ChBE Seminar Series: Joonil Seog
Tuesday, December 11, 2007
11:00 a.m.-12:00 p.m.
Room 2110 Chemical & Nuclear Engineering Bldg.
Professor Nam Sun Wang
Understanding molecular mechanisms in biology by measuring small forces
Presented by Assistant Professor Joonil Seog
Fischell Department of Bioengineering and the Department of Materials Science and Engineering
University of Maryland
Atomic Force Microscopy (AFM) and optical tweezers (OT) have capability of measuring molecular level interactions with high spatial resolution. Using AFM, the molecular origin of biomechanical properties of cartilage was probed by direct measurement of intermolecular repulsive forces between negatively charged glycosaminoglycan (GAG) macromolecules. The measured force showed a long-range, repulsive interaction that was significantly dependent on the ionic strength and pH, indicating the major role of electrostatic interaction between GAG layers. The contribution of the electrostatic component was quantified using a theoretical model of electrical double layer based on the Poisson-Boltzmann formulation and was compared with macroscopic tissue-level measurement. In another example of a biological system, mechanical unfolding and refolding behaviors of an adhesion molecule, integrin, were studied at single molecule level using an optical tweezer. The integrin is a heterodimer transmembrane protein that plays a critical role in cellular adhesion and migration during the inflammation and immune response. The I domain of integrin is a binding domain whose affinity is regulated by a conformational change. When the I domain is stretched, the two step unfolding behavior with different extension distances occurred in a specific order indicating that there are two sub-domains in the I domain with different mechanical properties. In force-clamping experiments, a quasi-stable intermediate state was observed with the same extension, which suggests that such intermediate is on-pathway to the folded state. The extension distances among three different states observed in force-clamping experiment were well correlated with the extension distances observed when two transitions were present in force-extension experiments, suggesting that the same intermediate is observed in these different types of experiments. The identity of the intermediate state and the force-bearing region of the I domain are discussed in detail based upon structural analysis of integrin I domain.