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: Germany 20.04.2015
• Subjects: Calcium - chemistry; Green Fluorescent Proteins - chemistry; Models, Molecular; Molecular Structure; Photochemical Processes; Protons; vibrational spectroscopy; excited state proton transport; femtochemistry; calcium ion sensing; fluorescent probes
• ISSN: 1521-3765
• DOI: 10.1002/chem.201500491

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.