Synthesis and Properties of the p-Sulfonamide Analogue of the Green Fluorescent Protein (GFP) Chromophore: The Mimic of GFP Chromophore with Very Strong N-H Photoacid Strength

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.

Unraveling the Mechanistic Pathway for the Dual Fluorescence in Green Fluorescent Protein (GFP) Chromophore Analogue: A Detailed Theoretical Investigation

The photophysical properties of the para-sulfonamide (p-TsABDI) analogue of the green fluorescent protein (GFP) chromophore with both proton donating and accepting sites have been exploited in polar solvents to understand the origin of the unusual dual fluorescence nature of the chromophore. In the polar solvents, the compound undergoes structural rearrangement upon photoexcitation, leading to the ultrafast excited-state intermolecular proton transfer (ESIPT) phenomenon at the S₁ surface. In this work, we employed both the static electronic structure calculations and on-the-fly molecular dynamics simulation to unravel the underlying reason for this peculiar behavior of the p-TsABDI analogue in polar solvents. To represent this adequately and provide extensive information on the ESIPT mechanism mediated by the solvent molecules, we considered explicit solvent molecules using the integral equation formalism variant of polarizable continuum (IEFPCM) model. From the static calculation analysis, we can conclude that the dual emissive behavior of the compound is ascribed to the proton transfer (PT) phenomena in the excited-state. However, based on the static calculation exclusively, it is hard to ascertain the mechanistic pathway of the PT phenomena. Hence, to investigate the dynamics and reaction mechanism for the ESIPT process, we performed the on-the-fly dynamics simulation for p-TsABDI in solvent clusters. Dynamics simulation results reveal that, based on the time lag between all the proton transfer processes, the ESIPT mechanism occurs in a stepwise manner from the benzylidene moiety of the chromophore to its imidazolinone moiety. However, the nonexistence of crossings between the S₁- and S₀-states confirms the PT characteristics of the reactions.

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.

Side-Chain Protonation States of a Fluorescent Arginine

An arginine derivative with a fluorescent side-chain, Boc-Arg(Nap)-OH, was prepared by palladium(0)-catalyzed coupling of Boc-Arg-OH with a 4-bromonaphthalimide. The presence of the fluorophore lowers the pK_a of the side-chain guanidinium group by several orders of magnitude, to 9.0 (±0.1), allowing the derivative to access an electrically neutral protonation state that is not generally available to arginine itself. Computational modeling (DFT) predicts that protonation takes place at the side-chain C═N atom that bears the fluorophore. Calculated electronic absorptions for the protonated (356 nm) and neutral species (440 nm) are in good agreement with experiment. When irradiated with light, excited-state proton transfer (ESPT) occurs from cationic side-chains to suitably basic solvents, resulting in fluorescence emission from the neutral species. Arg(Nap) can be incorporated into peptides with sterically accessible N-termini using specially adapted conditions of solid-phase peptide synthesis.

Why does a para-amino group make the green fluorescent protein chromophore non-fluorescent: coherent intramolecular charge transfer reduces the Z/E-photoisomerization barrier

A para-N-phenyl-amino group significantly increases the fluorescence quantum yield of N-phenyl-4-aminostilbene by the "amino conjugation effect", but, in contrast, a para-amino group in the para-amino analogue (p-ABDI) of the green fluorescent protein (GFP) chromophore makes p-ABDI non-fluorescent because the coherent photo-induced intramolecular charge transfer reduces the barrier of the Z/E-photoisomerization.