed by zeta potential measurements, dynamic light scattering (DLS) and FTIR study. Zeta potential measurements show a slight shift in the zeta potential of the nanoparticles upon incubation with GST-PTEN as compared to free nanoparticles indicating protein-nanoparticle interactions (Figure 4.19A and 4.19B). Hydrodynamic diameter of GST‐PTEN immobilized silica nanoparticles was found to be 133.4 nm. The increase in the diameter as compared to free silica nanoparticles (116.5 nm) was indicative of binding of protein with the nanoparticles (Figure 4.19C and 4.19D). The FTIR spectrum of silica nanoparticles demonstrated peaks at 3441 cm-1 , 1096 cm-1 and 803 cm-1 corresponding to Si‐OH stretching, Si‐O‐Si asymmetric stretching and Si‐O‐Si bending, respectively (Moncada et al., 2007). The FTIR spectrum of recombinant PTEN displayed signature peaks at 1642 cm-1 (Amide I), 1538 cm-1 (Amide II), 1318 cm-1 (Amide III), 668 cm-1 (Amide IV) arising from C=O stretching CN stretching, NH bending, OCN bending, respectively (Barth 2007, Kong and Yu 2007). Observation of peaks corresponding to characteristic bands of peptide linkage in the FTIR spectra of recombinant protein incubated with silica nanoparticles (Shang et al., 2007) indicated the binding of the recombinant PTEN protein to the nanoparticles (Figure 4.20).
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86 Figure 4.19 Immobilization of GST‐PTEN onto silica nanoparticles (A) Zeta potential of silica nanoparticles (B) Zeta potential recombinant GST-PTEN bound silica nanoparticles (C) Dynamic light scattering data of silica nanoparticles (Average diameter of 116.5 nm) (D) Dynamic light scattering data of GST-PTEN bound silica nanoparticles (Average diameter of 133.4 nm).
,
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87 Figure 4.20 FTIR spectra of silica nanoparticles, GST-PTEN and GST-PTEN SNP indicating protein loading.
4.7 Evaluation of Structural and Functional parameters of Recombinant PTEN