Revealing the Role of Electronic Effect to Modulate the Photophysics and Z-Scan Responses of o-Locked GFP Chromophores

Three novel core green fluorescent protein (GFP) chromophore analogues, based on a doubly locked conformation and variable electronic effects by replacing one hydrogen with bromine, iodine, and methyl, respectively, have been synthesized to modulate the push-pull effect. These chromophores exhibited intramolecular H-bonding, as evidenced by single-crystal X-ray and 1H NMR studies. The fluorescence quantum yields (ϕf) of all of the chromophores were found to be more than an order of magnitude higher (∼0.2) than the unlocked chromophores (∼0.01). It was found that the electronic effect did affect the nonradiative rates, as the quantum yields were found to vary with respect to different analogues in the same solvents. The effect of the push-pull effect was also evident by a higher Stokes-shifted emission in the case of the methyl derivative with respect to the bromo- and iodo-analogues. Furthermore, the emission spectra of these GFP chromophores were found to show positive solvatochromism, which was supported by a quantum chemical calculation. A detailed study, correlating the observed spectral changes with various solvent functions and supported by computational results, established a facile proton transfer, followed by twisted intramolecular charge transfer (TICT) to be the major nonradiative channels of these chromophores. Besides, a completely novel usage of these chromophores was explored for the first time by studying their third-order nonlinear optical characteristics in DMSO using a single-beam Z-scan technique. All of the chromophores exhibited tunable nonlinear refraction (NLR) and nonlinear absorption (NLA) properties that depend upon different substituent groups present in the chromophores. Here, the NLR was due to the effect of self-defocusing, whereas the NLA was triggered by reverse saturable absorption, which is attributed to the two-photon absorption (TPA) process. Surprisingly, the efficiency of the TPA ability of the chromophores was found to be a function of the induced electronic effect. Hence, this work opens a new route for the utility of the ortho-locked GFP chromophores in the field of nonlinear optical applications.

Trimethylguanosine cap-fluorescent molecular rotor (TMG-FMR) conjugates are potent, specific snurportin1 ligands enabling visualization in living cells

The trimethylguanosine (TMG) cap is a motif present inter alia at the 5' end of small nuclear RNAs, which are involved in RNA splicing. The TMG cap plays a crucial role in RNA processing and stability as it protects the RNA molecule from degradation by exonucleases and facilitates its export from the nucleus. Additionally, the TMG cap plays a role in the recognition of snRNA by snurportin, a protein that facilitates nuclear import. TMG cap analogs are used in biochemical experiments as molecular tools to substitute the natural TMG cap. To expand the range of available TMG-based tools, here we conjugated the TMG cap to Fluorescent Molecular Rotors (FMRs) to open the possibility of detecting protein-ligand interactions in vitro and, potentially, in vivo, particularly visualizing interactions with snurportin. Consequently, we report the synthesis of 34 differently modified TMG cap-FMR conjugates and their evaluation as molecular probes for snurportin. As FMRs we selected three GFP-like chromophores (derived from green fluorescent protein) and one julolidine derivative. The evaluation of binding affinities for snurportin showed unexpectedly a strong stabilizing effect for TMGpppG-derived dinucleotides containing the FMR at the 2'-O-position of guanosine. These newly discovered compounds are potent snurportin ligands with nanomolar KD (dissociation constant) values, which are two orders of magnitude lower than that of natural TMGpppG. The effect is diminished by ∼50-fold for the corresponding 3'-regioisomers. To deepen the understanding of the structure-activity relationship, we synthesized and tested FMR conjugates lacking the TMG cap moiety. These studies, supported by molecular docking, suggested that the enhanced affinity arises from additional hydrophobic contacts provided by the FMR moiety. The strongest snurportin ligand, which also gave the greatest fluorescence enhancement (Fm/F0) when saturated with the protein, were tested in living cells to detect interactions and visualize complexes by fluorescence lifetime monitoring. This approach has potential applications in the study of RNA processing and RNA-protein interactions.

A GFP Inspired 8-Methoxyquinoline-Derived Fluorescent Molecular Sensor for the Detection of Zn2+ by Two-Photon Microscopy

An effective, GFP-inspired fluorescent Zn2+ sensor is developed for two-photon microscopy and related biological application that features an 8-methoxyquinoline moiety. Excellent photophysical characteristics including a 37-fold fluorescence enhancement with excitation and emission maxima at 440 nm and 505 nm, respectively, as well as a high two-photon cross-section of 73 GM at 880 nm are reported. Based on the experimental data, the relationship between the structure and properties was elucidated and explained backed up by DFT calculations, particularly the observed PeT phenomenon for the turn-on process. Biological validation and detailed experimental and theoretical characterization of the free and the zinc-bound compounds are presented.

Excited-state dynamics and fluorescence lifetime of cryogenically cooled green fluorescent protein chromophore anions

Time-resolved action spectroscopy together with a fs-pump probe scheme is used in an electrostatic ion-storage ring to address lifetimes of specific vibrational levels in electronically excited states. Here we specifically consider the excited-state lifetime of cryogenically cooled green fluorescent protein (GFP) chromophore anions which is systematically measured across the S0-S1 spectral region (450-482 nm). A long lifetime of 5.2 ± 0.3 ns is measured at the S0-S1 band origin. When exciting higher vibrational levels in S1, the lifetime changes dramatically. It decreases by more than two orders of magnitude in a narrow energy region ∼250 cm-1 (31 meV) above the 0-0 transition. This is attributed to the opening of internal conversion over an excited-state energy barrier. The applied experimental technique provides a new way to uncover even small energy barriers, which are crucial for excited-state dynamics.

Charting the Chemical Reaction Space around a Multicomponent Combination: Controlled Access to a Diverse Set of Biologically Relevant Scaffolds

Charting the chemical reaction space around the combination of carbonyls, amines, and isocyanoacetates allows the description of new multicomponent processes leading to a variety of unsaturated imidazolone scaffolds. The resulting compounds display the chromophore of the green fluorescent protein and the core of the natural product coelenterazine. Despite the competitive nature of the pathways involved, general protocols provide selective access to the desired chemotypes. Moreover, we describe unprecedented reactivity at the C-2 position of the imidazolone core to directly afford C, S, and N-derivatives featuring natural products (e.g. leucettamines), potent kinase inhibitors, and fluorescent probes with suitable optical and biological profiles.

Charting the Chemical Reaction Space around a Multicomponent Combination: Controlled Access to a Diverse Set of Biologically Relevant Scaffolds

Charting the chemical reaction space around the combination of carbonyls, amines, and isocyanoacetates allows the description of new multicomponent processes leading to a variety of unsaturated imidazolone scaffolds. The resulting compounds display the chromophore of the green fluorescent protein and the core of the natural product coelenterazine. Despite the competitive nature of the pathways involved, general protocols provide selective access to the desired chemotypes. Moreover, we describe unprecedented reactivity at the C-2 position of the imidazolone core to directly afford C, S, and N-derivatives featuring natural products (e.g. leucettamines), potent kinase inhibitors, and fluorescent probes with suitable optical and biological profiles.

Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), Φ_PL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd(OAc)₂. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O₂CCF₃)(py)] (3), [Pd(C^N-oxazolone)(py)₂](ClO₄) (4), [Pd(C^N-oxazolone)(Cl)(py)] (5), and [Pd(C^N-oxazolone)(X)(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3-6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of Φ_PL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3-6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3-6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.

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.

Locking the GFP Fluorophore to Enhance Its Emission Intensity

The Green Fluorescent Protein (GFP) and its analogues have been widely used as fluorescent biomarkers in cell biology. Yet, the chromophore responsible for the fluorescence of the GFP is not emissive when isolated in solution, outside the protein environment. The most accepted explanation is that the quenching of the fluorescence results from the rotation of the aryl-alkene bond and from the Z/E isomerization. Over the years, many efforts have been performed to block these torsional rotations, mimicking the environment inside the protein β-barrel, to restore the emission intensity. Molecule rigidification through chemical modifications or complexation, or through crystallization, is one of the strategies used. This review presents an overview of the strategies developed to achieve highly emissive GFP chromophore by hindering the torsional rotations.

Designing Red-Shifted Molecular Emitters Based on the Annulated Locked GFP Chromophore Derivatives

Bioimaging techniques require development of a wide variety of fluorescent probes that absorb and emit red light. One way to shift absorption and emission of a chromophore to longer wavelengths is to modify its chemical structure by adding polycyclic aromatic hydrocarbon (PAH) fragments, thus increasing the conjugation length of a molecule while maintaining its rigidity. Here, we consider four novel classes of conformationally locked Green Fluorescent Protein (GFP) chromophore derivatives obtained by extending their aromatic systems in different directions. Using high-level ab initio quantum chemistry calculations, we show that the alteration of their electronic structure upon annulation may unexpectedly result in a drastic change of their fluorescent properties. A flip of optically bright and dark electronic states is most prominent in the symmetric fluorene-based derivative. The presence of a completely dark lowest-lying excited state is supported by the experimentally measured extremely low fluorescence quantum yield of the newly synthesized compound. Importantly, one of the asymmetric modes of annulation provides a very promising strategy for developing red-shifted molecular emitters with an absorption wavelength of ∼600 nm, having no significant impact on the character of the bright S-S1 transition.

Spectroscopic and computational investigations of organometallic complexation of group 12 transition metals by methanobactins from Methylocystis sp. SB2

Methanotrophic bacteria catalyze the aerobic oxidation of methane to methanol using Cu-containing enzymes, thereby exerting a modulating influence on the global methane cycle. To facilitate the acquisition of Cu ions, some methanotrophic bacteria secrete small modified peptides known as "methanobactins," which strongly bind Cu and function as an extracellular Cu recruitment relay, analogous to siderophores and Fe. In addition to Cu, methanobactins form complexes with other late transition metals, including the Group 12 transition metals Zn, Cd, and Hg, although the interplay among solution-phase configurations, metal interactions, and the spectroscopic signatures of methanobactin-metal complexes remains ambiguous. In this study, the complexation of Zn, Cd, and Hg by methanobactin from Methylocystis sp. strain SB2 was studied using a combination of absorbance, fluorescence, extended x-ray absorption fine structure (EXAFS) spectroscopy, and time-dependent density functional theory (TD-DFT) calculations. We report changes in sample absorbance and fluorescence spectral dynamics, which occur on a wide range of experimental timescales and characterize a clear stoichiometric complexation dependence. Mercury L3-edge EXAFS and TD-DFT calculations suggest a linear model for HgS coordination, and TD-DFT suggests a tetrahedral model for Zn2+ and Cd2+. We observed an enhancement in the fluorescence of methanobactin upon interaction with transition metals and propose a mechanism of complexation-hindered isomerization drawing inspiration from the wild-type Green Fluorescent Protein active site. Collectively, our results represent the first combined computational and experimental spectroscopy study of methanobactins and shed new light on molecular interactions and dynamics that characterize complexes of methanobactins with Group 12 transition metals.

Fluorescent Orthopalladated Complexes of 4-Aryliden-5(4H)-oxazolones from the Kaede Protein: Synthesis and Characterization

The goal of the work reported here was to amplify the fluorescent properties of 4-aryliden-5(4H)-oxazolones by suppression of the hula-twist non-radiative deactivation pathway. This aim was achieved by simultaneous bonding of a Pd center to the N atom of the heterocycle and the ortho carbon of the arylidene ring. Two different 4-((Z)-arylidene)-2-((E)-styryl)-5(4H)-oxazolones, the structures of which are closely related to the chromophore of the Kaede protein and substituted at the 2- and 4-positions of the arylidene ring (1a OMe; 1b F), were used as starting materials. Oxazolones 1a and 1b were reacted with Pd(OAc)₂ to give the corresponding dinuclear orthometalated palladium derivates 2a and 2b by regioselective C–H activation of the ortho-position of the arylidene ring. Reaction of 2a (2b) with LiCl promoted the metathesis of the bridging carboxylate by chloride ligands to afford dinuclear 3a (3b). Mononuclear complexes containing the orthopalladated oxazolone and a variety of ancillary ligands (acetylacetonate (4a, 4b), hydroxyquinolinate (5a), aminoquinoline (6a), bipyridine (7a), phenanthroline (8a)) were prepared from 3a or 3b through metathesis of anionic ligands or substitution of neutral weakly bonded ligands. All species were fully characterized and the X-ray determination of the molecular structure of 7a was carried out. This structure has strongly distorted ligands due to intramolecular interactions. Fluorescence measurements showed an increase in the quantum yield (QY) by up to one order of magnitude on comparing the free oxazolone (QY < 1%) with the palladated oxazolone (QY = 12% for 6a). This fact shows that the coordination of the oxazolone to the palladium efficiently suppresses the hula-twist deactivation pathway.

Access to highly functionalized imidazolones bearing α-amino acid esters via KOH-promoted annulation of amidines, nitrosoarenes and malonic esters

An efficient approach to obtain highly functionalized imidazolones bearing α-amino acid esters through KOH-mediated one-pot three-component annulation of amidines, nitrosoarenes and malonic esters is reported. This reaction features broad substrate scope, a cheap and readily available promoter, good to high yields for most substrates and mild reaction conditions. The mechanism study shows that the KOH-mediated formation of the imine intermediate via the reaction of nitrosoarenes with malonic esters is a key step.

Green fluorescent protein inspired fluorophores

Green fluorescence proteins (GFP) are appealing to a variety of biomedical and biotechnology applications, such as protein fusion, subcellular localizations, cell visualization, protein-protein interaction, and genetically encoded sensors. To mimic the fluorescence of GFP, various compounds, such as GFP chromophores analogs, hydrogen bond-rich proteins, and aromatic peptidyl nanostructures that preclude free rotation of the aryl-alkene bond, have been developed to adapt them for a fantastic range of applications. Herein, we firstly summarize the structure and luminescent mechanism of GFP. Based on this, the design strategy, fluorescent properties, and the advanced applications of GFP-inspired fluorophores are then carefully discussed. The diverse advantages of bioinspired fluorophores, such as biocompatibility, structural simplicity, and capacity to form a variety of functional nanostructures, endow them potential candidates as the next-generation bio-organic optical materials.

Five concomitant polymorphs of a green fluorescent protein chromophore (GFPc) analogue: understanding variations in photoluminescence with π-stacking interactions

The synthetically modified green fluorescent protein chromophore analogue 3,4,5-trimethoxybenzylidene imidazolinone (1) yielded five polymorphs (I, II, III, IV, V) concomitantly irrespective of the solvent used for crystallization. The pentamorphic modification of 1 is solely due to the interplay of iso-energetic weak intermolecular interactions in molecular associations as well as the conformational flexibility offered by a C-C single bond, which connects the electron-deficient moiety imidazolinone with the electron-rich trimethoxybenzylidene group. A common structural feature observed in all the polymorphs is the formation of a `zero-dimensional' centrosymmetric dimeric unit through a short and linear C-H...O hydrogen bond engaging phenyl C-H and imidazolinone carbonyl oxygen. However, the networking of these dimeric units showed a subtle difference in all the polymorphs. The 2D isostructurality was observed between polymorphs I, II and III, while the other two polymorphs IV and V revealed only `zero-dimensional' isostructurality. The different fluorescence emissions of Form I (blue) and Forms II to V (yellow) were attributed to the differences in π-stacking interactions. It shows that one can modulate the photophysical properties of these smart materials by slightly altering their crystal structure. Such an approach will aid in developing new multi-colour organic fluorescent materials of varying crystal structures for live-cell imaging and fluorescent sensing applications.

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.

A general fluorescent light-up probe for staining and quantifying protein

Proteins are the primary functional agents in all cellular processes, facilitating various functions such as enzymes and structure-forming or signal-transducing molecules. In this work, we report a fluorescent dye, PyMDI-Zn, which could specifically bind with proteins and provide a red-shifted fluorescent emission. The visual analysis of protein in sodium dodecyl sulfate-polyacrylamide gel electrophoresis could be realized in 5 min by using PyMDI-Zn as a light-up dye. Based on its cell penetration and low toxicity, PyMDI-Zn could also be applied to locate protein-rich regions and organelles in live cell imaging. Moreover, the direct protein quantitation can be realized based on PyMDI-Zn, providing a method of screening for food adulteration by nitrogen-rich compounds.

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.

Crystal structure and spectral studies of green fluorescent protein (GFP) chromophore analogue ethyl 2-[(4Z)-(6-hydroxy naphthalen-2-yl) methylene)-2-methyl-5-oxo-4,5-di hydro-1H-imidazol-1-yl] acetate

Synthetically modified green fluorescent protein chromophore derivative was prepared, its crystal structure and spectral properties were studied. Crystal data for C19H18N2O4: triclinic, space group P-1 (no. 2), a = 8.2506(17) Å, b = 11.934(2) Å, c = 17.461(4) Å, α = 102.89(3)°,   β = 94.62(3)°, γ = 96.68(3)°, V = 1654.5(6) Å3, Z = 4, T = 173(2) K, μ(MoKα) = 0.096 mm-1, Dcalc = 1.358 g/cm3, 7227 reflections measured (4.722° ≤ 2Θ ≤ 53.996°), 7227 unique (Rint = 0.0453, Rsigma = 0.0662) which were used in all calculations. The final R1 was 0.0561 (I > 2σ(I)) and wR2 was 0.1658 (all data). The single crystal structure showed, the benzylidine moiety adopts Z-conformation in solid state and the molecules were associated by various O−H···O and C−H···O non-covalent interactions. The UV absorption-emission spectral analysis indicated that a significant red shift of emission observed at alkaline pH indicating its utility for live cell imaging applications.

Gfp chromophore integrated conjugated microporous polymers: topological and ESPT effects on emission properties

A metal free topological approach is demonstrated to mimic the photophysical properties of natural gfp by synthesizing two gfp chromophore integrated conjugated microporous polymers (o-HBDI-TEB-CMP and o-MBDI-TEB-CMP). Interestingly, owing to the structural rigidity, the emission (λem = 515 nm) and excited state lifetime (4.1 ns) of hydroxy substituted o-HBDI-TEB-CMP are found to be similar to the natural gfp. The crucial role of the -OH group for the green emission is further supported by -OMe substituted o-MBDI-TEB-CMP (λem = 440 nm) and also validated theoretically.

Excited-state locked amino analogues of the green fluorescent protein chromophore with a giant Stokes shift

We design a new class of excited-state locked GFP chromophores which intrinsically exhibit a very large Stokes shift.

Site-dependent fluorescence enhanced polymers with a self-restricted GFP chromophore for living cell imaging

Amphipathic copolymers with a self-restricted GFP chromophore sited on different locations were successfully synthesized, characterized and applied in cell imaging.

Synthesis and properties of geometrical 4-diarylmethylene analogs of the green fluorescent protein chromophore

The E- and Z-isomers of E/Z-photoisomerizable diarylmethylene analogs of the GFP chromophore (geo-DAIN) produce different colors of aggregation-induced emission (AIE).

DNA mimics of red fluorescent proteins (RFP) based on G-quadruplex-confined synthetic RFP chromophores

Red fluorescent proteins (RFPs) have emerged as valuable biological markers for biomolecule imaging in living systems. Developing artificial fluorogenic systems that mimic RFPs remains an unmet challenge. Here, we describe the design and synthesis of six new chromophores analogous to the chromophores in RFPs. We demonstrate, for the first time, that encapsulating RFP chromophore analogues in canonical DNA G-quadruplexes (G4) can activate bright fluorescence spanning red and far-red spectral regions (Em = 583-668 nm) that nearly match the entire RFP palette. Theoretical calculations and molecular dynamics simulations reveal that DNA G4 greatly restricts radiationless deactivation of chromophores induced by a twisted intramolecular charge transfer (TICT). These DNA mimics of RFP exhibit attractive photophysical properties comparable or superior to natural RFPs, including high quantum yield, large Stokes shifts, excellent anti-photobleaching properties, and two-photon fluorescence. Moreover, these RFP chromophore analogues are a novel and distinctive type of topology-selective G4 probe specific to parallel G4 conformation. The DNA mimics of RFP have been further exploited for imaging of target proteins. Using cancer-specific cell membrane biomarkers as targets, long-term real-time monitoring in single live cell and two-photon fluorescence imaging in tissue sections have been achieved without the need for genetic coding.

A selective and sensitive fluorescent probe for the determination of HSA and trypsin

A simple fluorescent probe HBI-GR based on the combination of the fluorophore (p-HBI) in green fluorescent protein (GFP) and Guanine riboside (GR) for HSA was successfully synthesized. HBI-GR showed an obvious fluorescence enhancement toward HSA without interference from other proteins, amino acids, anions and commonly existing metal ions. HBI-GR exhibited high sensitivity towards HSA with a good linear relationship between the fluorescence intensity of HBI-GR and HSA concentration from 0 to 0.06mgmL^-1. The limit of detection, based on a signal-to-noise ratio of 3, was 15.09ngmL^-1, which was much lower than that of most other reported probes. HBI-GR was almost non-fluorescent because of the bond twisting in the exited state of chromophore HBI. After binding to the hydrophobic pocket of HSA, it showed an obvious fluorescence enhancement due to the rigidifying of the flexible chromophore HBI by the hydrophobic environment. The resulting HBI-GR/HSA system also showed a satisfactory sensing ability toward trypsin through decreased fluorescence intensity with the detection limit of 0.0282ngmL^-1. The fluorescence decreasing process was occurred as the lysine and arginine amino acids residues of HSA were cleaved by trypsin, which led to further exposure of HBI-GR to the PBS buffer phase and a concomitant decrease of the HBI-GR fluorescence intensity. Moreover, the probe HBI-GR was successfully used to detect HSA in healthy human urine and human blood serum samples. The practical application of the HBI-GR/HSA system for trypsin detection in healthy human urine also achieved satisfactory result.

A hybrid molecule of a GFP chromophore analogue and cholestene as a viscosity-dependent and cholesterol-responsive fluorescent sensor

Diarylmethylenated and cholestene-hybrid analogues of the GFP chromophore showed viscosity-dependent and cholesterol-responsive fluorescent properties.

Solvent H-bond accepting ability induced conformational change and its influence towards fluorescence enhancement and dual fluorescence of hydroxy meta-GFP chromophore analogue

The effect of structural rigidity towards enhancement of fluorescence quantum yield of GFP chromophore analogues has been documented. In the present study, a new way of enhancing the fluorescence quantum yield of two ortho-meta GFP chromophore analogues meta-methoxy-ortho-hydroxy-benzylimidazolidinone (abbreviated as mOMe-HBDI) and meta-diethylamino-ortho-hydroxyl imidazolidinone (abbreviated as MOHIM) has been reported. This enhancement is controlled by the H-bond accepting ability (denoted as β value) of the solvent and happens only in the case of GFP chromophore analogues having ortho (hydroxyl)-meta (electron donating group) and not in the case of analogues having a para electron donating group. The ground state (solid) conformation of mOMe-HBDI has been obtained from single crystal X-ray analysis, exhibiting the existence of strong intramolecular H-bonding. However, in solution phase, as the solvent β value increases, the strength of intramolecular H-bonding decreases. This process has strong influence on the relative conformational orientation of phenyl and imidazolidinone rings. For mOMe-HBDI, fluorescence quantum yield increases with increase in β value of the solvents. However, the effect of solvent polarity cannot be completely ruled out. The lower wavelength emission band (∼480 nm) has been assigned to the normal charge-transferred (CT) species, whereas the highly Stokes shifted emission band (∼660 nm) has been assigned to the proton-transferred (PT) tautomer species for mOMe-HBDI. In solvents of low β value (say hexane) only the PT band and in solvents of high β value (say DMSO) only the CT band is observed. Quite interestingly, in solvents of intermediate β value both CT and PT bands, thus dual emission, are observed. For mOMe-HBDI when fluorescence decay is monitored at the normal CT emission band, it is observed to be biexponential in nature. The short component increases from ∼0.2 ns to 0.6 ns and the long component increases from 1 to 3.6 ns as the β value of the solvent increases. For a particular solvent, when fluorescence decay is monitored at the normal CT band, as the monitoring wavelength increases the amplitude of the long lifetime component increases and that of the short lifetime component decreases. Time-resolved area-normalised emission spectral (TRANES) analysis confirms the possible existence of two conformers having differential stabilisation by solvent polarity. When fluorescence decay is monitored at the PT band an instrument response limited (<60 ps) decay is noted. Strong support in favour of the above-mentioned structural, steady state and time-resolved optical observations and analyses has been obtained from the methoxy derivative mOMe-MBDI and MOMIM.

Strong electron donation induced differential nonradiative decay pathways for para and meta GFP chromophore analogues

Z-E Isomerisation because of rotation around the exocyclic double bond (known as the τ-twist) and not any other internal conversion has been reported to be the major nonradiative decay channel for non-hydroxylic unconstrained para and meta GFP chromophore analogues. The equation Φf + 2ΦZE = 1 has been shown to hold well for both para and meta GFP chromophore analogues. If the above equation holds true, then upon reducing the extent of Z-E isomerisation (ΦZE), the fluorescence quantum yield (Φf) should increase. To probe the above proposition two sets of non-hydroxylic unconstrained para and meta GFP chromophore analogues were synthesized. Quite interestingly by introducing the strongly electron donating -NEt2 group to the benzenic moiety these para and meta GFP chromophore analogues were shown to exhibit differential optical behaviour w.r.t. the extent of the solvatochromic shift, Φf, ΦZE, and τf. For the first time it has been shown that the well accepted equation Φf + 2ΦZE = 1 does not hold at all for these non-hydroxylic unconstrained meta analogues. Although ΦZE has been shown to be <10%, Φf is much lower than the expected near unity value for these meta analogues. After detailed investigation into the nonradiative excited state decay channel, contrary to literature reports, energy gap law governed internal conversion and not Z-E isomerisation was shown to be the major nonradiative decay channel for these meta analogues. Two models are put forward to understand the differential optical behaviour of these para and meta GFP chromophore analogues. Support from X-ray crystal structures, NMR experiments, and computational calculations has also been provided.

Hidden photoinduced reactivity of the blue fluorescent protein mKalama1

We report a complete photocycle of the blue fluorescent protein exhibiting two delayed branches coupled to hidden proton transfer events.

Locked ortho- and para-core chromophores of green fluorescent protein; dramatic emission enhancement via structural constraint

We report the design strategy and synthesis of a structurally locked GFP core chromophore p-LHBDI, its ortho-derivative, o-LHBDI, and H2BDI possessing both para- and ortho-hydroxyl groups such that the inherent rotational motion of the titled compounds has been partially restricted. o-LHBDI possesses a doubly locked configuration, i.e., the seven-membered ring hydrogen bond and five-membered ring C(4-5-10-13-14) cyclization, from which the excited-state intramolecular proton transfer takes place, rendering a record high tautomer emission yield (0.18 in toluene) and the generation of amplified spontaneous emission. Compared with their unlocked counterparts, a substantial increase in the emission yield is also observed for p-LHBDI and H2BDI in anionic forms in water, and accordingly the structure versus luminescence relationship is fully discussed based on their chemistry and spectroscopy aspect. In solid, o-LHBDI exhibits an H-aggregate-like molecular packing, offers narrow-bandwidth emission, and has been successfully applied to fabricate a yellow organic light emitting diodes (λmax = 568 nm, ηext = 1.9%) with an emission full width at half-maximum as narrow as 70 nm.

Chemical introduction of the green fluorescence: imaging of cysteine cathepsins by an irreversibly locked GFP fluorophore

An activity-based probe, containing an irreversibly locked GFP-like fluorophore, was synthesized and evaluated as an inhibitor of human cathepsins and, as exemplified with cathepsin K, it proved to be suitable for ex vivo imaging and quantification of cysteine cathepsins by SDS-PAGE.

Zn(II)-coordination modulated ligand photophysical processes - the development of fluorescent indicators for imaging biological Zn(II) ions

Molecular photophysics and metal coordination chemistry are the two fundamental pillars that support the development of fluorescent cation indicators. In this article, we describe how Zn(II)-coordination alters various ligand-centered photophysical processes that are pertinent to developing Zn(II) indicators. The main aim is to show how small organic Zn(II) indicators work under the constraints of specific requirements, including Zn(II) detection range, photophysical requirements such as excitation energy and emission color, temporal and spatial resolutions in a heterogeneous intracellular environment, and fluorescence response selectivity between similar cations such as Zn(II) and Cd(II). In the last section, the biological questions that fluorescent Zn(II) indicators help to answer are described, which have been motivating and challenging this field of research.