International Collaboration in Chemistry on Control of Excited State Proton Transfer in GFP

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The 2008 Nobel Prize recognized the revolutionary biological applications of the green fluorescent protein (GFP) and related fluorescent proteins as tools for monitoring protein localization and trafficking in cells and tissues. However, the prototype GFP, from the jellyfish Aequorea victoria, is also unique in exhibiting highly efficient (>80%) proton transfer following absorption of a photon. Proton transfer contributes to most biological reactions, but typically takes place on time scales that are difficult to observe directly. The ability to trigger proton transfer with a femtosecond light pulse thus yields unequalled opportunities for understanding how proteins control this ubiquitous process. Our proposed investigations proposal is hypothesis-driven and aims to understand the dynamics and control of the ultrafast excited state proton transfer (ESPT) reaction. In particular, we hope to identify low frequency promoting vibrations that form an essential element of theoretical treatments of proton transfer. We present preliminary measurements of low frequency vibrational coherences that occur within the duration of the ESPT reaction. Control of ESPT in GFP is currently fervidly discussed and we propose to help resolve outstanding issues such as the enthalpic and entropic reaction barriers, the role of chromophore cooling, low frequency mode identification and assignment, and heterogeneity and interconversions of the protein-chromophore structure. Ultrafast kinetics, vibrational coherence spectroscopy (VCS), transient visible-visible and visible-infrared absorption spectroscopy, femtosecond stimulated Raman spectroscopy (FSRS), and coherent control spectroscopy will be used to dissect the microscopic details. Particular emphasis will be placed on temperature dependent studies. Further development of high level computational chemistry calculations that include the protein environment will aid in interpretation of the spectroscopic results. In turn, the experimental results will provide an essential calibration for the computational methods.