Unraveling Ultrafast Photoinduced Proton Transfer Dynamics in a Fluorescent Protein Biosensor for Ca 2+ Imaging

• Published in: Chemistry : a European journal Vol. 21; no. 17; pp. 6481 - 6490
• Main Authors: Tang, Longteng, Liu, Weimin, Wang, Yanli, Zhao, Yongxin, Oscar, Breland G., Campbell, Robert E., Fang, Chong
• Format: Journal Article
• Language: English
• Published: 20.04.2015
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201500491

Abstract Imaging Ca 2+ dynamics in living systems holds great potential to advance neuroscience and cellular biology. G‐GECO1.1 is an intensiometric fluorescent protein Ca 2+ biosensor with a Thr‐Tyr‐Gly chromophore. The protonated chromophore emits green upon photoexcitation via excited‐state proton transfer (ESPT). Upon Ca 2+ binding, a significant population of the chromophores becomes deprotonated. It remains elusive how the chromophore structurally evolves prior to and during ESPT, and how it is affected by Ca 2+ . We use femtosecond stimulated Raman spectroscopy to dissect ESPT in both the Ca 2+ ‐free and bound states. The protein chromophores exhibit a sub‐200 fs vibrational frequency shift due to coherent small‐scale proton motions. After wavepackets move out of the Franck–Condon region, ESPT gets faster in the Ca 2+ ‐bound protein, indicative of the formation of a more hydrophilic environment. These results reveal the governing structure–function relationship of Ca 2+ ‐sensing protein biosensors.