A Dark Excited State of Fluorescent Protein Chromophores, Considered as Brooker Dyes

TL;DR

This study reveals that the GFP chromophore's excited state is a dark state analogous to Brooker dyes, involving a higher stationary electronic state, which influences its excited state mechanisms.

Contribution

The paper demonstrates that the GFP chromophore is a resonant Brooker dye with a higher electronic state affecting its excitation properties, using correlated quantum chemistry.

Findings

The anionic GFP chromophore is a resonant Brooker dye.

A higher stationary electronic state exists below the first excitation energy of the neutral form.

Implications for GFP excited state mechanisms are discussed.

Abstract

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.