New tools to study biophysical properties of single molecules and single cells

Abstract

We present a review on two new tools to study biophysical properties of single molecules and single cells. A laser incident through a high numerical aperture microscope objective can trap small dielectric particles near the focus. This arrangement is named optical tweezers. This technique has the advantage to permit manipulation of a single individual object. We use optical tweezers to measure the entropic elasticity of a single DNA molecule and its interaction with the drug Psoralen. Optical tweezers are also used to hold a kidney cell MDCK away from the substrate to allow precise volume measurements of this single cell during an osmotic shock. This procedure allows us to obtain information about membrane water permeability and regulatory volume increase. Defocusing microscopy is a recent technique invented in our laboratory, which allows the observation of transparent objects, by simply defocusing the microscope in a controlled way. Our physical model of a defocused microscope shows that the image contrast observed in this case is proportional to the defocus distance and to the curvature of the transparent object. Defocusing microscopy is very useful to study motility and mechanical properties of cells. We show here the application of defocusing microscopy to measurements of macrophage surface fluctuations and their influence on phagocytosis.

Elasticity measurement of living cells with an atomic force microscope: data acquisition and processing

“…In magnetic tweezers experiments, a Young's modulus of ∼400 Pa was reported for human umbilical vein endothelial cells (HUVEC) [17], and ∼350 Pa was found for macrophages [3]. Optical tweezers studies revealed a Young's modulus of ∼450 Pa for macrophages [51] and ∼250 Pa for alveolar epithelial cells [33]. The more common use of sharp tips led to Young's moduli in the range of 1-50 kPa, as shown for platelets [49], HUVEC [25,26], chicken cardiomyocytes [28], and macrophages [52].…”

Modeling the entropic structural transition of DNA complexes formed with intercalating drugs

“…To perform these measurements, the techniques of optical tweezers [20,[22][23][24][25][26][27] and intensity autocorrelation spectroscopy [28] were used. The details about these two techniques and about the experimental procedure used can be found in [5,6,[27][28][29].…”

Real-Time Monitoring of Cell Elasticity Reveals Oscillating Myosin Activity

“…An AFM equipped with either a sharp tip or a colloidal probe in which the tip is replaced by a sphere (30), has been widely used to measure the mechanical properties of soft materials (4,31). Elasticity of cells are not only determined by AFM techniques but also by optical (32,33) and magnetic (34,35) tweezers. Optical tweezers use a focused laser beam and magnetic tweezers use a magnetic field gradient to provide the attractive or repulsive forces (typically in the order of pN) required to hold and move a bead.…”