pH-Dependent Interaction and Resultant Structures of Silica Nanoparticles and Lysozyme Protein

Small-angle neutron scattering (SANS) and UV–visible spectroscopy studies have been carried out to examine pH-dependent interactions and resultant structures of oppositely charged silica nanoparticles and lysozyme protein in aqueous solution. The measurements were carried out at fixed concentration (1 wt %) of three differently sized silica nanoparticles (8, 16, and 26 nm) over a wide concentration range of protein (0–10 wt %) at three different pH values (5, 7, and 9). The adsorption curve as obtained by UV–visible spectroscopy shows exponential behavior of protein adsorption on nanoparticles. The electrostatic interaction enhanced by the decrease in the pH between the nanoparticle and protein (isoelectric point ∼11.4) increases the adsorption coefficient on nanoparticles but decreases the overall amount protein adsorbed whereas the opposite behavior is observed with increasing nanoparticle size. The adsorption of protein leads to the protein-mediated aggregation of nanoparticles. These aggregates are found to be surface fractals at pH 5 and change to mass fractals with increasing pH and/or decreasing nanoparticle size. Two different concentration regimes of interaction of nanoparticles with protein have been observed: (i) unaggregated nanoparticles coexisting with aggregated nanoparticles at low protein concentrations and (ii) free protein coexisting with aggregated nanoparticles at higher protein concentrations. These concentration regimes are found to be strongly dependent on both the pH and nanoparticle size

Interaction of Cationic Lipid/DNA Complexes with Model Membranes As Determined by Neutron Reflectivity

Transfection of cells by DNA for the purposes of gene therapy can be effectively engineered through the use of cationic lipid/DNA “lipoplexes”, although the transfection efficiency of these complexes is sensitive to the neutral “helper” lipid included. Here, neutron reflectivity has been used to investigate the role of the helper lipid present during the interaction of these lipoplexes with model membranes composed primarily of zwitterionic lipid 1,2-dimyristoylphosphatidylcholine (DMPC) together with 10 mol % 1,2-dipalmitoylphosphatidylserine (DPPS). Dimethyldioctadecylammonium bromide (DDAB) vesicles were formed with two different helper lipids, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) and cholesterol, and complexed with a 1:1 charge ratio of DNA. The interaction of these complexes with the supported phospholipid bilayer was determined. DOPE-containing lipoplexes were found to interact faster with the model cell membrane than those containing cholesterol, and complexes containing either of the neutral helper lipids were found to interact faster than when DDAB alone was present. The interaction between the lipoplexes and the model membrane was characterized by an exchange of lipid between the membrane and the lipid/DNA aggregates in solution; the deposition of (additional) lipid on the surface of the model cell membrane was not apparent

Interaction of Cationic Lipid Vesicles with Model Cell MembranesAs Determined by Neutron Reflectivity

Transfection of cells by DNA (for the purposes of gene therapy) can be effectively engineered through the use of cationic lipid/DNA “lipoplexes”, although the transfection efficiency of these lipoplexes is sensitive to the neutral “helper” lipid included. Here, neutron reflectivity has been used to investigate the role of the helper lipid present during the interaction of cationic lipid vesicles with model cell membranes. Dimethyldioctadecylammonium bromide (DDAB) vesicles were formed with two different helper lipids, 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) and cholesterol, and the interaction of these vesicles with a supported phospholipid bilayer was determined. DOPE-containing vesicles were found to interact faster with the membrane than those containing cholesterol, and vesicles containing either of the neutral helper lipids were found to interact faster than when DDAB alone was present. The interaction between the vesicles and the membrane was characterized by an exchange of lipid between the membrane and the lipid aggregates in solution; the deposition of vesicle bilayers on the surface of the membrane was not apparent