Dynamical light scattering as a tool for studies of biopolymers

with Surendra P. Singh

Dynamical light scattering is a fast, convenient, and flexible way to study the structure and dynamics of a scattering medium [1]. It has been used to determine biopolymer diffusion coefficients, size, molecular weight, and internal motions [2,3]. It is also used to monitor polymerization and conformational transitions and to investigate a number of physiologically and biomedically important flow processes.

The principle underlying DLS is the scattering of light from refractive index fluctuations. In a solution of macromolecules, which includes aggregates and cells, these fluctuations arise due to the difference in the solute and solvent polarizability. The scattered light varies in time because translational, rotational, and internal motions of the particles cause their polarizability to change in time. Particle motions may be caused by Brownian diffusion, by externally applied fields, or by biological or chemical processes. The time variation of the scattered fight intensity is reflected in the autocorrelation function g(τ) or the power spectrum S(ω) of scattered light. For example, for translational Brownian diffusion, the autocorrelation function is g(τ)=exp(-2Fτ), where the relaxation rate F=q²D, q is the scattering vector, and D is the translational diffusion coefficient. This can be related to molecular weight, size, and shape [4]. Similarly, if the motion is uniform translation with velocity v, then the frequency of the incident light is Doppler-shifted by Δω =q.v and we can use DLS for velocimetry. DLS has also been used to measure rotational and internal motion of DNA molecules using polarization of scattered light [2].

The experiments described here will use dynamical light scattering (DLS) as a tool for studying the behavior of biopolymers under a variety of conditions. These will include microcell and microfluidic environments where ionic concentration or pH and temperature can be varied. Initial experiments will use known molecules for "calibration" purpose. These studies will then be extended to include unknown molecules. These projects we will use many of the same experimental techniques that we have developed for fast photon counting and correlation measurements of weak light beams in the context of laser physics and quantum optics [5] .

1. B. J. Berne and R. Pecora, Dynamic Light Scattering with Applications to Chemistry, Biology, and Physics (Wiley-Interscience, New York, 1976).
2. Maria A. Ivanova, Alexander V. Arutyunyan, Aleksey V. Lomakin, and Valentine A. Noskin, Appl. Opt. 36, 7657 (1997).
3. N. C. Santos and M. A. R. B. Castanho, Biophys. J. 71, 1641 (1996).
4. R. Mhatre, I. S. Krull, and H. H. Stuting, J. Chromatogr. 502, 21-46 (1990). I. S. Krull, H. H. Stuting, and S. C. Krzysko, J. Chromatogr. 442, 29-52 (1988). H. H. Stuting, Krull, I. S. Krull Anal. Chem. 62, 2107-2114 (1990).
5. Y. Qu and S. Singh, Phys. Rev. A 51, 2530 (1995); Y. Qu, S. Singh, and C. D. Cantrell, Phys. Rev. Lett. 76, 1236 (1996); S. Singh, M. Mortazavi, K. J. Phillips, and M. R. Young in Laser Noise (Proc. SPIE 1376), edited by R. Roy (SPIE, Bellingham, WA, 1991), pp 143-152.