Binding of immunoglobulin molecules to preadsorbed protein A layers as observed by surface forces measurements

Versatile Protein-A Coated Photoelectric Immunosensors with a Purple-Membrane Monolayer Transducer Fabricated by Affinity-Immobilization on a Graphene-Oxide Complexed Linker and by Shear Flow

Bacteriorhodopsin-embedded purple membranes (PM) have been demonstrated to be a sensitive photoelectric transducer for microbial detection. To efficiently prepare versatile BR-based immunosensors with protein A as antibody captures, a large, high-coverage, and uniformly oriented PM monolayer was fabricated on an electrode as an effective foundation for protein A conjugation through bis-NHS esters, by first affinity-coating biotinylated PM on an aminated surface using a complex of oxidized avidin and graphene oxide as the planar linker and then washing the coating with a shear flow. Three different polyclonal antibodies, each against Escherichia coli, Lactobacillus acidophilus, and Streptococcus mutans, respectively, were individually, effectively and readily adsorbed on the protein A coated electrodes, leading to selective and sensitive quantitative detection of their respective target cells in a single step without any labeling. A single-cell detection limit was achieved for the former two cells. AFM, photocurrent, and Raman analyses all displayed each fabricated layer as well as the captured bacteria, with AFM particularly revealing the formation of a massive continuous PM monolayer on aminated mica. The facile cell-membrane monolayer fabrication and membrane surface conjugation techniques disclosed in this study may be widely applied to the preparation of different biomembrane-based biosensors.

Monte Carlo simulations of antibody adsorption and orientation on charged surfaces

Monte Carlo simulations were performed to study the adsorption and orientation of antibodies on charged surfaces based on both colloidal and all-atom models. The colloidal model antibody consists of 12 connected beads representing the 12 domains of an antibody molecule. The structure of the all-atom antibody model was taken from the protein databank. The effects of the surface charge sign and density, the solution pH and ionic strength on the adsorption and orientation of different colloidal model antibodies with different dipole moments were examined. Simulation results show that both the 12-bead and the all-atom models of the antibody, for which the dipole moment points from the Fc to (Fab)2 fragments, tend to have the desired "end-on" orientation on positively charged surfaces and undesired "head-on" orientation on negatively charged surfaces at high surface charge density and low solution ionic strength where electrostatic interactions dominate. At low surface charge density and high solution ionic strength where van der Waals interactions dominate, 12-bead model antibodies tend to have "lying-flat" orientation on surfaces. The orientation of adsorbed antibodies results from the compromise between electrostatic and van der Waals interactions. The dipole moment of an antibody is an important factor for antibody orientation on charged surfaces when electrostatic interactions dominate. This charge-driven protein orientation hypothesis was verified by our simulations results in this work. It was further confirmed by surface plasmon resonance biosensor and time-of-flight secondary ion mass spectrometry experiments reported elsewhere.

Mannosylerythritol lipids, yeast glycolipid biosurfactants, are potential affinity ligand materials for human immunoglobulin G

Three mannosylerythritol lipids (MEL-A, -B, and -C), yeast glycolipid biosurfactants, were independently attached to poly (2-hydroxyethyl methacrylate) beads (PHEMA), and the three obtained MEL-PHEMA composites were examined for their binding affinity to human immunoglobulin G (HIgG). Of the three composites, the composite bearing MEL-A exhibited the highest binding capacity for HIgG. The binding amount of HIgG increased with increased applied concentration, reaching 106 mg HIgG (per g of composite), with a binding yield of 81%. Interestingly, the protein binding to the composite appeared to follow two different modes (Langmuir type and Freundlich type) depending on the applied concentration. The binding amount of human serum albumin to the composite was much smaller than that of HIgG. The bound human serum albumin, however, had minimal effect on the subsequent binding of HIgG, indicating that the two proteins have different binding sites onto the composite. More significantly, the bound HIgG was efficiently recovered under significantly mild elution conditions: Approximately 90% of the protein was eluted from the composite with phosphate buffer at pH 7. These results indicate that the glycolipid biosurfactant may have great potential as an affinity ligand material for HIgG.