FIGURE 7.

Chromophore forms and internal conversion pathways. (a) Benzoidal and quinoidal mesomers of the GFP chromophore in its deprotonated form. The benzoidal character is increased if the negative charge on the oxygen of Y66 is stabilized by hydrogen bonds. The benzoidal structure prevails in the neutral chromophore form. (b) The one-bond rotation around the angle φ. With increasing single-bond character of φ, this movement becomes likelier. In contrast, increasing quinoidal character as in the I-state reduces its contribution to internal conversion. (c) The hula-twist motion, which is less sensitive to the bond-order of the exocyclic bonds. However, this concerted two-bond rotation requires space near the heterocycle and is therefore probed by S65G mutations and photoconversion.