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OSTC

Optical Science and
Technology Center

OSTC Materials Seminar - "Developing Femtosecond Raman Spectroscopy to Elucidate Reaction Mechanisms of Biomolecules and Materials in Solution"

S401 Pappajohn Business Building

"Developing Femtosecond Raman Spectroscopy to Elucidate Reaction Mechanisms of Biomolecules and Materials in Solution"
OSTC MATERIALS SEMINAR
Tuesday, April 14, 2015
1:30 p.m.
S401 Pappajohn Business Building

Individuals with disabilities are encouraged to attend all University of Iowa sponsored events. If you are a person with a disability who requires an accommodation in order to attend this program, please contact the Optical Science & Technology Center in advance at 353-0974 or email OSTC@uiowa.edu.
www.ostc.uiowa.edu

Dr. Chong Fang
Department of Chemistry, Oregon State University

Abstract
Photochemical reactions power numerous biological and energe-related processes and their importance cannot be overstated. Photosynthesis, vision, and bioluminiscence all rely on structural dynamics of chromophores,
commonly a conjugated organic moiety in condensed phase from water to protein pocket, which are responsible for light absorption and emission. To reveal atomic choreography determining the fate of photoexcited
chromophore in a range of local environements particularly involving water, we develop an emerging structural dynamics tool called femtosecond stimulated Raman spectroscopy (FSRS) with broadly tunable pulses in
conjunction with femtosecond transient absorption, cascaded four-wave mixing, time-resolved third-harmonic generation, vibrational normal mode calculations, and molecular dynamics simulation to dissect the
multidimensional reaction coordinate of photoacid in solution and fluorescent protein Ca2+ biosensors in water.  Following 400 nm electronic excitation, the photoacid pyranine (HPTS) undergoes characteristic nuclear motions
to either facilitate excited-state proton transfer (ESPT) when proton acceptors are nearby, or perform vibrational cooling in solvents lacking proton accepting capability such as methanol. In analogy, FSRS results on genetically
encoded Ca2+ sensors for optical imaging (GECOs) with the three-residue SYG or TYG chromophore reveal dramatically different structural evolution pathways following photoexcitation in the Ca2+-free vs. bound state.
The gating motions for green fluorescence in these biosensors are retrieved from Fourier transform analysis of vibrational quantum beats, whereas blue fluorescence is correlated with inhibition of ESPT by location change of
key residues. Besides crucial design principles for molecular device functionalization from structural dynamics insights, FSRS is proven to be a powerful optical tool to elucidate hidden reaction cooridnate during
photochemical reactions in action, with simultaneously high spectral and temporal resolutions to effectively map excited-state potential energy surface of a wide range of functional materials and biomolecules. Recent advances
using broadband up-converted multicolor array to generate laser sidebands and time-resolved third harmonic generation to reveal phonon dynamics will also be discussed.

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