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- Biothiol-specific fluorescent probes with aggregation-induced emission characteristics;
- Proteomics Sample Preparation.
- Slavery behind the Wall: An Archaeology of a Cuban Coffee Plantation.
At pH 6. Consequently, at this pH, only Hcy resulted in fluorescence. In addition to the aldehyde-bearing probes, a selective colorimetric method for the detection of Hcy over Cys and GSH was described46,53—55 based on the generation of the Hcy thiyl radical by photolysis followed by the intramolecular hydrogen atom transfer HAT reaction to form an a-amino carbon-centred radical. This sensing mechanism of generating Hcy radicals may inspire chemists to design colorimetric probe for the differentiation of Hcy over Cys and GSH.
Lysosome-Targeted Single Fluorescence Probe for Two-Channel Imaging Intracellular SO2 and Biothiols
The Michael addition of Cys to acrylates initially generates thioethers, which cyclize to 7-membered S,N heterocycles 1,4-thiazepane. This reaction has been used since in syntheses. In it was pioneered by the Strongin group, and then exploited Fig. With Cys and Hcy this is followed by cyclization with the release of free fluorophores, whereas with GSH the thioether is stable Fig.
Differentiation between Cys and Hcy typically results from the much faster intramolecular cyclization of the Cys adducts compared to the Hcy analogs, which likely reflects the lower activation entropy for the formation of a 7- vs. Probe 28 and 29 were designed for the detection of Cys and Hcy by exploiting the phenomenon of excited-state intramolecular proton transfer ESIPT. Probe 28 lacks ionizable protons does not participate in ESIPT and manifests a single weak enol emission band at nm due to the alkene-induced PET quenching.
The detection limits of Cys and Hcy were 0. Similar phenomenon was observed in probe 29 Fig. The potential of 28 for clinical applications was illustrated by demonstrating the selective detection of Cys or Hcy in diluted deproteinized human plasma; probe 29 was used for the fluorescent imaging of intracellular Cys.
Interaction of Small Biothiols with Silver and Gold Nanoparticles
The acrylation of fluorescein derivatives resulted in colorless and non-fluorescent spirocyclic molecules 30—32, which allowed the selective detection of Cys Fig. The detection limit of 30 for sensing Cys was evaluated to be 77 nM. Probe 32 was constructed based on long-wavelength emitting seminaphthofluorescein.
The reaction of 30—32 with Cys. The probe was used to detect Cys in calf serum and in living cells. The absorption and emission maxima of Fig. The reason of such a large shift is the keto— enol tautomerization upon Cys-triggered deesterification. Because of the very slow intramolecular cyclization of Hcy and GSH adducts of 36, the probe was highly selective to Cys. Ratiometric imaging of Cys in MCF-7 cells with probe 36 was demonstrated.
Those probes 29—36 were highly selective to Cys because the very slow intramolecular cyclization of the Hcy and GSH adducts did not generate any free fluorophores on the timescale of the measurements. The implementation of the masked fluorophore strategy using chloroacetate 37 ,64 bromoacetate65 or chloropropionate66 instead of acrylate as the thiol reactive masking group was demonstrated.
In these probes, first the halide is displaced by Cys or Hcy, followed by cyclization releasing free fluorophore Fig. In an interesting departure from the pattern described above, probes 38—40 showed different selectivities towards thiols Fig. Native chemical ligation NCL is an important and powerful tool widely used for the synthesis of proteins. Classical NCL is a reaction cascade starting with the trans-thioesterification of a thioester at a C-terminus of a peptide by an N-terminal Cys residue of another peptide.
A label-free fluorimetric detection of biothiols based on the oxidase-like activity of Ag+ ions.
The abovementioned sequence is fast due to the proximity of the amino group to the thioester and the involvement of the five-membered intramolecular transition state Fig. Review Article Fig. Upon excitation at nm, FRET dyad 42 emitted at nm from the rhodamine acceptor. Treatment of 42 with Cys produced spirocyclic rhodamine 42a incapable of FRET, and resulted in the fluorescence of the energy donor at nm; a similar sensing behavior was also observed for Hcy. The measured rate constants kobs were 0.
Almost no changes in the emission were observed in the presence GSH, which generated thioester 42b without further intramolecular rearrangement. Probe 42 was used to detect aminothiols in newborn calf and human serum samples and for the ratiometric fluorescent imaging of Cys in living HepG2 cells. Ratiometric fluorescent probe 43 exploited a hybrid NIR fluorophore of coumarin and benzopyrylium Fig.
A fused phenyl thioester group served as the recognition group, thanks to its rapid NCL reaction than alkyl thioesters. The fluorescence band at nm decreased with the concomitant growth of a new emission band at nm coumarin emission. GSH only caused the thiol—thioester Chem. Chem Soc Rev exchange, resulting in no change in fluorescence. Only very limited spectral changes were observed in the presence of other biologically relevant species amino acids, metal ions.
Yang et al. In addition, the nitrothiophenol group served both as the leaving group and as a fluorescence quencher by the PET process. Upon the addition of biothiols to 44, the weak fluorescence at nm rhodol emission was turned on. The probe was applied for the selective sensing of GSH and Cys in human breast cancer cells and reduced human serum. Dual-channel imaging of intracellular Cys and GSH was also demonstrated. Fluorescence of the amines was used for the detection of Cys by replacing the Cl substituent of 45 with nitrophenol or nitrothiophenol moieties 46, 47 , which suppressed the fluorescence of the BODIPY core through PET.
The addition of Cys to 46 yielded N through fast substitution rearrangement due to the favored 5-membered transition state and turned on the fluorescence. Chem Soc Rev Fig. Probe 47 was designed by attaching an electron-withdrawing imidazolium group to BODIPY core to increase the reactivity of nucleophilic aromatic substitution. The same chemistry was exploited by Zhao and Ahn group to design BODIPY based probes and 49,80 which were used for the selective bioimaging of biothiols in living cells and in living zebrafish Fig.
Two commercially available dyes, 7-nitro-2,1,3-benzoxadiazole NBD-Cl and heptamethine cyanine IR , themselves are useful for the selective detection of biothiols. NBD and coumarin, were covalently linked. Treatment of 50 with GSH released the coumarin fluorophore and generated S-bound NBD, which was essentially nonfluorescent, and no fluorescence enhancement at nm was observed. Fluorescent probe 52 containing a tunable lipophilic cation unit as the biomarker for mitochondria was used for the detection of mitochondrial GSH in living cells.
Probe 52 may be applicable as a mitochondrial GSH tracker superior to the commercially available MitoTracker rhodamine Fig. Zhang and co-worker reported a series of fluorescent probes based on nitro-naphthalimide derivatives Fig. In their early work, probe 53 was employed to detect Cys, but only at 50 1C and in DMF because of the low reactivity of 53, which precluded its use in biological systems. Chemical structures of 51 and It allowed the discrimination between Cys and GSH from different emissions at nm and nm. Fluorescent probe 56 with three potential reaction sites was exploited to selectively detect Cys and Hcy by further elaboration of this strategy Fig.
The thiol derived from Cys underwent rapid addition to the Michael receptor site 2 yielding highly fluorescent amino-coumarin Cys. The same reaction was negligibly slow in the Hcy derivative, presumably due to higher energy of an 8-member cyclic transition state, relative to the 7-member analog in the Cys case.
GSH also could replace the chlorine of 56 yielding M3, which subsequently underwent macrocyclization at reaction site 3 to form the final membered ring product GSH. Supramolecular interactions such as hydrogen bonding and electrostatic interactions are commonly used strategies to construct fluorescent probes. The selective detection of GSH was demonstrated with bisspiropyran 57 Fig.
Probe 57 permeated cell membranes well, enabling its use for GSH imaging in living cells. However, probes that rely only on reversible non-covalent interactions tend to display moderate selectivity over other competitive species and are susceptible to interference in complex biological environments.
Supramolecular interactions e. As discussed in Section 2. By combining Michael addition with electrostatic attraction, fluorescent probe 58 displayed high selectivity for Cys Fig. Upon addition of Cys, a fold fluorescence enhancement at nm was observed. Chem Soc Rev resulted in only and 9-fold enhancements, respectively. The kinetic analysis revealed that 58 reacted with Cys fold faster than with Hcy, and fold faster than with GSH.
The selectivity was attributed to electrostatic interactions between the cationic probe and negatively charged Cys. The isomers of 58, 58 0 were synthesized for evaluating the spatial influence of electrostatic interactions. The results indicated that the proper spatial electrostatic interaction played a significant role in the selectivity and kinetics during the sensing process. Probe 58 displayed satisfactory cell permeability and was employed to imaging Cys in living cells.
Fluorescent probe 59 showed high selectivity for Cys over Hcy using similar chemistry to 58 Fig. In contrast, the addition of Hcy and GSH increased the ratiometric value only 1. Cys was expected to undergo a NCL native chemical ligation reaction with 59 to produce the intermediate M1, followed by an intramolecular cyclization reaction to generate cyclic Cys. However, the reaction of 59 and Hcy resulted in thioester Hcy, which was attributed to an electrostatic attraction between cationic hemicyanine and negatively charged Cys that inhibited the subsequent S,N-acyl shift see Fig.
Probe 59 was utilized to selectively image Cys in living cells. Probes 60—62 were designed for the selective detection of GSH or Cys assisted by intramolecular hydrogen bonding. Probe 60 bears a nitroolefin moiety as the reaction site for thiols, and a N-phenylazacrown receptor as an additional supramolecular interaction site for recognizing GSH Fig. Review Article typical pH value for tumor tissues buffered solution of 60, the addition of GSH generated a stronger emission signal than the addition of the equivalent amount of Cys or Hcy, presumably resulting from the interaction of the ammonium group in GSH with the N-phenylazacrown moiety interfering with fluorescencequenching PET.
The short linker connecting the thioether and ammonium groups in Cys or Hcy adduct precludes the formation of an equivalent supramolecular complex with these thiols. Probe 60 allowed visualization of GSH distribution in the cytosol of human breast adenocarcinoma cells.
Probe 61 was weakly fluorescent due to the rapid CQN isomerization in the excited state. A similar strategy was utilized in probe The emission intensity of 62 at nm increased linearly with Cys concentrations in the 0—30 mM range, with the detection limit of 0. Probe 62 was used to selectively image Cys in human renal cells. This review covers the highly topical problem of the design and properties of fluorescent probes for selective discrimination among cysteine Cys , homocysteine Hcy and glutathione GSH.
Most such probes have an electrophilic site carrying a labile substituent susceptible to nucleophilic displacement by the thiolate of these analytes or electron-poor CQC and CQO bonds susceptible to Michael addition at the S—H bond. Differentiation between GSH and Cys or Hcy exploits the propensity of aminothioethers derived from the last two thiols to undergo subsequent intramolecular rearrangement by the adjacent NH2 group to yield amino derivatives.
We classified these probes by their reaction types, including cyclization with aldehydes, conjugate addition—cyclization with acrylates, native chemical ligation and aromatic substitution-rearrangement. Integration of covalent bond-forming reactions with supramolecular interactions improved selectivity and increased reactivity of certain probes. Despite impressive progress, challenges in designing probes for discrimination among biothiols in complex biological environments abound. Review Article understanding how to exploit the high substrate selectivity of enzymes to yield highly specific probes or the limitations that the use of enzymes may impose.
Further investigation should not be restricted in the detection of Cys, Hcy and GSH, but also the thiol-containing species in specific organelles of cell or organs.