Margaret Shun Cheung
Assistant Professor
629C SRI Department of Physics
University of Houston
Houston, TX, 77204
Tel: 713-743-8358
Fax: 713-743-3589
email: mscheung_at_uh.edu
physics website

Research Interests:
Theoretical Biological Physics, Nanobiophysics, and Soft Condensed Matter

Research projects in my group focus on development of theoretical models and physico-chemical principles to assess macromolecular dynamics under various stress conditions that will in turn shed lights on better designs of probes to manipulate these materials of interest. To tackle macromolecular dynamics that spans multiple orders of magnitudes in space and time, we develop and apply state-of-the-art multi-scaled simulation approaches that integrate high-performance computing resources nationwide to achieve a complete understanding of macromolecular interactions that dictate functionality.

Projects in my group strongly tie to interdisciplinary research activities in the city of Houston that is a nationally recognized hub of medical centers, petroleum industry, and material sciences. We work on various issues in biological physics, nanobiology, and soft condensed matter.

Protein dynamics in vivo

Biopolymer dynamics in nano-sized confinement

Simulation of surfactant self-assembly

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Protein dynamics in vivo:

To investigate the consequences of macromolecular crowding on the behavior of a globular protein, we performed a combined experimental and computational study on the 148-residue, single-domain protein, Desulfovibrio desulfuricans apo-flavodoxin. In vitro thermal unfolding experiments, as well as assessment of native and denatured structures, were probed using far-UV circular dichroism (CD) in the presence of various amounts of Ficoll 70, an inert spherical crowding agent. Ficoll 70 has a concentration-dependent effect on the thermal stability of apo-flavodoxin ( Tm of 20 C at 400 mg/ml; pH 7). As judged by CD, addition of Ficoll 70 causes an increase in the amount of secondary structure in the native-state ensemble (pH 7, 20C) but only minor effects on the denatured state. Theoretical calculations, based on an off-lattice model and hard-sphere particles, are in good agreement with the in vitro data. The simulations demonstrate that, in the presence of 25 % volume occupancy of spheres, native flavodoxin is thermally stabilized and the free energy landscape shifts to favor more compact structures in both native and denatured states. The difference contact map reveals that the native-state compaction originates in stronger interactions between the helices and the central -sheet, as well as by less fraying in the terminal helices. This is the first study to demonstrate that macromolecular crowding has structural effects on the folded ensemble of polypeptides.

L. Stagg, S.-Q.Zhang, M. S. Cheung, P. Wittung-Stafshede, "Molecular crowding enhances native structure and stabilityof alpha/beta protein flavodoxin", Proc. Natl. Acad. Sci. U. S. A., 104, 18976-18981, (2007). [PDF]

Biopolymer dynamics in nano-sized confinement:

The behavior of biopolymers in nano-sized confinement is investigated using coarse-grained models and molecular simulations. We address the effects of geometry of a confinement and the wall-protein interactions on protein folding dynamics. By measuring folding rates and dissecting structural properties of the transition states in nano-sized spheres and ellipsoids, we are able to justify the best form of a confinement in which the rates of folding kinetics are most enhanced. This knowledge in identifying optimal conditions for reactions will have a broad impact in nanotechnology and pharmaceutical sciences.

S.-Q.Zhang and M. S. Cheung. "Manipulating Biopolymer Dynamics by Anisotropic Nanoconfinement", Nano Letters 7, 3438-3442(2007). [PDF]

Simulation of surfactant self-assembly:

An understanding of how hydrocarbons and surfactants interact in wet and porous media is essential to improve petroleum production rates. We investigate hydrophobic interactions in hydrocarbons and surfactants at multiple scales ranging from a nano-sized pore in an aquifer to a meter-sized reservoir. Specifically, this project will examine principles of how surfactants transfer hydrocarbons from solid surfaces to solutions as a function of the pore size and material composition using molecular simulations. Because these dynamics occur at multiple length and time scales all-atomistic models are not viable for simulations of a large complex system that involves hydrocarbons, surfactants, and water molecules. To circumvent these difficulties, we develop and apply multi-grained molecular simulations to investigate dynamics of hydrocarbons and surfactants in bedrocks.