SANS and UV–vis Spectroscopy Studies of Resultant Structure from Lysozyme Adsorption on Silica Nanoparticles

Abstract

The interaction of lysozyme protein (M.W. 14.7 kD) with two sizes of silica nanoparticles (16 and 25 nm) has been examined in aqueous solution using UV-vis spectroscopy and small-angle neutron scattering (SANS). The measurements were performed on fixed concentration (1 wt %) of nanoparticles and varying concentration of protein in the range 0 to 2 wt %. The adsorption isotherm as obtained using UV-vis spectroscopy suggests strong interaction of the two components and shows an exponential behavior. The saturation values of adsorption are found to be around 90 and 270 protein molecules per particle for 16 and 25 nm sized nanoparticles, respectively. The adsorption of protein on nanoparticles leads to the aggregation of particles and these structures have been studied by SANS. The aggregates are characterized by fractal structure coexisting with unaggregated particles at low protein concentrations and free proteins at higher protein concentrations. Further, contrast variation SANS measurements have been carried out to differentiate the adsorbed and free protein in these systems.

IgG and fibrinogen driven nanoparticle aggregation

“…The same trend has been observed for transferrin or serum albumin mixed with sulfonated polystyrene NPs [16,17], while other studies show that gold NPs are stabilized by serum albumin and cytochrome C [18][19][20][21]. Lysozyme, however, can induce the strong aggregation of silica, polystyrene, and gold NPs [17,[22][23][24][25], which also is observed when fibrinogen is incubated with polystyrene and silica NPs [26]. Interestingly, fibrinogen induces aggregation of poly-(acrylic acid)-conjugated gold NPs [27] in a protein concentration-dependent manner.…”

Formation of Silica-Lysozyme Composites Through Co-Precipitation and Adsorption

“…However, what we can deduct from the current data is the fact that neither of the inter-particle distances determined for the composite samples could accommodate for a naturally folded lysozyme molecules in between silica particles (since the lysozyme molecules are ellipsoidal and 3 nm × 4.5 nm in size, and the correlation peak would thus be expected at q ~ 0.8 nm −1 ). The same was observed by Kumar et al (2011) when analyzing silica-lysozyme aggregates by small-angle neutron scattering. These authors attributed the "missing" space between particles as an indication for the near-complete loss of structural coherence of the lysozyme molecule upon adsorption to multiple silica spheres.…”

“…It consists of 129 amino acids, including 6 lysine and 11 arginine residues exposed at the surface of the molecule [Figure S1 in Supplementary Material, Canfield (1963), Jollès et al (1963)]. This gives the molecule an overall positive surface charge over a large pH range [pHIEP = 11.1, Haynes and Norde (1994)] making electrostatic interactions with negatively charged silica monomers, nanoparticles, and surfaces possible (Coradin et al, 2003;Bharti et al, 2011;Kumar et al, 2011). The interaction between these organic macromolecules and silica occurs mainly by electrostatic interaction of primary amino groups (-NH2) with silica (Sumper and Brunner, 2008;Otzen, 2012), although hydrophobic interactions may also contribute (see below).…”

Size dependent fractal aggregation mediated through surfactant in silica nanoparticle solution

“…1). The data clearly show a high scattering with linear dependence in the low Q region indicating the formation of fractal structure particle aggregates in the system [4]. The scattering at high Q region matches to that from the DTAB micelles which could be either adsorbed on nanoparticle or free in the mixed system.…”

“…In SANS, one measures the coherent scattering intensity as a function of Q which can be given by [4] …”