title: Structure and Interaction in the pH-Dependent Phase Behavior of Nanoparticle-Protein Systems.
abstract: The pH-dependent structure and interaction of anionic silica nanoparticles (diameter 18 nm) with two globular model proteins, lysozyme and bovine serum albumin (BSA), have been studied. Cationic lysozyme adsorbs strongly on the nanoparticles, and the adsorption follows exponential growth as a function of lysozyme concentration, where the saturation value increases as pH approaches the isoelectric point (IEP) of lysozyme. By contrast, irrespective of pH, anionic BSA does not show any adsorption. Despite having a different nature of interactions, both proteins render a similar phase behavior where nanoparticle-protein systems transform from being one-phase (clear) to two-phase (turbid) above a critical protein concentration (CPC). The measurements have been carried out for a fixed concentration of silica nanoparticles (1 wt %) with varying protein concentrations (0-5 wt %). The CPC is found to be much higher for BSA than for lysozyme and increases for lysozyme but decreases for BSA as pH approaches their respective IEPs. The structure and interaction in these systems have been examined using dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The effective hydrodynamic size of the nanoparticles measured using DLS increases with protein concentration and is related to the aggregation of the nanoparticles above the CPC. The propensity of the nanoparticles to aggregate is suppressed for lysozyme and enhanced for BSA as pH approached their respective IEPs. This behavior is understood from SANS data through the interaction potential determined by the interplay of electrostatic repulsion with a short-range attraction for lysozyme and long-range attraction for BSA. The nanoparticle aggregation is caused by charge neutralization by the oppositely charged lysozyme and through depletion for similarly charged BSA. Lysozyme-mediated attractive interaction decreases as pH approaches the IEP because of a decrease in the charge on the protein. In the case of BSA, a decrease in the BSA-BSA repulsion enhances the depletion attraction between the nanoparticles as pH is shifted toward the IEP. The morphology of the nanoparticle aggregates is found to be mass fractal.
authors.affiliation: Solid State Physics Division, Bhabha Atomic Research Centre , Mumbai 400 085, India.|Homi Bhabha National Institute , Mumbai 400 094, India.
authors.forename: Indresh
authors.initials: I
authors.lastname: Yadav
authors.affiliation: Solid State Physics Division, Bhabha Atomic Research Centre , Mumbai 400 085, India.
authors.forename: Sugam
authors.initials: S
authors.lastname: Kumar
authors.affiliation: Solid State Physics Division, Bhabha Atomic Research Centre , Mumbai 400 085, India.|Homi Bhabha National Institute , Mumbai 400 094, India.
authors.forename: Vinod K
authors.identifier: 0000-0002-2020-9026
authors.initials: VK
authors.lastname: Aswal
authors.affiliation: Laboratory for Neutron Scattering, Paul Scherrer Institut , CH-5232 PSI Villigen, Switzerland.
authors.forename: Joachim
authors.initials: J
authors.lastname: Kohlbrecher
mesh_terms: D000818:Animals; D002417:Cattle; D000067493:Dynamic Light Scattering; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D009113:Muramidase / Q000737:chemistry* / Q000378:metabolism; D053758:Nanoparticles / Q000737:chemistry*; D010316:Particle Size; D053838:Scattering, Small Angle; D012710:Serum Albumin, Bovine / Q000737:chemistry*; D012822:Silicon Dioxide / Q000737:chemistry*; D013499:Surface Properties; D014961:X-Ray Diffraction
chemical_list: D012710:Serum Albumin, Bovine; D012822:Silicon Dioxide; D009113:Muramidase