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Charge transfer in green fluorescent protein.
Jasper J van Thor1, J Timothy Sage
1Laboratory of Molecular Biophysics, University of Oxford, Rex Richards Building, South Parks Road, Oxford OX1 3QU, UK. jasper@biop.ox.ac.uk
Summary
The protein environment is crucial for green fluorescent protein (GFP) photoreactions, enabling efficient fluorescence and suppressing damaging pathways. Even at low temperatures, GFP exhibits significant structural changes in response to light.
Area of Science:
- Biophysics
- Photochemistry
- Structural Biology
Background:
- Green fluorescent protein (GFP) is a vital tool in molecular biology, but its photoreactions and the role of its protein environment are not fully understood.
- Charge transfer reactions are key to GFP's photocycle, yet their behavior in isolated chromophores differs significantly from the intact protein.
Purpose of the Study:
- To investigate the role of the protein environment in mediating charge transfer reactions within the green fluorescent protein (GFP) photocycle.
- To correlate structural dynamics and protein environmental effects with the high quantum efficiency of GFP fluorescence and competing phototransformation reactions.
Main Methods:
- Ultrafast spectroscopy was employed to study structural dynamics and charge transfer events.
- Cryo-trapping techniques were used to capture early reaction products at low temperatures (100 K).
Main Results:
- Charge transfer reactions crucial for GFP photoreactions do not occur in the isolated chromophore, highlighting the protein's essential role.
- The protein environment suppresses non-radiative pathways and facilitates excited state proton transfer, contributing to high fluorescence quantum efficiency.
- A competing phototransformation reaction, involving proton and electron transfer, mimics charge redistribution in the fluorescence cycle.
- Significant protein structural responses, including displacements of chromophore, solvent, and a photogenerated CO2 molecule, were observed even at 100 K.
Conclusions:
- The protein environment is critical for the efficient and stable functioning of the green fluorescent protein (GFP) photocycle.
- GFP exhibits strong conformational dynamics and responses to light-induced events even below its dynamical transition temperature (~200 K).
- Understanding these light-induced charge transfer events and conformational dynamics has implications for protein thermodynamics and the design of light-sensitive proteins.