The Physics Department has recently acquired a top-notch research grade light scattering apparatus from the Brookhaven Instruments Corporation.  The purchase has been made possible by grants from HHMI. The instrument allows students to perform a variety of basic experiments which demonstrate physical principles at work in inter-disciplinary fields such as molecular biology, biotechnology, biophysics and physical chemistry.

First-year students operate the light scattering apparatus.

The instrument is capable of both static and dynamic light scattering, allowing students to measure not only parameters such as molecular weight of an unknown macromolecule (biological molecule or a polymer), but also its hydrodynamic radius, the translation diffusion coefficient, and conformational stability. An emphasis is placed on demonstrating the basic principles behind the laws of probability, friction, and diffusion that are central to understanding Brownian motion and are so fundamental to understanding many aspects of the molecular machinery of living cells and other biological systems. In addition, upper level students can learn about the light scattering theory, easily extendable to other scattering methods (x-ray, neutron, etc.) and about photon correlation spectroscopy, involving mathematical skills ubiquitous in statistical methods.

Typical experiments allow students to collect and process average light intensity data in order to determine the molecular weight of an “unknown” protein. The students can also measure, explain, and predict phenomena concerning both the diffusion processes (e.g. Maxwell-Boltzmann distribution of molecular speeds, Einstein-Stokes formulas, Fick’s Law, etc.) and scattering methods (e.g. basic scattering theory, correlation functions, etc.). In a basic implementation, this experiment involves measurements of light intensity time-correlation functions on several sizes of polystyrene spheres at various scattering angles, temperatures, and concentrations. The analysis requires non-trivial data processing (inverting data from time to frequency domain, non-linear fitting, etc.) in order to obtain the hydrodynamic radius of these particles from the translational diffusion coefficient.

In the Near Future

We are purchasing an apparatus for optical tweezing.  Optical tweezing has become a widespread technique used in biophysics laboratories to trap small dielectric particles with a tightly focused laser beam. We now propose a professional quality integrated system capable of performing real experiments on molecules of biological interest. The most common applications of optical tweezers are found in studies of the physical properties of a single DNA molecule (e.g. its elastic response), and molecular rotors (e.g. kinesin or myosin-V stepping kinetics). They can also be used to measure local viscoelastic properties of the medium surrounding the particle (e.g. inside a cell).