Are flourescent proteins exotic?

"Biochemistry is the search for the chemistry that works."

Just how special, unique, and fine-tuned are biomolecules?

There are a wide range of proteins which have functionalities based on their optical properties. One outstanding example are flourescent proteins. Seth Olsen and I just completed a paper which uses high-level quantum chemistry calculations to show that the low-lying excited states of the chromophore

molecule in the green flourescent protein has a natural description in terms of the resonant colour theory of organic dyes developed in the middle of the twentieth century by Brooker, Platt, and Moffitt.

[Aside: this is the same Platt of multiple alternative hypothesis fame!].

Brooker showed how one could relate the absorption wavelength of an asymmetric methine dye molecule to the absorption wavelengths of two symmetric parent dyes.

The figure above shows that the anion of the

chromophore for GFP (which is what is responsible for the light emission) is a Brooker dye which is "on resonance", i.e. its ground state and first excited state contain equal superpositions of two valence bond states with electrons displaced to the left and the right of the methine bridge (central carbon atom).