  The green fluorescent protein (GFP) chromophore is an asymmetric monomethine
dye system. In the resonance color theory of dyes, a strong optical excitation
arises from interactions of two valence-bond structures with a third, higher
structure. We use correlated quantum chemistry to show that the anionic
chromophore is a resonant Brooker dye, and that the third structure corresponds
to a higher stationary electronic state of this species. The excitation energy
of this state should be just below the first excitation energy of the neutral
form. This has implications for excited state mechanism in GFPs, which we
discuss.
