Biomedical subjects

Xiubo Zhao

Publications and source records attributed to Xiubo Zhao.

4 recordsLinked to original sources

PubMed2006

Coadsorption of human milk lactoferrin into the dipalmitoylglycerolphosphatidylcholine phospholipid monolayer spread at the air/water interface.

The coadsorption of human milk lactoferrin into a spread monolayer of dipalmitoylglycerol phosphatidylcholine (DPPC) at the air/water interface has been studied by neutron reflection. The system is a good model of the preocular tear film outer interface, which was the motivation for the study. The association of the protein with the surface was indicated by an increase of the surface pressure exerted by the DPPC monolayer. The extent of lactoferrin coadsorption was found to decrease with increasing surface pressure in the lipid monolayer, a trend consistent with the observation reported for other proteins, such as lysozyme and beta-lactoglobulin. The neutron reflectivity measurements were subsequently carried out at the three surface pressures of 8, 15, and 35 mN/m to examine the structure and composition of lactoferrin coadsorbed at the interface. Whereas the DPPC monolayer effectively prevented lactoferrin insertion at the high surface pressure, a measurable amount of lactoferrin was found at the air/water interface at the two lower surface pressures. At 15 mN/m it was difficult to identify the distribution of lactoferrin with respect to the DPPC monolayer, due to its relatively low adsorbed amount and much broader distribution. At the lowest surface pressure of 8 mN/m, the lactoferrin coadsorption was found to increase with time over the first few hours. After 5 h the distribution of the lactoferrin layer became similar to, though quantitatively lower than, that adsorbed in the absence of the DPPC monolayer. It is characterized by a top dense sublayer of 15 A with a bottom diffuse sublayer of 60 A, indicating structural unfolding induced by surface adsorption under these conditions.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2006

Orientation of a monoclonal antibody adsorbed at the solid/solution interface: a combined study using atomic force microscopy and neutron reflectivity.

Conformational orientations of a mouse monoclonal antibody to the beta unit of human chorionic gonadotrophin (anti-beta-hCG) at the hydrophilic silicon oxide/water interface were investigated using atomic force microscopy (AFM) and neutron reflectivity (NR). The surface structural characterization was conducted with the antibody concentration in solution ranging from 2 to 50 mg.L(-1) with the ionic strength kept at 20 mM and pH = 7.0. It was found that the antibody adopted a predominantly "flat-on" orientation, with the Fc and two Fab fragments lying flat on the surface. The AFM measurement revealed a thickness of 30-33 A of the layer formed in contact with 2 mg.L(-1) antibody in water, but, interestingly, the flat-on antibody molecules formed small nonuniform clusters equivalent to 2-15 antibody molecules. Parallel AFM scanning in air revealed even larger surface clusters, suggesting that surface drying induced further aggregation. The AFM study thus demonstrated that the interaction between protein and the hydrophilic surface is weak and indicated that surface aggregation can be driven by the attraction between neighboring protein molecules. NR measurements at the solid/water interface confirmed the flat-on layer orientation of adsorbed molecules over the entire concentration range studied. Thus, at 2 mg.L(-1), the adsorbed antibody layer was well represented by a uniform layer with a thickness of 40 A. This value is thicker than the 30-33 A observed from AFM, suggesting possible layer compression caused by the tip tapping. An increase in the antibody concentration to 10 mg.L(-1) led to increasing surface adsorption. The corresponding layer structure was well represented by a three-layer model consisting of an inner sublayer of 10 A, a middle sublayer of 30 A, and an outer sublayer of 25 A, with the protein volume fractions in each sublayer being 0.22, 0.42, and 0.10, respectively. The structural transition can be interpreted as a twisting and tilting of segments of the adsorbed molecules, driven by an electrostatic repulsion between them that increases with the surface packing density. Hindrance of antigen access to antibody binding sites, resulting from the change in surface packing, can account for the decrease in antigen binding capacity (AgBC) with increasing surface density of the antibody that is observed.

Animals↗

PubMed2005

Solution pH-regulated interfacial adsorption of diblock phosphorylcholine copolymers.

Spectroscopic ellipsometry has been used to examine the pH-responsive interfacial adsorption of a series of biocompatible diblock copolymers incorporating 2-methacryloyloxyethyl phosphorylcholine-based (MPC) residues and 2-(dialkylamino)ethyl methacrylate residues, with a specific focus on 2-(diethylamino)ethyl groups (referred to as MPCm-DEAn, where m and n refer to the mean degrees of polymerization of each block) at the hydrophilic silicon oxide/water interface. For all the copolymers studied the surface excess shows only weak concentration dependence. Increasing the length of the DEA block has little effect on the dynamic or equilibrated adsorption at pH 7, indicating that the DEA block adopts a flat conformation on the silicon oxide surface at this pH. With increasing pH, however, the surface excess shows a dramatic increase, followed by a subsequent decline. The observed maximum in surface excess represents a balance between charge over-compensation of the copolymer with the oppositely charged surface and the subsequently reduced charge density of the copolymer. Variations in the observed maxima for various MPCm-DEAn diblock copolymers indicate different surface conformations at high pH. Salt addition does not affect copolymer adsorption. This behavior is attractive for biomedical applications in which the ionic strength is variable. It was also found that the preadsorbed diblock copolymers immobilized DNA from solution to an extent that is proportional to the relative charge ratio between the anionic DNA and the cationic DEA block of the copolymer.

Adsorption↗

PubMed2005

Surface-induced unfolding of human lactoferrin.

We have determined the structural conformations of human lactoferrin adsorbed at the air/water interface by neutron reflectivity (NR) and its solution structure by small angle neutron scattering (SANS). The neutron reflectivity measurements revealed a strong structural unfolding of the molecule when adsorbed at the interface from a pH 7 phosphate buffer solution (PBS with a total ionic strength at 4.5 mM) over a wide concentration range. Two distinct regions, a top dense layer of 15-20 angstroms on the air side and a bottom diffuse layer of some 50 angstroms into the aqueous subphase, characterized the unfolded interfacial layer. At a concentration around 1 g dm(-3), close to the physiological concentration of lactoferrin in biological fluids, the adsorbed amount was 5.5 x 10(-8) mol m(-2) in the absence of NaCl, but the addition of 0.3 M NaCl reduced protein adsorption to 3.5 x 10(-8) mol m(-2). Although the polypeptide distributions at the interface remained similar, quantitative analysis showed that the addition of NaCl reduced the layer thickness. Parallel measurements of lactoferrin adsorption in D2O instead of null reflecting water confirmed the unfolded structure at the interface. Furthermore, the D2O data indicated that the polypeptide in the top layer was predominantly protruded out of water, consistent with it being hydrophobic. In contrast, the scattering intensity profiles from SANS were well described by a cylindrical model with a diameter of 47 angstroms and a length of 105 angstroms in the presence of 0.3 M NaCl, indicating a retention of the globular framework in the bulk solution. In the absence of NaCl but with the same amount of phosphate buffer, the length of the cylinder increased to some 190 angstroms and the diameter remained constant. The length increase is indicative of changes in distance and orientation between the bilobal monomers due to the change in charge interactions. The results thus demonstrate that the surface structural unfolding was caused by the exposure of the protein molecule to the unsymmetrical energetic balance following surface adsorption.

Adsorption↗