Mechanical forces play a pivotal role in cellular adhesions, where a vast array of proteins interacts with the cell cytoskeleton, affecting focal adhesions and adherens junctions, and regulating cell behavior and fate. To directly observe such events, we developed an experimental assay that combines several advanced single molecule techniques. Here, ultrafast force-clamp spectroscopy is employed to directly probe the force-dependence of molecular interactions between a single actin filament and a binding protein with sub-ms time resolution. Stabilization of the microscope through local gradient localization enables the resolution of protein conformational changes and binding position with sub-nm accuracy. An experimental arrangement, termed oriented dumbbell, allows us to determine the actin filament orientation and, thus, asymmetries in the force response of the interacting proteins. We applied our methodology to the interaction between α-catenin and F-actin, revealing that α-catenin switches between a slip and an asymmetric cooperative catch-bond with F-actin. This mechanism may underlie fluid-to-solid phase transitions that occur at the membrane-cytoskeleton interface.
We use dynamically controlled annular beam optical tweezers to orientate non-spherical swimming E.coli. Typically elliptically shaped particles in a Gaussian optical trap will align themselves with the direction of beam propagation. This orientation makes determining the dynamics of swimming particles difficult, as most optical tweezers systems are only able to capture information in the focal plane, but not the axial direction. We use simulations and experimental measurements to show that we are able to orientate motile E.coli from a vertical to horizontal position with a spatial light modulator in as little as one intermediate step without reduction in trap stiffness.
Structured light is readily used to control and investigate microsystems. Dynamic manipulation of microscopic particles of matter has enabled non-contact investigations of a large number of complex physical, biological and chemical systems. In recent years, ever more sophisticated techniques to control microsystems have been developed, shadowing rapid advances in computer technology. This work utilises computational and other signal processing techniques to dynamically control input light fields and modulate the scattered output light fields to improve the specificity of measurements using either optical switching or beam shaping. The aggregate properties of structured light such as linear and angular momentum are changed when scattered from a particle. We present improvements to measurement of signals otherwise hidden in noise or other larger signals by selectively filtering considering the amplitude, phase or wave function symmetry. Measurements are demonstrated using both liquid crystal spatial light modulators and digital micromirror devices. Further challenges that occur in development of these techniques will be discussed as well as their use when applied to specific experiments. We will give an overview of both past successes in measurement of position, force and angular momentum transfer and other current research developments.
Accurate centroid position measurements of a light spot are vital for optical tweezers. To get quantitative measurements we can find the optical force by measuring the change in momentum of the trapping beam and track the position of the trapped particle using its image. We propose to use a filter with a linear transmittance to modulate the amplitude of the input beam in such a way that the transmitted intensity of the light will be proportional to the displacement of the spot. Our method is compared with existing methods and we find that it provides accurate measurements at high bandwidth.
Precise temperature measurements in optical traps are crucial for biological objects as they are highly sensitive to the temperature. We shift the trapped particle from the equilibrium position by moving the stage and record the position and the optical force synchronously. The absolute nature of the force detection method makes it suitable for measuring optical forces in a non-linear region of the optical trap increasing the precision to 0.2K. Moreover, the proposed method does not require a precalibration of the force detector and, therefore, can be used for temperature independent calibration.
Particle–wall interactions are important in biology, micromachining, coagulation studies, and many other areas of science. As a contactless tool, optical tweezers are ideal for measuring these kind of interactions. Here we will present a new method for calculating the non-optical forces acting on a trapped particle using simultaneous position and force detection. Analysis of the particle's Brownian motion when trapped gives a measure of all the forces experienced by the particle. In contrast, measuring only the light's momentum change directly gives the solely optical force. This is achieved measuring the changes in the scattered light. The difference between the forces recorded by the two techniques reveals the external forces acting on the trapped particle. Therefore, by trapping the particle close to a wall, one can study the particle-wall interaction force in details. The simulation were done using the optical tweezer toolbox [1] to find the optical force acting on a particle. The net force was calculated from a Brownian motion’s statistics of a trapped particle in the presence of the exponential external force. By using the proposed method, we were able to successfully reconstruct the external force. The experiment was done on a trapped spherical PMMA particle (d=2.2um) close to the 3D-printed wall. For the particle-wall distance ~0.7um the non-optical force is ~100fN . The experiment and simulation results confirm the efficiency of the proposed method for an external force measurements. [1] Nieminen et al., J. Opt. A 9, S196-S203 (2007).
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.