Resonantly Enhanced Multiphoton Ionization Spectrum of the Neutral Green Fluorescent Protein Chromophore

The photophysics of the green fluorescent protein is governed by
the electronic structure of the chromophore at the heart of its β-barrel
protein structure. We present the first two-color, resonance-enhanced,
multiphoton ionization spectrum of the isolated neutral chromophore
in vacuo with supporting electronic structure calculations. We find
the absorption maximum to be 3.65 ± 0.05 eV (340 ± 5 nm),
which is blue-shifted by 0.5 eV (55 nm) from the absorption maximum
of the protein in its neutral form. Our results show that interactions
between the chromophore and the protein have a significant influence
on the electronic structure of the neutral chromophore during photoabsorption
and provide a benchmark for the rational design of novel chromophores
as fluorescent markers or photomanipulators.