The Department of Chemistry invites you to its departmental seminar on Monday, May 4, at 4:10 p.m. in Fulmer Hall, room 438.
Dr. Chong Fang from Oregon State University will present, Developing Femtosecond Raman Spectroscopy to Capture Transient Snapshots of Photosensitive Molecules and Biosensors.
Abstract: Photochemical reactions power numerous biological and energe-related processes and their importance cannot be overstated. Photosynthesis, vision, and bioluminescence all rely on structural dynamics of chromophores, commonly a conjugated organic moiety in condensed phase, which are responsible for light absorption and emission. To reveal transient structural evolution of photoexcited chromophore in various environment from water to protein pocket, we develop a structural dynamics tool called femtosecond stimulated Raman spectroscopy (FSRS) with broadly tunable pulses together with transient absorption, cascaded four-wave mixing, time-resolved third-harmonic generation, vibrational normal mode calculation, and molecular dynamics simulation to dissect multidimensional reaction coordinate of photoacids and fluorescent protein Ca2+ biosensors in solution. Following 400 nm excitation, the photoacid pyranine undergoes skeletal 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 with the three-residue 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 of vibrational quantum beats, while blue fluorescence is correlated with inhibition of ESPT by location change of key residues. Besides crucial design principles for molecular devices from structural dynamics insights, FSRS is proven to be a powerful tool to elucidate hidden reaction coordinate during photochemistry 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.
Contact: Chelsea (Pickett) Gao, chelsea.m.pickett@wsu.edu.
