Synthesis and properties of the para-trimethylammonium analogues of green fluorescence protein (GFP) chromophore: The mimic of protonated GFP chromophore

7-membered-ring effect on fluorescence quantum yield: does metal-complexation-induced twisting-inhibition of an amino GFP chromophore derivative enhance fluorescence?

To investigate two aspects, namely, (1) the 7-membered-ring effect on fluorescence quantum yield and (2) whether metal-complexation-induced twisting-inhibition of an amino green fluorescent protein (GFP) chromophore derivative is bound to enhance fluorescence, a novel GFP-chromophore-based triamine ligand, (Z)-o-PABDI, is designed and synthesized. Before complexation with metal ions, the S₁ excited state of (Z)-o-PABDI undergoes τ-torsion relaxation (Z/E photoisomerization) with a Z/E photoisomerization quantum yield of 0.28, forming both ground-state (Z)- and (E)-o-PABDI isomers. Since (E)-o-PABDI is less stable than (Z)-o-PABDI, it is thermo-isomerized back to (Z)-o-PABDI at room temperature in acetonitrile with a first-order rate constant of (1.366 ± 0.082) × 10^-6 s^-1. After complexation with a Zn²⁺ ion, (Z)-o-PABDI as a tridentate ligand forms a 1 : 1 complex with the Zn²⁺ ion in acetonitrile and in the solid state, resulting in complete inhibition of the φ-torsion and τ-torsion relaxations, which does not enhance fluorescence but causes fluorescence quenching. (Z)-o-PABDI also forms complexes with other first-row transition metal ions Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺ and Cu²⁺, generating almost the same fluorescence quenching effect. By comparison with the 2/Zn²⁺ complex, in which a 6-membered ring of Zn²⁺-complexation enhances fluorescence significantly (a positive 6-membered-ring effect on fluorescence quantum yield), we find that the flexible 7-membered rings of the (Z)-o-PABDI/Mⁿ⁺ complexes trigger their S₁ excited states to relax through internal conversion at a rate much faster than fluorescence (a negative 7-membered-ring effect on fluorescence quantum yield), leading to fluorescence quenching regardless of the type of transition metal that complexes with (Z)-o-PABDI.

Photoswitchable Fluorescent Proteins: Mechanisms on Ultrafast Timescales

The advancement of super-resolution imaging (SRI) relies on fluorescent proteins with novel photochromic properties. Using light, the reversibly switchable fluorescent proteins (RSFPs) can be converted between bright and dark states for many photocycles and their emergence has inspired the invention of advanced SRI techniques. The general photoswitching mechanism involves the chromophore cis-trans isomerization and proton transfer for negative and positive RSFPs and hydration-dehydration for decoupled RSFPs. However, a detailed understanding of these processes on ultrafast timescales (femtosecond to millisecond) is lacking, which fundamentally hinders the further development of RSFPs. In this review, we summarize the current progress of utilizing various ultrafast electronic and vibrational spectroscopies, and time-resolved crystallography in investigating the on/off photoswitching pathways of RSFPs. We show that significant insights have been gained for some well-studied proteins, but the real-time "action" details regarding the bidirectional cis-trans isomerization, proton transfer, and intermediate states remain unclear for most systems, and many other relevant proteins have not been studied yet. We expect this review to lay the foundation and inspire more ultrafast studies on existing and future engineered RSFPs. The gained mechanistic insights will accelerate the rational development of RSFPs with enhanced two-way switching rate and efficiency, better photostability, higher brightness, and redder emission colors.

Against the NEER principle: the third type of photochromism for GFP chromophore derivatives

Photochromism is the heart of photochromic fluorescent proteins. Excited-state proton transfer (ESPT) is the major cause of photochromism for the green fluorescent protein (GFP) and Z-E photoisomerization through τ-torsion is the major cause of photochromism for Dronpa (a GFP mutant). In this article, s-E-1 opens a third type of photochromism for GFP chromophore derivatives, which involves light-driven φ-torsion with no τ-torsion, followed by excited-state intramolecular proton transfer (ESIPT), and is gated by environmental polarity. Since s-E-1 does not follow Z-E photoisomerization through τ-torsion but undergoes light-driven φ-torsion, which involves equilibration of the excited-state rotamers, it is clearly against the NEER (Non-Equilibration of Excited-state Rotamers) principle.

E-Z Isomerization Mechanism of the Green Fluorescent Protein Chromophore: Remote Regulation by Proton Dissociation of the Phenol Group

Dronpa, a GFP (green fluorescent protein)-like fluorescent protein, allows its fluorescent and nonfluorescent states to be switched to each other reversibly by light or heat through E-Z isomerization of the GFP chromophore. In this article, a GFP chromophore (p-HBDI) in water is used as a model to explore this E-Z isomerization mechanism. Based on the experimental solvent isotope effect (k_H2O/k_D2O = 2.30), the E-Z isomerization of p-HBDI in water is suggested to go through the remote-proton-dissociation-regulated direct mechanism with a proton transfer in the rate-determining step. The fractionation factor (ϕ) of the water-associated phenol proton of p-HBDI in the transition state is found to be 0.43, which is exactly in the range of 0.1-0.6 for the fractionation factor (ϕ) of the transferring proton in the transition state of R₂O···H···O⁺H₂ in water. This means that the phenol proton of E-p-HBDI in the transition state is on the way to the associated water oxygen during the E-Z isomerization. The proton dissociation from the phenol group of p-HBDI remotely regulates its E-Z isomerization. Less proton dissociation from the phenol group (pK_a = 8.0) at pH = 1-4 results in a modest reduction in the E-Z isomerization rate of p-HBDI, while complete proton dissociation from the phenol group at pH = 11-12 also reduces its E-Z isomerization rate by one order of magnitude because of the larger charge separation in the transition state of the p-HBDI anion. All of these results are consistent with the remote-proton-dissociation-regulated direct mechanism but against the water-assisted addition/elimination mechanism.

S1/S0 Potential Energy Surfaces Experience Different Types of Restricted Rotation: Restricted Z/ E Photoisomerization and E/ Z Thermoisomerization by an Out-of-Plane Benzyl Group or In-Plane m-Pyridinium Group?

Any method that can enhance the fluorescence of fluorophores is highly desirable. Fluorescence enhancement accomplished by restricted Z/ E photoisomerization through intramolecular steric hindrance or relatively high bond order of a C═C double bond in a S₁ excited state has rarely been studied. In this article, we used green fluorescent protein (GFP) chromophore analogues as a model to get new physical insights into the restricted Z/ E photoisomerization and E/ Z thermoisomerization phenomena. We found that the S₁ and S₀ potential energy surfaces (PESs) of the GFP chromophore analogues experience two dramatically different types of restricted rotation, and 2b can be a representative example. In its S₁ PES, it is not the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group but the relatively high bond order of the I-bond in the S₁ excited state of 2b that makes it have a higher barrier for the Z/ E photoisomerization, a smaller Z/ E photoisomerization quantum yield, and a higher fluorescence quantum yield. In its S₀ PES, it is not the reduced bond order of the I-bond in the S₀ ground state of 2b but the intramolecular steric hindrance between the out-of-plane benzyl group and the in-plane m-pyridinium group that makes it have an extra higher barrier for E/ Z thermoisomerization and a much smaller E/ Z thermoisomerization rate constant.