Pyridinium Analogues of Green Fluorescent Protein Chromophore: Fluorogenic Dyes with Large Solvent-Dependent Stokes Shift

Annulated oxazolium anion-π+ AIEgens

A new class of anion-π+ AIEgens based on ring-fused annulated oxazolium architectures is reported, which can be readily synthesized via a single-step dual C-H activation annulative π-extension (APEX) protocol from simple oxazolium salts. The crucial and decisive role of anion-π+ interactions in their solid-state structural arrangement was analyzed to correlate their tunable AIE features and solid-state quantum yields.

Monitoring GPCR conformation with GFP-inspired dyes

Solvatochromic compounds have emerged as valuable environment-sensitive probes for biological research. Here we used thiol-reactive solvatochromic analogs of the green fluorescent protein (GFP) chromophore to track conformational changes in two proteins, recoverin and the A2A adenosine receptor (A2AAR). Two dyes showed Ca2+-induced fluorescence changes when attached to recoverin. Our best-performing dye, DyeC, exhibited agonist-induced changes in both intensity and shape of its fluorescence spectrum when attached to A2AAR; none of these effects were observed with other common environment-sensitive dyes. Molecular dynamics simulations showed that activation of the A2AAR led to a more confined and hydrophilic environment for DyeC. Additionally, an allosteric modulator of A2AAR induced distinct fluorescence changes in the DyeC spectrum, indicating a unique receptor conformation. Our study demonstrated that GFP-inspired dyes are effective for detecting structural changes in G protein-coupled receptors (GPCRs), offering advantages such as intensity-based and ratiometric tracking, redshifted fluorescence spectra, and sensitivity to allosteric modulation.

Bright NIR-Emitting Styryl Pyridinium Dyes with Large Stokes' Shift for Sensing Applications

Two NIR-emitting donor-π-acceptor (D-π-A) type regioisomeric styryl pyridinium dyes (1a-1b) were synthesized and studied for their photophysical performance and environment sensitivity. The two regioisomers, 1a and 1b, exhibited interesting photophysical properties including, longer wavelength excitation (λex ≈ 530-560 nm), bright near-infrared emission (λem ≈ 690-720 nm), high-fluorescence quantum yields (ϕfl ≈ 0.24-0.72) large Stokes' shift (∆λ ≈ 150-240 nm) and high-environmental sensitivity. Probe's photophysical properties were studied in different environmental conditions such as polarity, viscosity, temperature, and concentration. Probes (1a-1b) exhibited noticeable changes in absorbance, emission and Stokes' shift while responding to the changes in physical environment. Probe 1b exhibited a significant bathochromic shift in optical spectra (∆λ ≈ 20-40 nm) compared to its isomer 1a, due to the regio-effect. Probes (1a-1b) exhibited an excellent ability to visualize bacteria (Bacillus megaterium, Escherichia coli), and yeast (Saccharomyces cerevisiae) via fluorescence microscopy.

Meta-CF3-Substituted Analogues of the GFP Chromophore with Remarkable Solvatochromism

In this work, we have shown that the introduction of a trifluoromethyl group into the me-ta-position of arylidene imidazolones (GFP chromophore core) leads to a dramatic increase in their fluorescence in nonpolar and aprotic media. The presence of a pronounced solvent-dependent gradation of fluorescence intensity makes it possible to use these substances as fluorescent polarity sensors. In particular, we showed that one of the created compounds could be used for selective labeling of the endoplasmic reticulum of living cells.

Keto-Analogs of Arylidene-Imidazolones as Fluorogenic Dyes

Abstract

We report a series of (Z)-2-acetyl-4-benzyliden-1-methyl-1Н-imidazol-5(4Н)-ones with a pronounced solvent-dependent intensity of fluorescence variation. The introduction of the 2-acetyl group allows one to shift the absorption and emission maxima to the long-wavelength region. We have shown that these compounds can be used for staining the endoplasmic reticulum for fluorescent microscopy.

Regulation of mitochondrial proteostasis by the proton gradient

Mitochondria adapt to different energetic demands reshaping their proteome. Mitochondrial proteases are emerging as key regulators of these adaptive processes. Here, we use a multiproteomic approach to demonstrate the regulation of the m‐AAA protease AFG3L2 by the mitochondrial proton gradient, coupling mitochondrial protein turnover to the energetic status of mitochondria. We identify TMBIM5 (previously also known as GHITM or MICS1) as a Ca²⁺/H⁺ exchanger in the mitochondrial inner membrane, which binds to and inhibits the m‐AAA protease. TMBIM5 ensures cell survival and respiration, allowing Ca²⁺ efflux from mitochondria and limiting mitochondrial hyperpolarization. Persistent hyperpolarization, however, triggers degradation of TMBIM5 and activation of the m‐AAA protease. The m‐AAA protease broadly remodels the mitochondrial proteome and mediates the proteolytic breakdown of respiratory complex I to confine ROS production and oxidative damage in hyperpolarized mitochondria. TMBIM5 thus integrates mitochondrial Ca²⁺ signaling and the energetic status of mitochondria with protein turnover rates to reshape the mitochondrial proteome and adjust the cellular metabolism.

Fluorescent probes for targeting endoplasmic reticulum: design strategies and their applications

Advances in developing organic fluorescent probes and fluorescence imaging techniques have enhanced our understanding of cell biology. The endoplasmic reticulum (ER) is a dynamic structure that plays a crucial role in protein synthesis, post-translational modifications, and lipid metabolism. The malfunction of ER contributes to several physiological and pathological conditions. Therefore, the investigations on the imaging and role of ER have attracted a lot of attention. Due to their simplicity, synthetic tunability, photostability, high quantum yields, easier cellular uptake, and lower cytotoxicity, organic fluorophores offer invaluable tools for the precision targeting of various cellular organelles and probe ER dynamics. The precision staining is made possible by incorporating specific functional groups having preferential and local organelle biomolecular interactions. For instance, functional moieties such as methyl sulfonamide, sulfonylurea, and pentafluorophenyl assist in ER targeting and thus have become essential tools to probe a deeper understanding of their dynamics. Furthermore, dual-function fluorescent probes that simultaneously image ER and detect specific physiological parameters or biological analytes were achieved by introducing special recognition or chemically reactive sites. This article attempts to comprehensively capture various design strategies currently employed by researchers utilizing small organic molecules to target the ER and detect specific analytes.

Practicable Applications of Aggregation-Induced Emission with Biomedical Perspective

Considerable efforts have been made into developing aggregation-induced emission fluorogens (AIEgens)-containing nano-therapeutic systems due to the excellent properties of AIEgens. Compared to other fluorescent molecules, AIEgens have advantages including low background, high signal-to-noise ratio, good sensitivity, and resistance to photobleaching, in addition to being exempt from concentration quenching or aggregation-caused quenching effects. The present review outlines the major developments in the biomedical applications of AIEgens-containing systems. From a literature survey, the recent AIE works are reviewed and the reasons why AIEgens are chosen in various biomedical applications are highlighted. The research activities on AIEgens-containing systems are increasing rapidly, therefore, the present review is timely.

Tailoring C-6-Substituted Coumarin Scaffolds for Novel Photophysical Properties and Stimuli-Responsive Chromism

A judicious strategy was utilized to envision the substantial regio-positional effects of substituents on the photophysical properties of the 2H-chromen-2-one-3-benzothiazole scaffold-based push-pull framework, named 6-X-CUMs. Among them, 6-NEt₂-CUM reveals prominent excited-state intramolecular charge transfer with a large change of dipole moment (Δμ ∼ 18.23 D), hence displaying remarkable emission solvatochromism from the green (536 nm in cyclohexane) to far-red region (714 nm in dimethyl sulfoxide) and a high-temperature sensitivity (-0.23 nm °C^-1). These, together with unique basicity and acido-/vaporchromism upon acidification elucidated by NMR and photospectroscopic studies, show stark contrast to the conventional 7-NEt₂-CUM. The new series of these tailored 6-X-CUMs represents a new dimension in tailoring the photophysical properties for the development of a promising class of multistimuli-responsive materials.

Ab-initio molecular dynamics and hybrid explicit-implicit solvation model for aqueous and nonaqueous solvents: GFP chromophore in water and methanol solution as case study

Solute-solvent interactions are proxies for understanding how the electronic density of a chromophore interacts with the environment in a more exhaustive way. The subtle balance between polarization, electrostatic, and non-bonded interactions need to be accurately described to obtain good agreement between simulations and experiments. First principles approaches providing accurate configurational sampling through molecular dynamics may be a suitable choice to describe solvent effects on solute chemical-physical properties and spectroscopic features, such as optical absorption of dyes. In this context, accurate energy potentials, obtained by hybrid implicit/explicit solvation methods along with employing nonperiodic boundary conditions, are required to represent bulk solvent around a large solute-solvent cluster. In this work, a novel strategy to simulate methanol solutions is proposed combining ab initio molecular dynamics, a hybrid implicit/explicit flexible solvent model, nonperiodic boundary conditions, and time dependent density functional theory. As case study, the robustness of the proposed protocol has been gauged by investigating the microsolvation and electronic absorption of the anionic green fluorescent protein chromophore in methanol and aqueous solution. Satisfactory results are obtained, reproducing the microsolvation layout of the chromophore and, as a consequence, the experimental trends shown by the optical absorption in different solvents.

Mechanism of Color and Photoacidity Tuning for the Protonated Green Fluorescent Protein Chromophore

The neutral or A state of the green fluorescent protein (GFP) chromophore is a remarkable example of a photoacid naturally embedded in the protein environment and accounts for the large Stokes shift of GFP in response to near UV excitation. Its color tuning mechanism has been largely overlooked, as it is less preferred for imaging applications than the redder anionic or B state. Past studies, based on site-directed mutagenesis or solvatochromism of the isolated chromophore, have concluded that its color tuning range is much narrower than its anionic counterpart. However, as we performed extensive investigation on more GFP mutants, we found that the color of the neutral chromophore can be more sensitive to protein electrostatics than can the anionic counterpart. Electronic Stark spectroscopy reveals a fundamentally different electrostatic color tuning mechanism for the neutral state of the chromophore that demands a three-form model as compared to that of the anionic state, which requires only two forms ( J. Am. Chem. Soc. 2019, 141, 15250-15265). Specifically, an underlying zwitterionic charge-transfer state is required to explain its sensitivity to electrostatics. As the Stokes shift is tightly linked to excited-state proton transfer (ESPT) of the protonated chromophore, we infer design principles of the GFP chromophore as a photoacid through the color tuning mechanisms of both protonation states. The three-form model could also be applied to similar biological and nonbiological dyes and complements the failure of the two-form model for donor-acceptor systems with localized ground-state electronic distributions.

Unraveling the Effect on Luminescent Properties by Postsynthetic Covalent and Noncovalent Grafting of gfp Chromophore Analogues in Nanoscale MOF-808

Here, we demonstrate mimicking of photophysical properties of native green fluorescent protein (gfp) by immobilizing the gfp chromophore analogues in nanoscale MOF-808 and further exploring the bioimaging applications. The two virtually nonfluorescent gfp chromophore analogues carrying different functionalities, BDI-AE (COOH/COOMe) and BDI-EE (COOMe/COOMe) were immobilized in nanosized MOF-808 via postsynthetic modification. An ¹H NMR and IR study confirms that BDI-AE was coordinated in NMOF-808, whereas BDI-EE was just noncovalently encapsulated. Interestingly, the extremely weakly fluorescent monomers BDI-AE and BDI-EE (QY = 0.01-0.03%, lifetime = 0.01-0.03 ns) showed a 10²-fold increase in quantum efficiency with a significantly longer excited-state lifetime (QY = 1.8-5.6%, lifetime 0.89-1.49 ns) after immobilization in the NMOF-808 scaffold. Moreover, BDI-AE@MOF-808 has 4 times higher quantum efficiency as well as longer excited-state lifetime in comparison to BDI-EE@NMOF-808 due to the rigidity imposed in the chromophore upon coordination with Zr⁴⁺ in the former case. Further, a cell viability test performed for BDI-AE@NMOF-808 in HeLa cells confirmed the nontoxic nature of the material and, more importantly, bioimaging applications have also been explored successfully.

Dual-Self-Restricted GFP Chromophore Analogues with Significantly Enhanced Emission

The tremendous gap of fluorescence emission of synthetic green fluorescent protein (GFP) chromophore to the protein itself makes it impossible to use for applications in molecular and cellular imaging. Here, we developed an efficient methodology to enhance the photoluminescence response of synthetic GFP chromophore analogues by constructing dual-self-restricted chromophores. Single self-restricted chromophores were first generated with 2,5-dimethoxy substitution on the aromatic ring, which were further modified with phenyl or 2,5-dimethoxy phenyl to form dual-self-restricted chromophores. These two chromophores showed an obvious solvatofluorochromic color palette across blue to yellow with a maximum emission Stokes shift of 95 nm and dramatically enhanced fluorescence emission in various aprotic solvents, especially in hexane, where the QY reached around 0.6. Importantly, in acetonitrile and dimethyl sulfoxide, the fluorescence QYs of both chromophores were over 0.22, which were the highest reported so far in high polar organic solvents. Meanwhile, the fluorescence lifetimes also improved obviously with the maximum of around 4.5 ns. Theoretical calculations revealed a more favorable Mülliken atomic charge translocation over the double-bond bridge and illustrated much higher energy barriers for the Z/E photoisomerization together with larger bond orders compared with the GFP core chromophore, p-HBDI. Our work significantly improved the fluorescence emission of synthetic GFP chromophore analogues in polar solvents while reserved the multicolor emitting function, providing a solid molecular motif for engineering high-performance fluorescent probes.

Photostable Voltage-Sensitive Dyes Based on Simple, Solvatofluorochromic, Asymmetric Thiazolothiazoles

A family of asymmetric thiazolo[5,4-d]thiazole (TTz) fluorescent dye sensors has been developed, and their photophysical sensing properties are reported. The π-conjugated, TTz-bridged compounds are synthesized via a single-step, double condensation/oxidation of dithiooxamide and two different aromatic aldehydes: one with strong electron-donating characteristics and one with strong electron-accepting characteristics. The four reported dyes include electron-donating moieties (N,N-dibutylaniline and N,N-diphenylaniline) matched with three different electron-accepting moieties (pyridine, benzoic acid, and carboxaldehyde). The asymmetric TTz derivatives exhibit strong solvatofluorochromism with Stokes shifts between 0.269 and 0.750 eV (2270 and 6050 cm^-1) and transition dipole moments (Δμ = 13-18 D) that are among the highest reported for push-pull dyes. Fluorescence quantum yields are as high as 0.93 in nonpolar solvents, and the fluorescence lifetimes (τ_F) vary from 1.50 to 3.01 ns depending on the solvent polarity. In addition, thermofluorochromic studies and spectrophotometric acid titrations were performed and indicate the possibility of using these dyes as temperature and/or acid sensors. In vitro cell studies indicate good cell membrane localization, negligible cytotoxicity, promising voltage sensitivities, and photostabilities that are 4 times higher than comparable dyes. Their ease of synthesis and purification, remarkable photophysical properties, and chemically sensitive TTz π-bridge make these asymmetric dye derivatives attractive for environmental and biological sensing or similar molecular optoelectronic applications.

Red-Shifted Substrates for FAST Fluorogen-Activating Protein Based on the GFP-Like Chromophores

A genetically encoded fluorescent tag for live cell microscopy is presented. This tag is composed of previously published fluorogen-activating protein FAST and a novel fluorogenic derivative of green fluorescent protein (GFP)-like chromophore with red fluorescence. The reversible binding of the novel fluorogen and FAST is accompanied by three orders of magnitude increase in red fluorescence (580-650 nm). The proposed dye instantly stains target cellular proteins fused with FAST, washes out in a minute timescale, and exhibits higher photostability of the fluorescence signal in confocal and widefield microscopy, in contrast with previously published fluorogen:FAST complexes.

A “turn-on” Michler's ketone–benzimidazole fluorescent probe for selective detection of serum albumins

Enhanced emission and selective binding with albumins.