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Materials Science at Oregon State University

Developing Femtosecond Raman Spectroscopy to Characterize Photosensitive Molecules and Biosensors in Solution

Date: Thursday, May 14th
Presenter: Prof. Chong Fang, OSU Chemistry,

Abstract


Understanding transient molecular structural transformation from the reactant to product state during chemical reactions has been a grand challenge for physical chemists, biophysicists, and materials scientists. The main issue lies in the spatial and/or time resolution to spectroscopically characterize reaction pathways because the intrinsic atomic motions occur on the ultrafast (i.e., femtosecond to picosecond) timescale. To guide the rational design of functional materials and biomolecules, we have improved the emerging structural dynamics tool called femtosecond stimulated Raman spectroscopy (FSRS)1-4 with broadly tunable Raman pump and probe pulses to capture structural snapshots of molecular systems in condensed phase, from aqueous solution, organic solvent to protein environment. With low-frequency FSRS based on broadband up-converted multicolor array (BUMA) technology,4,5 we track the aqueous aluminum speciation as a function of pH during the electrolytic synthesis of flat Al13 nanoclusters that are versatile solution precursors to environment friendly Al2O3 thin film coating.6 Upon incorporation of an actinic 400 nm pump pulse, we study the initial structural evolution of photoexcited chromophore inside the protein pocket of non-invasive, genetically encodable fluorescent protein based biosensors for Ca2+ imaging.7,8 The fluorescence modulation mechanism has been revealed to intimately correlate with the conformational dynamics of the embedded three-residue chromophore that exhibits different excited-state proton transfer capabilities in various microenvironments. The versatility and unique resolving power of broadband FSRS, aided by femtosecond electronic spectroscopy and our newly developed time-resolved third-harmonic generation, quantum mechanical calculations and molecular dynamics simulations, provide a new powerful toolset to elucidate the multidimensional reaction mechanisms in the electronic ground and excited state.

References
(1) McCamant, D. W.; Kukura, P.; Yoon, S.; Mathies, R. A. Rev. Sci. Instrum. 2004, 75, 4971-4980.
(2) Frontiera, R. R.; Fang, C.; Dasgupta, J.; Mathies, R. A. Phys. Chem. Chem. Phys. 2012, 14, 405-414.
(3) Liu, W.; Han, F.; Smith, C.; Fang, C. J. Phys. Chem. B 2012, 116, 10535-10550.
(4) Zhu, L.; Liu, W.; Fang, C. Appl. Phys. Lett. 2014, 105, 041106.
(5) Liu, W.; Zhu, L.; Wang, L.; Fang, C. Opt. Lett. 2013, 38, 1772-1774.
(6) Wang, W.; Liu, W.; Chang, I.-Y.; Wills, L. A.; Zakharov, L. N.; Boettcher, S. W.; Cheong, P. H.-Y.; Fang, C.; Keszler, D. A. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 18397-18401.
(7) Oscar, B. G.; Liu, W.; Zhao, Y.; Tang, L.; Wang, Y.; Campbell, R. E.; Fang, C. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 10191-10196.
(8) Tang, L.; Liu, W.; Wang, Y.; Zhao, Y.; Oscar, B. G.; Campbell, R. E.; Fang, C. Chem. Eur. J. 2015, 21, 6481-6490.

Bio:
Dr. Chong Fang joined the OSU Faculty in September 2010 as Assistant Professor of Chemistry. Since 2011, he is Adjunct Graduate Faculty of Physics and Materials Science, and an ONAMI Researcher. He did postdoctoral research in Prof. Richard Mathie's group at the University of California, Berkeley developing femtosecond Raman spectroscopy. Before that, he earned his Ph.D. under the tutelage of the late Prof. Robin Hochstrasser at the University of Pennsylvania focusing on two-dimensional infrared spectroscopy and biophysics, where he was the School of Arts and Sciences Dean’s Scholar in 2005. Dr. Fang obtained dual B.S. in Chemical Physics and Applied Computer Sciences from the University of Science and Technology of China, where he was the recipient of the highest undergraduate award, Guo Moruo Scholarship in 2000.

At OSU, Dr. Fang’s research program focuses on the structure-function relationships of molecular systems ranging from fluorescent protein based biosensors — which render microscopic cell organelles visible and are of immense value in bioengineering and biomedicine — to metal-organic complexes that are solution precursors to develop high-quality thin films in an environment-friendly way. His group consists of one postdoctoral fellow, six graduate students, and a number of undergraduate students. His advanced ultrafast spectroscopy laboratory to capture molecular “movies” is housed on the second floor of Linus Pauling Science Center, which consists of a wide range of nonlinear spectroscopic setups from femtosecond stimulated Raman, transient absorption, two-dimensional infrared to time-resolved third-harmonic generation. Dr. Fang has received the OSU General Research Fund, Research Equipment Reserve Fund, Faculty Release Time Grant, and most recently the NSF CAREER Award with a five-year grant co-funded by the Chemistry of Life Processes and the Chemical Structure, Dynamics, and Mechanisms Programs.