Protonic Gating of Excited-State Twisting and Charge Localization in GFP Chromophores: A Mechanistic Hypothesis for Reversible Photoswitching

Abstract

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.

Analytical Derivative Coupling for Multistate CASPT2 Theory

“…Anionic pHBDI is considered to be an emitting species of the green fluorescent protein (GFP) and its variants. 6,[66][67][68][69][70][71][72][73][74][75][76] While GFP exhibits strong fluorescence with a lifetime on the order of nanoseconds, 6,66,[68][69][70][71][72][73] the nonadiabatic transition is known to occur in about a few picoseconds when the chromophore is not embedded in the protein environment. 77 As a resonant monomethine dye, it is widely accepted that the anionic GFP chromophore undergoes nonadiabatic transitions when the chromophore is twisted along the bridge.…”

“…77 As a resonant monomethine dye, it is widely accepted that the anionic GFP chromophore undergoes nonadiabatic transitions when the chromophore is twisted along the bridge. 6,69,74,78 There are two available bridge channels in this molecule, which are the imidazolinone (I) and phenolate (P) channels, named after the moiety connected to the bridge bond that twists. 6 We optimized the planar equilibrium geometry of the ground state, the geometries of the I-and P-twisted minima of the first excited state, and the MECIs between these states near the twisted geometries.…”

“…1 Non-radiative deactivation is an example of a process that plays a vital role in photo-induced structural changes of molecules that are used as photochromic and photomechanical materials. [2][3][4][5] Non-radiative transitions also act as a competitive deactivation pathway in light emission devices, 6 reducing the quantum yield of emission.…”

Optimizing conical intersections of solvated molecules: The combined spin-flip density functional theory/effective fragment potential method

“…[91][92][93][94][95][96][97][98][99][100][101][102][103][104][105] The protein environment plays a crucial role in the fluorescence of the chromophore (HBI). It has been argued that the surrounding residues prevent the chromophore from twisting around the phenoxy ("P-bond": τ P ) or imidazolinone ("I-bond": τ I ) bonds of the bridge (see Fig.…”

“…In aqueous solution, however, a strong fluorescence is quenched because the chromophore can undergo a twisting motion and reach the CI point near the twisted conformation. Olsen et al 99 performed a nonadiabatic simulation for HBI and predicted that the I-bond twisting occurs exclusively for the neutral form of HBI. In the second example, the equilibrium and CI points of GFP chromophore, neutral HBI (cis isomer shown in Fig.…”