Dark-State-Mediated Efficient Energy Trapping in a Model GFP Chromophore

Area of Science:

• Photochemistry
• Biophysics
• Quantum Chemistry

Background:

• Photoactive proteins' function depends on bright and dark excited states.
• Dark states are crucial for photostability and light harvesting but hard to study.

Purpose of the Study:

• To directly observe and characterize the optically dark, low-lying singlet excited state in the isolated anion of the meta green fluorescent protein (GFP) chromophore.
• To understand the photoprotective mechanisms in biomolecular anions.

Main Methods:

• Ultrafast time-resolved action absorption spectroscopy.
• Photoelectron spectroscopy.
• High-level ab initio calculations.

Main Results:

• Direct observation and full characterization of a dark singlet excited state.
• Captured state formation in 100 fs with a lifetime of 94 ps.
• Unambiguous assignment of charge-transfer character and elucidation of the trapping mechanism.

Conclusions:

• Identified a photoprotective mechanism in biomolecular anions.
• Ultrafast internal conversion quenches electron emission, stabilizing long-lived electronic excitation.
• This mechanism operates even above the electron detachment threshold.