title: Torsion stiffness of a protein pair determined by magnetic particles.
abstract: We demonstrate the ability to measure torsion stiffness of a protein complex by applying a controlled torque on a magnetic particle. As a model system we use protein G bound to an IgG antibody. The protein pair is held between a magnetic particle and a polystyrene substrate. The angular orientation of the magnetic particle shows an oscillating behavior upon application of a rotating magnetic field. The amplitude of the oscillation increases with a decreasing surface coverage of antibodies on the substrate and with an increasing magnitude of the applied field. For decreasing antibody coverage, the torsion spring constant converges to a minimum value of 1.5 × 10(3) pN·nm/rad that corresponds to a torsion modulus of 4.5 × 10(4) pN·nm(2). This torsion stiffness is an upper limit for the molecular bond between the particle and the surface that is tentatively assigned to a single protein G-IgG protein pair. This assignment is supported by interpreting the measured stiffness with a simple mechanical model that predicts a two orders of magnitude larger stiffness for the protein G-IgG complex than values found for micrometer length dsDNA. This we understand from the structural properties of the molecules, i.e., DNA is a long and flexible chain-like molecule, whereas the antibody-antigen couple is orders of magnitude smaller and more globular in shape due to the folding of the molecules.
authors.affiliation: Eindhoven University of Technology, Eindhoven, The Netherlands.
authors.forename: X J A
authors.initials: XJ
authors.lastname: Janssen
authors.forename: J M
authors.initials: JM
authors.lastname: van Noorloos
authors.forename: A
authors.initials: A
authors.lastname: Jacob
authors.forename: L J
authors.initials: LJ
authors.lastname: van Ijzendoorn
authors.forename: A M
authors.initials: AM
authors.lastname: de Jong
authors.forename: M W J
authors.initials: MW
authors.lastname: Prins
mesh_terms: D007074:Immunoglobulin G / Q000737:chemistry* / Q000378:metabolism; D008280:Magnetics / Q000379:methods*; D009419:Nerve Tissue Proteins / Q000737:chemistry* / Q000378:metabolism; D013997:Time Factors; D019415:Torque; D054159:Torsion, Mechanical
chemical_list: C029152:G-substrate; D007074:Immunoglobulin G; D009419:Nerve Tissue Proteins