Professor Joonil Seog

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Research Interests

Self-assembly and molecular mechanics of biological materials, biomaterials for gene delivery, stimuli-responsive peptide-based materials, optical tweezers.

Current Research

Adam working with the optical mini tweezers.

Optical mini tweezers.

In my lab, we are interested in studying the structure-function relationship of biological molecules at the molecular level using high resolution force spectroscopy. Biological molecules such as proteins, glycoprotein, and polysaccharides are designed such a way to perform their unique function which in turn determines macroscopic tissue level properties. With the development of tools that can measure very small forces, we can directly probe molecular level forces to find out molecular origin of macroscopic behavior. For example, the study of the giant muscle protein titin using atomic force microscopy showed how a single titin molecule provides a muscle with passive elasticity during muscle stretching in its physiological condition. In another example, the intermolecular interaction of the highly charged polysaccharides molecules that exist abundantly in cartilage was directly measured using the same technique. By comparing the data obtained at molecular level with the data obtained from a macroscopic tissue level experiment, it was found out that electrostatic repulsive interactions between these molecules contributes to about half of compressive resistance of the cartilage tissue. These are very exciting moments for scientists since we can now explain macroscopic mechanical behaviors of tissue from molecular viewpoint, obtained from direct force measurement at molecular level. This molecular viewpoint also provides insight about the structure-function relationship of biological molecules as well as the guidance about how to engineer biological materials or synthetic biomaterials to enhance or optimize their performance.

Atomic force microscope (AFM)

Atomic force microscope (AFM).

We utilize high resolution force spectroscopy such as atomic force microscopy or optical tweezer to study intermolecular interactions of biological molecules and mechanical properties of single biological molecules. When you unfold a single protein by stretching it, the unfolding force is usually in the range of 10 to 200 piconewton (pN, 10-12 newton), depending on pulling rate and when you compress densely packed molecular layer on the surface, the compressive resistance can go up to several nanonewton (nN, 10-9 newton) depending on the environment and the nature of the intermolecular interaction. Thanks to the development of high resolution force spectroscopy, these tiny forces can be measured at a great accuracy with high spatial resolution. The force measurement at the single or multimolecular level gives important information about kinetics and thermodynamics as well as the nature of the interaction. This fundamental information is very important to our ability to control the interactions between the molecules, and it will eventually provide a useful insight to improve medical devices and biomaterials for tissue engineering or drug delivery applications by optimizing molecular level interactions. In addition, the information obtained from single molecule experiments will help prepare nanostructural materials with tailored nanomechanical properties to be used as building blocks for nanoscale devices, or to probe cell-biomaterial interaction at a molecular level in well-defined environments.

Movie: DNA condensation at the single molecule level monitored using optical tweezers

This movie shows DNA condensation by a positively charged peptide polymer. The top bead is "tweezed" by a laser light and the bottom bead is fixed onto the pipette tip. There is a single DNA molecule connecting the two beads at a 3 pN force level. As the positively charged peptide polymer binds to the negatively charged DNA, it condenses the DNA into a compact form, which is demonstrated by reduction of the distance between the beads.