We describe a simple optical method that creates structured illumination of a photoactivatable probe and apply this method to characterize chromatin motions in nuclei of live cells. A laser beam coupled to a diffractive optical element at the back focal plane of an excitation objective generates an array of near diffraction-limited beamlets with FWHM of 340 ± 30 nm, which simultaneously photoactivate a 7 × 7 matrix pattern of GFP-labeled histones, with spots 1.70 μm apart. From the movements of the photoactivated spots, we map chromatin diffusion coefficients at multiple microdomains of the cell nucleus. The results show correlated motions of nearest chromatin microdomain neighbors, whereas chromatin movements are uncorrelated at the global scale of the nucleus. The method also reveals a DNA damage-dependent decrease in chromatin diffusion. The diffractive optical element instrumentation can be easily and cheaply implemented on commercial inverted fluorescence microscopes to analyze adherent cell culture models. A protocol to measure chromatin motions in nonadherent human hematopoietic stem and progenitor cells is also described. We anticipate that the method will contribute to the identification of the mechanisms regulating chromatin mobility, which influences most genomic processes and may underlie the biogenesis of genomic translocations associated with hematologic malignancies.
Here we report on the motion of microscopic, optically trapped glass rods suspended in water and experiencing a light torque. The motion consists of two distinct regimes: a linear regime where the rod angle increases linearly with time and a nonlinear regime where the rod angle changes nonlinearly, experiencing accelerations and rapid reversals. These regimes depend on whether the rotation frequency of the linearly polarized driving light is above or below a critical frequency, Ωc. We will present experimental data that spans both regimes. We will also provide a theoretical model that agrees with the observed motion. We are working on extending this effort on the optical trapping and rotation of rods to smaller scales, where the diameter is 100 nm or less. This scaling down will allow us to study the nonlinear motion near a surface. Such studies can help us to understand surface effects that are important in micro- and nanofluidics. Toward this end, we report results on our attempts to trap silver nanorods of diameter close to 100 nm suspended in acetone.
We use polarized light to generate light torques for controlling nanoparticles and for producing nanomotors. We report on experiments where we apply light torques to glass nanorods in an optical trap at a known distance from a nearby surface. We tried two different optical traps: (1) a standing wave trap using 20 mW and (2) a single beam trap using 80 mW of light at wavelength 514 nm from an Ar+ laser. The rods studied here are 250-500 nm in diameter and are 1-4 microns long. The motion of the rotating rods is studied and a theoretical model of the motion is presented. The motion can be heavily affected by the presence of a nearby surface. For example, past studies have provided evidence that rotatory motion near a surface can change to rocking motion and vice versa. In this study, we present results of motion free of such surface effects. Studies of the motion of nano-objects are useful in understanding nanorheological phenomena in both biological and inorganic systems.