Protonic gating of excited-state twisting and charge localization in GFP chromophores: a mechanistic hypothesis for reversible photoswitching.

Reversible photoswitching fluorescent proteins can be photoswitched between fluorescent and nonfluorescent states by different irradiation regimes. Accumulating spectroscopic and crystallographic evidence suggest a correlated change in protonation state and methine bridge isomerism of the chromophore. The anion can decay by photoisomerization of either of the methine bonds, but only one channel can act as a switch. Using ab initio multiple spawning dynamics simulations, we show that protonation is sufficient to change the photoisomerization channel in the chromophore. We propose that this behavior can underlie a switch given certain other conditions. We also propose a basis for coupling between excited-state basicity changes and selection of the photoisomerization channel based on the polarity of twisted charge-transfer states for neutral and anionic forms of the chromophore.