Nanoscale Measurements of Biochemical Interactions at the Surface of Optically Trapped Particles

Abstract

Molecular interactions at biological surfaces are central to numerous physiological functions, such as cell-cell communication, adhesion, or the immune response to pathogens. Most analyses of these interactions require the use of purified proteins, which are studied in suspension or after immobilization on artificial carrier objects. To characterize instead biomolecular dynamics directly at the surface of small biological particles, we have combined optical tweezers with a multi-channel microfluidics chamber. This instrument allows us to expose a laser-trapped particle to a series of microenvironments that are created without walls by laminar flow. We monitor biochemical reactions between the particle surface and supplied ligands by measuring minuscule changes in the particle's response to a well-defined fluidic drag force. Among others, this response is highly sensitive to the particle size. Once a particle is trapped in a typical experiment, the chamber is moved perpendicularly to the main flow direction in a sinusoidal wave to apply a periodic drag force. Using custom-developed software, the resulting particle motion is tracked with a resolution of ∼5 nm at a rate of up to 3,200 frames per second. A linear-systems analysis allows us to monitor the particle radius in different environments with a resolution of a few nanometers. In first measurements, we used this setup to examine the interaction between protein-A and human IgG. We found that at saturation, bound IgG added an apparent thickness of 12.6±1.4(SD) nm to protein-A-coated beads, in agreement with the expected size of human IgG of ∼12 nm. Moreover, we measured an equilibrium constant of KA = 0.115 nM−1 for the protein-A:IgG interaction, in excellent agreement with literature values.