Laser-tweezer experiments

Recent laser-tweezer experiments used antibody pairs to demarcate tandem Ig segments within single titin molecules (Leake et al, 2004)... 

There are other major objections to Crocker and Grier s claim [13, 14], one of which we consider in Chapter 7. First, there is the timescale of the measurements. The laser tweezer experiments measure transient collisions, whereas equilibrium in these dispersions usually takes weeks or months to achieve [17], Inspection of Figure 3.8 reveals that any transient experiment is going to be dominated by the medium-range strong repulsion. The minimum is so shallow that the true equilibrium nature of the forces takes weeks to reveal itself against the background of the Brownian motion. We also observed that when an n-butylammonium vermiculite gel had been compressed... 

In this chapter, we will show how nonequilibrium methods can be used to calculate equilibrium free energies. This may appear contradictory at first glance. However, as was shown by Jarzynski [1, 2], nonequilibrium perturbations can be used to obtain equilibrium free energies in a formally exact way. Moreover, Jarzynski s identity also provides the basis for a quantitative analysis of experiments involving the mechanical manipulation of single molecules using, e.g., force microscopes or laser tweezers [3-6]. 

In optical tweezer experiments, the optical scattering force is used to trap particles, but the force can also be used to control the shape of liquid droplets26. An infrared laser with 43-mW power focused onto a microdroplet on a superhydrophobic surface enabled up to 40% reversible tuning of the equatorial diameter of the droplet26. Such effects must naturally also be taken into account when exciting laser modes in droplets in experiments with levitated drops. 

Since most of the variance in the data arises from thermal motion of the beads held in the optical tweezers, the standard deviation (a) is governed by the optical tweezer stiffness (Figure 12.7). Therefore, if for example experiments are carried out at a combined laser tweezer stiffness of 0.04 pN nm, the standard deviation of the thermal noise will be 15 nm, so if 100 events are collected the step size estimate will have a standard error of the mean of (15/vT00) =1.5 nm. 

Manipulation of single polymer chains has become a major direction of research in polymer science because such experiments can yield valuable insight into the relationship between the chemical structure and physical properties of macromolecules [1-3]. Such experiments do also depend on the conditions of the environment of the polymers, e.g., in an experiment where colloidal beads attached to the chain ends of a biopolymer are moved away from each other with laser tweezers. Then, the extension X) versus force (/) relation will depend on the quality of the solvent in which the polymer has been dissolved. Also, the structure of a polymer adsorbed on a substrate from solution will depend on the solvent quality [4, 5]. 

In the laser tweezing experiments by Pantina and Furst [17], a linear chain of polymethylmethacrylate (PMMA) particles were immersed in an aqueous salt solution. Aided by optical tweezers, terminal particles of the chain were held and a central... 

As noted in the text, special care should be taken to avoid stray reflections of the laser beam used in this experiment (see Appendix C also). The pulse energies are high enough to cause serious eye injury and safety goggles should be worn at all times except when the instructor has determined that it is safe to remove them. Iodine is corrosive to the skin and should be handled with a spatula or tweezers in adding a few crystals to the fluorescence ceU. 

The major impact of optical tweezers in biology arises from their use in the rapidly expanding field of single-molecule research. By chance, the optical forces produced by commonly available lasers (with output powers of a few hundred milliwatts) happen to be in the piconewton (pN) range, which is just right to experiment with... 

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