Mapping GFP Structural Evolution during Excited-State Proton Transfer with Femtosecond Stimulated Raman

We use femtosecond stimulated Raman spectroscopy to view transient structural changes of the photoexcited GFP-chromophore, identifying low-frequency ring-wagging motions that gate excited-state proton transfer. Mechanistic insights on reactive systems require structural probing with femtosecond resolution.