Force Spectroscopy with Optical and Magnetic Tweezers

Abstract

Micromanipulation of individual cells and molecules is increasingly important for a wide range of biophysical research because, although ensemble biochemical analysis provides excellent qualitative and quantitative descriptions, it seldom describes phenomena at the molecular level. By observing the force spectroscopy of single molecules, the kinetics, mechanics, and variation of structure, function, and interactions can be fully explored to provide a more complete physiological picture. The use of electric and magnetic fields for manipulating particles dates back more than a century, with a rich tapestry of applications in separation, filtering and trapping. Recognizing the non-contact advantages of magnetic manipulation, Crick and Hughes probed the physical properties of a cell’s cytoplasm more than fifty years ago using magnetic particles [1]. Two decades later, with the development of intense electromagnetic fields from lasers, the manipulation of latex particles with light was experimentally demonstrated by Ashkin in 1970 in his “levitation traps” [2]. Ashkin went on to pioneer optical trapping of both atoms and biomolecules, leading to one of the most successful technology transfers from a physics lab to cell biology. For many applications, in particular for characterizing biomolecules and their interactions, it is desirable to have a non-contact technique for exerting a force. A non-contact technique allows the behavior of a single molecule under stretching or torsional forces to be measured and manipulated without complicating surface effects or material response limitations. Non-contact techniques also benefit from being easier to multiplex into exerting force on multiple sites of the same molecule or multiple heterogeneous molecules, or to collect parallel statistics on homogeneous copies of the same system. In general they are not limited by access constraints to the interaction volume, and therefore integrate more readily with the desired environmental conditions and other imaging and spectroscopic techniques. For these reasons, and practical reasons such as low cost and biocompatibility, optical and magnetic tweezers have become prominent methods for manipulating and measuring single biological entities and their interactions. To experience a force in an optical or magnetic field, a molecule must possess either dielectric or magnetic contrast against the surrounding medium. Often the entity under observation does not have favorable intrinsic properties either for imaging or for generating a force, and it is necessary or desirable to label the molecule with a particle or tag to improve contrast. These particles or tags can be multifunctional, acting passively as a position and force sensor and actively as a handle through which a force can be exerted on the attached molecule. 2