Olson Research Group
Sub–Molecular Tunneling Spectroscopy
The Olson Group, Fall 2015. Left-to-right: Elizabeth Stewart (undergraduate), Nathan Price (undergraduate, B.S. May 2016), Joel A. Olson (Advisor), Zhuo Li (graduate student, M.S. May 2016), Shivagee Govind (masters graduate student), Xixuan Guo (doctoral graduate student).
The Olson Group is an interdisciplinary research team that resides in the Chemistry Department at Florida Institute of Technology in Melbourne Florida. Our research program is primarily directed toward fundamental and applied studies of surface phenomena and materials sciences, utilizing numerous analysis methods including scanning tunneling microscopy (STM), scanning electron microscopy (SEM), optical microscopy, and direct analysis in real time (DART) mass spectrometry, to name a few. We utilize these tools to learn fundamental information about important chemical and materials systems. This information can then be used to understand fundamental phenomena (such as the quantum-mechanical behaviors of individual molecules) and to use this understanding for useful applications such as computer aided drug design (CADD), as well as the design of other important chemical systems (such as catalysts, herbicides, pesticides, etc.).
Our group involves students in research endeavors. Ranging in experience from high school students, through undergraduate students, to doctoral graduate students, our purpose is to provide hands-on research training to prepare students for their next career steps, all-the-while pushing back the boundaries of human scientific knowledge. Our research students (both graduate and undergraduate) are included as co-authors on publications in the scientific literature. We also have students from diverse major areas of study involved in our research, including students from Chemistry, Chemical Engineering, Physics, and Electrical and Computer Engineering. If you would like more information about any of Florida Tech’s chemistry degree programs, please use the appropriate links on the Chemistry Department website. If you would like to contact Dr. Olson directly, his contact information may be found here.
FAME is the Florida meeting of the American Chemical Society. In May of 2016, presentations were given by Dr. Olson, Graduate Student Xixuan Guo, and Undergraduate Student Elizabeth Stewart. Abstracts for their presentations are included below.
PRESENTATION TITLE: Sub-Molecular QSAR: Using Scanning Tunneling Microscopy Simulations for Rational Drug Design.
Dr. Olson presented the group’s recent breakthrough drug design research in the development of a novel sub-molecular computational quantum-mechanical descriptor for quantitative structure activity relationship analysis. This research represents an entirely novel approach toward molecular design that is based on computational simulations of tunneling spectroscopy measurements of individual molecules.
ABSTRACT: Tryptanthrins represent a class of compounds of interest for their anti-parasitic properties versus organisms that cause malaria, leishmania, trypanosomiasis, tuberculosis, and fungal infections. However, little is known of their mode(s) of action at the molecular level. To investigate their geometric and electronic behavior, STM has been used to observe monolayers of these compounds at the solution-graphite and vacuum-graphite interfaces. Sub-molecular resolution has allowed the direct observation of individual lobes of the HOMO and LUMO states, correlating very closely with DFT-calculated MOs. Additionally, tunneling barrier height studies reveal that the origin of the relative apparent tunneling barriers for MO-mediated surfaces differ markedly from those of non-orbital mediated surfaces, representing a novel phenomenon. These tunneling barrier measurements were then used to develop a computational STM simulation based on the 3-dimensional convolution of the tip state with the molecular states. The simulation displays remarkable agreement with experimental results. Furthermore, the simulation results were used to perform a first-ever tunneling barrier-based QSAR analysis of tryptanthrins. The results suggest a previously unknown pharmacophore versus P. falciparum, the pathogen that causes the lethal form of malaria.
PRESENTATION TITLE: Semiquantitative Electron Tunneling Barrier Height Measurements of Molecular Monolayers at the Solution−Graphite Interface: Nonorbital-Mediated Tunneling.
Guo presented his recent work on barrier height measurements of stearic acid monolayers on graphite. This work provides a fundamental basis for our understanding of barrier height measurements of more complex molecular adlayer systems. Guo is the first author on the paper that describes this research, which is reported in the Journal of Physical Chemistry C.
ABSTRACT: The semi-quantitative apparent tunneling barrier height of stearic acid adsorbed onto highly oriented pyrolytic graphite (HOPG) in 1-phenyloctane was measured. The topographic images indicated the different conformers of a single monolayer, which had not been previously reported, although large number of publications were on these and similar systems. The apparent barrier height images presented two individual effects. The apparent barrier height map is dominated by the distribution of surface charges, known as electrostatic potential (ESP), in regions where significant polarity exists like the place near carboxyl group. On the other hand, in regions where little variations in ESP like the place near the alkane chains exist, the manifestation of the topography dominates the apparent barrier height. In the case of imaging these types of monolayers, the effect by adlayer molecular orbitals was excluded from these tunnelings since the energies of the molecular orbitals fall outside of the Fermi levels of the substrate and the tip under the described tunneling conditions. These studies improve a basis for further investigations into barrier height tunneling spectroscopy of molecular monolayers on HOPG.
POSTER TITLE: Tracking Molecular Conformations of Stearic Acid in Surface-Adsorbed Monolayers.
Elizabeth presented her poster that describes time-lapse STM tracking of individual stearic acid molecules to asses whether adlayer molecules undergo conformational changes at room temperature. Elizabeth is a co-author on the paper that describes this research, which is reported in the Journal of Physical Chemistry C.
ABSTRACT: When stearic acid molecules are absorbed onto highly oriented pyrolytic graphite, they adsorb in such a way as to create both syn and anti conformers in similar proportions. This is an unexpected observation, since the syn conformer is lower in energy by ~13.5 kJ/mol according to DFT calculations, which would thermodynamically favor the syn conformation by a 99:1 ratio. It is possible to determine the conformer of individual molecules in scanning tunneling microscopy (STM) topography images by analyzing the characteristics of the dimer ring. In a low drift setting, it was possible to identify and track individual molecules of stearic acid. The images were analyzed individually in order to identify conformers, and then placed chronologically in order to track each molecule and determine if there were any conformational interconversions over time. A total of 159 STM images were collected for this experiment in which a select few molecules were tracked. Tracking molecules was accomplished by correlating similar characteristics of molecules for consecutive scans. The molecules that were selected to be tracked were based on the overall clarity and distinction of shape throughout the sequence of images. The overall duration of the scanning sequence where molecules were tracked lasted for a little less than half an hour. The results showed that the dimer rings do not interconvert between conformations over the time period measured. Therefore, it may be inferred that an initial adsorption occurs wherein the dimer ring may adsorb in either of the two alignments; then the molecules pack in such a way that the dimer ring remains strongly adsorbed to the surface; and once packed on the surface, the barrier to rotation of the dimer ring becomes large enough to preclude additional conformational interconversion.
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