Imidazolinone Ring Research Articles

Photocatalytic Intramolecular Alkene Hydroamination of N-Alkoxy Ureas: An Approach to Imidazolinones.

Imidazolinones were obtained in good yields by intramolecular hydroamination of N-alkoxy ureas in the presence of an organic photocatalyst and an inorganic base. In this reaction, the N-alkoxy urea anion generated by deprotonation undergoes photocatalyzed single-electron-transfer oxidation to generate the corresponding radical, which cyclizes to afford the imidazolinone ring. This new protocol grants access to an array of complex molecules containing a privileged imidazolinone core.

A novel violet fluorescent protein contains a unique oxidized tyrosine as the simplest chromophore ever reported in fluorescent proteins.

We describe an engineered violet fluorescent protein from the lancelet Branchiostoma floridae (bfVFP). This is the first example of a GFP-like fluorescent protein with a stable fluorescent chromophore lacking an imidazolinone ring; instead, it consists of oxidized tyrosine 68 flanked by glycine 67 and alanine 69. bfVFP contains the simplest chromophore reported in fluorescent proteins and was generated from the yellow protein lanFP10A2 by two synergetic mutations, S148H and C166I. The chromophore structure was confirmed crystallographically and by high-resolution mass spectrometry. The photophysical characteristics of bfVFP (323/430 nm, quantum yield 0.33, and Ec 14,300 M-1 cm-1 ) make it potentially useful for multicolor experiments to expand the excitation range of available FP biomarkers and Förster resonance energy transfer with blue and cyan fluorescent protein acceptors.

Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach.

Methine-bridged conjugated bicyclic aromatic compounds are common constituents of a range of biologically relevant molecules such as porphyrins, dipyrrinones, and pharmaceuticals. Additionally, restricted rotation of these systems often results in highly to moderately fluorescent systems as observed in 3H,5H-dipyrrolo[1,2-c:2',1'-f]pyrimidin-3-ones, xanthoglows, pyrroloindolizinedione analogs, BODIPY analogs, and the phenolic and imidazolinone ring systems of Green Fluorescent Protein (GFP). This manuscript describes an inexpensive and operationally simple method of performing a Claisen-Schmidt condensation to generate a series of fluorescent pH dependent pyrazole/imidazole/isoindolone dipyrrinone analogs. While the methodology illustrates the synthesis of dipyrrinone analogs, it can be translated to produce a wide range of conjugated bicyclic aromatic compounds. The Claisen-Schmidt condensation reaction utilized in this method is limited in scope to nucleophiles and electrophiles that are enolizable under basic conditions (nucleophile component) and non-enolizable aldehydes (electrophile component). Additionally, both the nucleophilic and electrophilic reactants must contain functional groups that will not inadvertently react with hydroxide. Despite these limitations, this methodology offers access to completely novel systems that can be employed as biological or molecular probes.

Local Electric Field Controls Fluorescence Quantum Yield of Red and Far-Red Fluorescent Proteins.

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Computational Modeling Reveals the Mechanism of Fluorescent State Recovery in the Reversibly Photoswitchable Protein Dreiklang.

The unique properties of the photoswitchable protein Dreiklang are attributed to a reversible hydration/dehydration reaction at the imidazolinone ring of the chromophore. Recovery of the fluorescent state, which is associated with a chemical reaction of the chromophore's dehydration, is an important part of the photocycle of this protein. Here we characterize the fluorescent (ON) and nonfluorescent (OFF) states of Dreiklang and simulate the thermal recovery reaction OFF → ON using computational approaches. By using molecular modeling methods including the quantum mechanics/molecular mechanics (QM/MM) technique, we characterize the structures and spectra of the ON- and OFF-states. The results are consistent with available experimental data. The computed reaction profile explains the observed recovery reaction and clarifies the mechanism of chemical transformations in the chromophore-containing pocket in Dreiklang.

Glucopyranosylidene-spiro-imidazolinones, a New Ring System: Synthesis and Evaluation as Glycogen Phosphorylase Inhibitors by Enzyme Kinetics and X-ray Crystallography.

Epimeric series of aryl-substituted glucopyranosylidene-spiro-imidazolinones, an unprecedented new ring system, were synthesized from the corresponding Schiff bases of O-perbenzoylated (gluculopyranosylamine)onamides by intramolecular ring closure of the aldimine moieties with the carboxamide group elicited by N-bromosuccinimide in pyridine. Test compounds were obtained by Zemplén O-debenzoylation. Stereochemistry and ring tautomers of the new compounds were investigated by NMR, time-dependent density functional theory (TDDFT)-electronic circular dichroism, and DFT-NMR methods. Kinetic studies with rabbit muscle and human liver glycogen phosphorylases showed that the (R)-imidazolinones were 14-216 times more potent than the (S) epimers. The 2-naphthyl-substituted (R)-imidazolinone was the best inhibitor of the human enzyme (Ki 1.7 μM) and also acted on HepG2 cells (IC50 177 μM). X-ray crystallography revealed that only the (R) epimers bound in the crystal. Their inhibitory efficacy is based on the hydrogen-bonding interactions of the carbonyl oxygen and the NH of the imidazolinone ring.

Protonation of the imino nitrogen deactivates the excited state of imidazolin-5-one in the solid state

The imino nitrogen of p-methoxybenzylideneimidazolinone (pMBDI) was protonated using aqueous $$\hbox {HClO}_{4}$$ and HCl acid and crystallized in monoclinic crystal system with $$\textit{P2}_{\textit{1}}/n$$ space group. Interestingly, the protonated compounds are fluorescent in polar solvents but non-fluorescent in non-polar solvent as well as in the solid state. These results point to the existence of a non-radiative pathway involving the imidazole nitrogen in the quenching of excited states in these compounds. SYNOPSIS The imino nitrogen of p-methoxybenzylideneimidazolinone (pMBDI) was protonated and crystallized in monoclinic crystal system with $$\textit{P2}_{\textit{1}}/n$$ space group. Interestingly, the protonation at imino nitrogen of imidazolinone ring results in the loss of the fluorescence property of gfp chromophore analogue (p-methoxybenzylideneimidazolinone).

Synthesis and biological activity of imidazo[4,5-c]quinoline derivatives as PI3K/mTOR inhibitors

A series of imidazo[4,5-c]quinoline derivatives(12a—12m) was synthesized with 2-amino-5-bromobenzoic acid and 4-nitrophenylacetonitrile as starting materials, 6-bromo-4-chloro-3-nitroquinoline as intermediate and Suzuki reaction and closure of the imidazolinone ring with triphosgene as key steps. The structures of the key intermediate and target compounds were confirmed by means of 1H NMR, 13C NMR and HRMS. These compounds show an interesting kinase profile as dual PI3K/mTOR tool compounds.

Photoinduced Chromophore Hydration in the Fluorescent Protein Dreiklang Is Triggered by Ultrafast Excited-State Proton Transfer Coupled to a Low-Frequency Vibration.

Because of growing applications in advanced fluorescence imaging, the mechanisms and dynamics of photoinduced reactions in reversibly photoswitchable fluorescent proteins are currently attracting much interest. We report the first time-resolved study of the photoswitching of Dreiklang, so far the only fluorescent protein to undergo reversible photoinduced chromophore hydration. Using broadband femtosecond transient absorption spectroscopy, we show that the reaction is triggered by an ultrafast deprotonation of the chromophore phenol group in the excited state in 100 fs. This primary step is accompanied by coherent oscillations that we assign to its coupling with a low-frequency mode, possibly a deformation of the chromophore hydrogen bond network. A ground-state intermediate is formed in the picosecond-nanosecond regime that we tentatively assign to the deprotonated water adduct. We suggest that proton ejection from the phenol group leads to a charge transfer from the phenol to the imidazolinone ring, which triggers imidazolinone protonation by nearby Glu222 and catalyzes the addition of the water molecule.

PH-Sensitive fluorophores from locked GFP chromophores by a non-alternant analogue of the photochemical meta effect.

We report the synthesis and characterization of a pH-sensitive fluorescence switch based on a conformationally-locked green fluorescent protein (GFP) chromophore. The chromophore differs from difluoroboryl-locked parent by the addition of a titratable alcohol group on the imidazolinone ring. The chromophore is fluorescent at pH ≤ 5, but becomes non-fluorescent at higher pH, where the substituent is ionized. We use a quantum chemical model to show that the mechanism of the fluorescence turn-off is electronically analogous to photochemical meta effects in aryl-containing systems.

Mechanism Behind the Apparent Large Stokes Shift in LSSmOrange Investigated by Time-Resolved Spectroscopy.

LSSmOrange is a fluorescent protein with a large energy gap between the absorption and emission bands (5275 cm(-1)). The electronic structure of the LSSmOrange chromophore, 2-[(5-)-2-hydroxy-dihydrooxazole]-4-(p-hydroxybenzylidene)-5-imidazolinone, is affected by deprotonation of the p-hydroxybenzylidene group. We investigated LSSmOrange by time-resolved spectroscopy in the femtosecond and nanosecond range. The ground state chromophore was almost exclusively in the neutral form, which had a main absorption band at 437 nm with a small shoulder at 475 nm. The absorption at a wavelength within the former band promoted the protein to the excited state where excited state proton transfer (ESPT) could lead to deprotonation in 0.8 ps. Following ESPT, the chromophore emitted fluorescence with a maximum at 573 nm and a decay time of 3500 ps. Although deprotonation by ESPT occurs, we unexpectedly found a slow accumulation of the anionic form in the ground state upon repeated high intensity excitation. This accumulation of the anionic form was accompanied by a shift of the absorption band to 553 nm without changing the emission band. MALDI-MS revealed that this shift is accompanied by decarboxylation of E222, which is interacting with the imidazolinone ring of the chromophore. We concluded that the photoinduced decarboxylation induced a conformational change that affected local environment around the hydroxyl group, resulting in a stable deprotonated form of the chromophore.

Imazamox-clay complexes with chitosan- and iron(III)-modified smectites and their use in nanoformulations.

Imazamox is an ionisable herbicide, weakly retained and with high soil vertical mobility, that is used for the control of the root-parasitic plants Orobanche spp. A natural smectite (SW) modified with the biopolymer chitosan (Ch) or with Fe(3+) cation was assayed as adsorbent or carrier for imazamox controlled-release formulations (CRFs). The greatest adsorption (74%) was observed for SWFe at high initial concentration (500 µM) and low pH (4.3). The interaction mechanism of imazamox on SWFe implies interlayer polar adsorption, followed by protonation of the imidazolinone ring, whereas ionic, polar and hydrophobic interactions seemed to occur in imazamox adsorption on SWCh. The herbicide release into water was inversely related to the strength of the imazamox-clay interactions and ranged in the first 10 min for imazamox-SWFe and imazamox-SWCh complexes from 27 to 75%, whereas commercial imazamox released 86%. The imazamox-SWCh weak complex (SWCh6 WC) showed similar herbicidal activity to the commercial formulation but produced a reduction of 15% in the total soil leaching losses and a reduction of 40% in the peak maximum concentration in soil column leachates. The imazamox-clay weak complex (WC) of SWFe and SWCh and the strong complex (SC) with SWCh showed appropriate behaviour as nanopesticides or smart delivery systems to be incorporated in CRFs. © 2015 Society of Chemical Industry.

Role of zwitterions in kindling fluorescent protein photochemistry.

Kindling fluorescent protein (KFP), one of the chronologically first members of photoswitchable colored proteins from the green fluorescent protein (GFP) family, increasingly attracts efforts from experimental and theoretical sides. Ambiguous conclusions in solving puzzles of photochemistry of KFP and of its parent natural protein asFP595 are partially explained by lack of reliable theoretical data on chromophore properties in the electronically excited state. We report the results of state-of-the-art quantum chemistry calculations of the structure and energy of the KFP chromophore, 2-acetyl-,4-(p-hydroxybenzylidene)-1-methyl-5-imidazolone (AHBMI), both in the ground and excited states. Ground state equilibrium structures of anionic and zwitterionic protonation states of AHBMI were computed by the conventional MP2 method while excited state structures were characterized by the extended multireference perturbation theory method XMCQDPT2 including optimization of geometry parameters at this level. In particular, the computational results demonstrate that the basicity of the N2 nitrogen atom of the imidazolinone ring should noticeably increase upon electronic excitation, thus affecting excited state proton transfer events in proteins. The results of these simulations as well as of quantum mechanical-molecular mechanical (QM/MM) calculations for model protein systems evidence that KFP conformations with the zwitterionic chromophore are hardly expected to occur in the ground state, but may be populated upon excitation.

Tuning the solid-state emission of the analogous GFP chromophore by varying alkyl chains in the imidazolinone ring

Tuning the solid-state emission of the analogous GFP chromophore by varying alkyl chains in the imidazolinone ring

Tuning the solid-state emission of the analogous GFP chromophore by varying alkyl chains in the imidazolinone ring

New analogues of green fluorescent protein (GFP) chromophore mGFP-Cn (n = 1, 3, 5, 11) with alkyl chains of different lengths in the imidazolinone rings were synthesized and their crystal structures were determined. These GFP-like chromophores are all emissive in the solid state. And the solid-state emission quantum yields of increase by extending the lengths of alkyl chains, owing to the fact that the intermolecular pi-pi interactions are significantly weakened based on their crystal structures.

Unusual Emitting States of the Kindling Fluorescent Protein: Appearance of the Cationic Chromophore in the GFP Family

The kindling fluorescent protein (KFP), the Ala143Gly variant of the natural chromoprotein asFP595, is a prospective biomarker in live cells. Following the results of QM/MM calculations, we predict that excitation of the protein under certain conditions, favoring formation of KFP fractions with the neutral chromophore, should result in fluorescence from the cationic form of the chromophore which is unusual for the members of the green fluorescent protein family. Occurrence of the neutral form is due to a water wire connecting the chromophore with the exterior of the protein. Occurrence of the cationic form is due to the excited-state proton transfer from the conserved Glu215 to the imidazolinone ring nitrogen of the chromophore. The emission band from conformations with the trans cationic chromophore should be noticeably shifted to the blue side around 520 nm compared to the well-known red fluorescence around 600 nm arising from the cis anionic species.

Synthesis, in silico metabolic and toxicity prediction of some novel imidazolinones derivatives as potent anticonvulsant agents

A series of 1,2,4-trisubstituted 5-imidazolinone derivatives were synthesized by Erlenmeyer condensation of benzoylglycine (hippuric acid) with different aldehydes in the presence of sodium acetate and acetic anhydride. The derivatives of the compounds were prepared by condensation of some known sulpha drugs with 5-oxazolone derivatives. The anticonvulsant activity of the compounds was determined by the protection of pentylenetetrazole-induced convulsions that was ranged from 10 to 60%. The compounds with p-OCH3, p-OH and o-Cl substitutions in the phenyl ring on 4th position of the imidazolinone ring exhibited good anticonvulsant activity. In silico metabolic and toxicity studies showed that all the compounds in the series are not likely to exhibit toxicity except the compounds IIIa, IIIb, VIa and VIb, that is predicted to show 29% mutagenicity and 53% irritation in comparison to the other compounds. The predicted lethal effect and hERG toxicity of the compounds showed that IIa, IVa, Va and Vb might be toxic at higher concentrations. The results successfully establish the synthesized imidazolinone derivatives as novel compounds with anticonvulsant properties, low predicted cardiotoxicity and lethal effects thus can be promising leads for further development as novel anticonvulsants.

Imidazolinone herbicides in strongly acidic media: Speciation and electroreduction

From UV-visible measurements and potentiometric titrations it follows that the lowest pK values (pK1) of imidazolinone herbicides correspond to the simultaneous protonation/dissociation equilibria of both the pyridinic (or quinolinic) nitrogen and the carboxyl group, the following pK (pK2) to the imminium nitrogen and the basic pK (pK3) to the dissociation of the imido nitrogen. Below pH 6 and down to pH c.a. 2.5, the dominant form of the herbicide is a double ion having both positive and negative charges, this being important in discussing the effect of pH in the natural dynamics of imidazolinone herbicides, especially in their soil sorption. Electrochemical studies of the reduction of the herbicides were made on mercury and carbon electrodes in strongly acidic media (0.1 to 2.7 M H2SO4) as well up to pH 7. The reduction signals were all attributed to the reduction of the imidazolinone ring except the second peak/wave that was found to have originated by the reduction of the pyridine/quinoline ring. A signal observed in strongly acidic media and at highly negative overpotentials was attributed to the reduction of the imidazolinone ring of the product of the previous reduction in a process consisting of two reversible electron transfers followed by a protonation reaction. À partir de mesures UV-visible et des titrages potentiométriques il s’ensuit que la plus basse valeur de pK (pK1) des herbicides de type imidazolinone correspond aux équilibres de dissociation/protonation simultanées d’azote pyridinique (ou quinolinique) et le groupe carboxyle, le pK suivant (pK2) à l’azote imminium et le pK basique (pK3) à la dissociation de l’azote imido. En dessous de pH 6 et pH jusqu’à C.A. 2,5, la forme dominante de l’herbicide est un ion double ayant charges positive et négative, ce qui est important dans la discussion l’effet du pH dans la dynamique naturelle des herbicides, en particulier dans leur sorption dans le sol. Des études électrochimiques de la réduction des herbicides ont été faites sur des électrodes de mercure et de charbon dans des milieux fortement acides (de 0,1 à 2,7 M H2SO4) ainsi jusqu’à pH 7. Les signaux de réduction ont été attribuées à la réduction de l’anneau de type imidazolinone, sauf le deuxième pic, les vagues causées par la réduction de l’anneau de pyridine/quinoline. La signal observé en milieu fortement acide et à surtensions très négative a été attribuée à la réduction de l’anneau d’imidazolinone du produit de la réduction précédente dans un processus composé de deux transferts d’électrons réversible suivie d’une réaction de protonation.

Primary Events of Photodynamics in Reversible Photoswitching Fluorescent Protein Dronpa

Reversible photoswitching fluorescent protein Dronpa can reversibly switch between fluorescent on-state and nonfluorescent off-state by two radiations. The primary events of photodynamics in Dronpa were elucidated by nonadiabatic ONIOM (CASSCF:AMBER) molecular dynamics simulations. All radiationless decay processes are found mainly to result from one bond flip of a bridge C−C bond of the chromophore in the protein, regardless of its protonation state or conformation, rather than hula twisting. In the off-state protein, trans−cis photoisomerization of the neutral trans chromophore takes place via a rotation around the imidazolinone ring. In the wild-type on-state protein, the anionic cis chromophore mostly remains planar for at least 20 ps. In contrast, in the H193T mutant on-state, faster decay via a rotation of the phenoxy ring or imidazolinone ring of the anionic cis chromophore was found, suggesting that flexibility of the chromophore and its immediate protein environment is the key to radiationless decays.

Electrochemical reduction of imazamethabenz methyl on mercury and carbon electrodes

Abstract This paper presents polarographic and voltammetric studies of the reduction of the herbicide imazamethabenz methyl (2/3-methyl-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-p-toluate), on mercury and carbon electrodes. The electrochemical studies were performed in strongly acidic media (0.1–2.7 M H2SO4) as well as in the pH range of 1–12. The overall reduction process involves the uptake of two electrons. The results obtained in polarography show that there is the reduction of two species, related via an acid–base equilibrium, and having very close reduction potentials. The voltammetric results obtained with a glassy carbon electrode were very similar to those observed on mercury electrodes. The reducible group in the molecule is the imidazolinone ring. In strongly acidic media (pH pKa the neutral form of the herbicide is reduced and the second electron transfer becomes reversible or quasi-reversible. In basic media, the species reduced is the deprotonated imazamethabenz methyl and the r.d.s. is the second electron transfer.