A dark excited state of fluorescent protein chromophores, considered as Brooker dyes

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 valencebond 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.

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

“…We have previously argued that the presence of this low-lying double excitation can be anticipated by considering the Hückel-theoretic model of the corresponding odd-alternant hydrocarbon reference. 45 Our results here are consistent with a lowering of the energy of the double excitation when the electronic correlation model is improved, such as also occurs for the dark 2 1 A g state of polyenes. 76 It is not obvious why the angle that characterizes the changes in the character of |B should be the same as the angle θ that characterizes the mixing of |B with |L ± R .…”

“…Figure 1) also establishes an analogy between these dyes and the chromophore of the green fluorescent protein. 33,45,48 A similar, even broader, analogy can also be found within the active space of the analogous two-state SA2-CAS(4,3) solutions. 44,60,72 Both these dyes and GFP chromophores display a common binding-dependent fluorescence enhancement associated with the suppression of a common double-bond photoisomerization chemistry in the excited state.…”

“…The fact that SA3-CAS(4,3) solutions with the same structure can be found for these distinct molecules suggests that the solutions are stable, and that they are a general feature of dyes from this class. 45,47,48 Benchmark studies 49,50 have shown that the excitation energies of anionic GFP chromophore models calculated with multireference perturbation theory and SA-CASSCF models are not sensitive to basis set or active space size within a range of 0.2 eV, which is within quoted reliability limits 43,51,52 of such models. The systems studied here should be easier to deal with than this, because they are all cations.…”

“…This time scale separation is a natural consequence of the chemical structure of the dyes. The occurrence of similar self-consistent field solutions for these dyes 33 and others 45,48 is equally a consequence of the common chemical structure.…”

“…The protocol we use here is essentially the same as that we have used in our studies of the chemically related green fluorescent protein (GFP) chromophore system. 44, 45 The importance and utility of three-orbital models of the electronic structure of GFP chromophores have been highlighted in the work of independent research groups via distinct electronic structure approaches. 46 Their work provides a strong support for the contention that the four-electron three-orbital problem is a good model of the electronic structure of diarylmethanes.…”