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Cite This: Chem. Mater. 2018, 30, 1467−1471
Phenolic Pyrogallol Fluorogen for Red Fluorescence Development in a PAS Domain Protein Ju-Kang Kim,†,§ Haesung A. Lee,‡,§ Haeshin Lee,*,‡ and Hyun Jung Chung*,†,⊥ †
Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, 291 University Road, Daejeon 34141, Korea ‡ Department of Chemistry, Korea Advanced Institute of Science and Technology, 291 University Road, Daejeon 34141, Korea ⊥ Department of Biological Sciences, Korea Advanced Institute of Science and Technology, 291 University Road, Daejeon 34141, Korea S Supporting Information *
P
tunning. Considering the biochemical difficulties in developing red fluorescence, this study demonstrates the unique role of gallol (trihydoxyphenol) derivatives in redshifting of fluorescence emission. We also demonstrated fluorescent color switching for multiple cycles by sequentially adding and removing these fluorogens in E. coli expressing Y-FAST. As shown in Scheme 1, Y-FAST fluorogens were spectrally tuned by modifying the fluorophore chemical structures. The
yrogallol (C6H3(OH)3) is a ubiquitous chemical moiety found in the plant kingdom.1 Examples include tannic acid,2 a well-known polyphenol found in plants such as chestnuts, oaks, and grapes; epigallocatechin gallate (EGCG)3 from green/black/white teas; and gallic acid,4 which is found in strawberries and bananas. These chemicals are antioxidants that effectively absorb reactive oxygen species in biological systems, providing anticancer benefits.5−7 In addition to traditional antioxidant studies, pyrogallol-containing derivatives have recently been used for surface functionalization,8−10 biomedical hydrogels,11−13 plant-inspired adhesives,14−16 nanoparticles for drug delivery,17,18 and self-sealing/healing materials.19−24 Along with the adhesive properties, assembly of colloidal polyphenol particles used for structural color,25,26 and diverse other structures27,28 also has been reported. In-plane hydrogen bonds, metal-hydroxyl coordination, and benzene-mediated hydrophobic/π−π/π-cation interactions are mechanisms found in water-resistant adhesives and coatings.29−32 In addition to interfacial adhesions and coatings, phenolates are key functional groups in protein biochemistry, particularly related to color development. Photoactive yellow protein (PYP) contains a phenolic chromophore called p-coumaric acid covalently linked to Cys69 by a thioester bond.33 Subsequently, a 3,4-dihydroxyphenolic chromophore was used to study PYP spectral tuning,34,35 resulting in a red-shift in absorption from 446 to 488 nm. Green fluorescent protein (GFP) also contains a tyrosine-derived phenolic fluorophore, with an emission wavelength of 508 nm. A catechol (3,4dihydroxyphenol)-containing fluorogen was recently used to tune emission spectra (λem = 511 to 531 nm) and aluminum ion selective detection.36 Until now, no protein spectral tuning studies using 2,3,4-trihydroxylphenolic derivatives have been reported. Interestingly, red fluorescence was developed by physically binding a gallic aldehyde−rhodanine conjugate to a PAS domain protein, Y-FAST. Y-FAST is the outcome of extensive protein engineering of PYP, which contains a phenolic chromophore, p-coumaric acid. Subsequently, hydroxylated or methoxy phenol derivatives were used for spectral tuning of PYP.34,35 Similarly, the original fluorogen of the engineered Y-FAST was also a phenolic compound called (Z)5-(4-hydroxybenzylidene)-2-thioxo-1,3-thiazolidin-4-one (HBR). Therefore, we made a hypothesis using gallolcontaining compounds as new phenolic fluorogen candidates which can bind to Y-FAST with an effect of fluorescent © 2018 American Chemical Society
Scheme 1. Overall Scheme for the Synthetic Step of the Spectrally Tuned Fluorogens
details of synthetic procedures are described in the Supporting Information. Similar to the synthetic route between rhodanine and benzaldehyde (known as HBR, [5-(4-hydroxybenzalidin)rhodanine]),37 we prepared fluorophore derivatives by reacting other benzaldehyde derivatives with rhodanine. Moreover, we hypothesized that these newly synthesized fluorophores would have different fluorescent properties depending on the chemical structure of the benzaldehyde derivatives. It is widely known that fluorescence spectra are red-shifted by lowering the potential energy when the π-electron resonance structure of the fluorophore is elongated.38,39 In addition, the spectra can be red-shifted by adding electron-delocalizing functional groups to the fluorophore.40 Thus, we selected 4-hydroxy-3-methoxycinnamaldehyde, which has a longer electron resonance doubleReceived: December 11, 2017 Revised: February 21, 2018 Published: February 27, 2018 1467
DOI: 10.1021/acs.chemmater.7b05136 Chem. Mater. 2018, 30, 1467−1471
Communication
Chemistry of Materials
cells. As shown in Figure 2a, E. coli expressing Y-FAST proteins associated with an exogenous fluorogen emitted the corre-
bond chain than benzaldehyde, and 3,4,5-trihydroxybenzaldehyde, which has further extended electron delocalization paths on the aromatic ring than benzaldehyde, as color-tuning reagents for fluorogen synthesis. As shown in Figure 1, we synthesized two spectrally tuned fluorogens, gallic aldehyde-rhodanine (GA-Rho) conjugate (i.e.,
Figure 1. Characterization of emission spectra for Y-FAST proteins: 527 nm (green color, HBR), 564 nm (yellow color, mCA-Rho), and 604 nm (red color, GA-Rho).
5-(3,4,5-trihydroxybenzylidine)−rhodanine) and 3-methylcatechol aldehyde−rhodanine (mCA-Rho) conjugate (i.e., 5-(4hydroxy-3-methyoxycinamylidine)−rhodanine). As the original fluorophore, HBR, contains a single hydroxyl group in Y-FAST, we varied the number of hydroxyl groups to dihydroxy and trihydroxyphenyl fluorogens. The dihydroxyphenyl-rhodanine conjugate failed to emit fluorescence; thus, the methyldihydroxyphenyl-containing fluorogen was used in this study (Scheme 1). The two fluorogens, GA-Rho and mCA-Rho, were added to the Y-FAST protein for spectral characterization. As expected, both new fluorogens showed red-shifted fluorescence emission wavelengths compared to that of the original fluorogen: maximum emission at 527 nm for HBR (single −OH) to 564 nm for mCA-Rho (two −OH with 3-Omethyl) to 604 nm for GA-Rho (three −OH). mCA-Rho fluoresced yellow and GA-Rho fluoresced red when the fluorogens were bound to the Y-FAST protein. The spectral shift of the mCA-Rho fluorogen to the yellow region is due to the additional double bond between the rhodanine and benzene ring structure, which forms an elongated electron resonance structure, resulting in π-electron delocalization and shifting the fluorescence to a longer wavelength. The GA-Rho fluorogen showed a further redshift to the red region due to the presence of three hydroxyl groups, which provide an elongated electron resonance structure to the aromatic ring. Many studies have shown that pyrogallol can be easily deprotonated and oxidized to become a quinone under physiological conditions.41 The detailed photochemical properties of the fluorogen/YFAST complexes are listed in Table 1. We also demonstrated that the spectrally tuned fluorogens could turn on Y-FAST fluorescence emission in living bacterial
Figure 2. (a) Living E. coli staining by phenolic fluorogens by adding HBR (top), mCA-Rho (middle), and GA-Rho (bottom). Scale bar = 10 μm. (b) Fluorescence turning off with PBS washing. Scale bar = 50 μm. (c) Zoom-in cell fluorescence images highlighted for the emphasized squares in panel b. Scale bar = 10 μm.
sponding colors of the fluorogen. This demonstrates that the three fluorogens HBR, mCA-Rho, and GA-Rho easily penetrated the bacterial cell wall, providing a great advantage for live imaging applications. Furthermore, Figure 2b shows that bacterial fluorescence activation can be easily turned off by a simple washing step. The fluorescence rapidly disappeared within 2 s after adding fresh PBS buffer. Subsequently, the fluorescence was turned on again by adding each fluorogen. Using this reversible turn-on and off phenomena, we hypothesized that facile fluorescent color switches in living E. coli are possible regardless of the starting emission. This is interesting because fluorescence switching typically requires two or more proteins to be coexpressed within a cell. When YFAST is present, the colors of yellow, green, or red fluorescence can be developed in any direction by adding the corresponding fluorophores. Furthermore, each color can be switched from yellow to green to red or from green to yellow to red or from red to yellow to green (Figure 3a). In practice, fluorescent color in living E. coli cells was circularly switched by sequentially adding and washing fluorogens. In addition to the single-cycle turnover of the three fluorescent colors, we further attempted to achieve multicycle color switching in live bacteria. As shown in Figure 3c, multiple colors in E. coli were switched for 5 consecutive cycles (i.e., green (G) to yellow (Y) to red (R) to G to Y to R to G and so forth). Each bacterial fluorescence signal was stably retained during the multicycle experiments. Furthermore, mammalian cell labelings by Y-FAST were also feasible, exhibiting three emitting fluorescent colors. A549 cells showed green (HBR), yellow (mCA-Rho), and red (GA-Rho) similarly to E. coli labeling (Figure 3d). This study clearly
Table 1. Photochemical Properties of Fluorogen upon Binding with Y-FAST
Y-FAST + HBR Y-FAST + mCA-Rho Y-FAST + GA-Rho
λabs, nm
λem, nm
ε, M−1·cm−1
Φ, %
467 493 444
527 564 604
44,000 40,200 41,900
9 2.7 3.4 1468
DOI: 10.1021/acs.chemmater.7b05136 Chem. Mater. 2018, 30, 1467−1471
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Chemistry of Materials
Figure 3. (a) Schematic description of fluorescence emission switches. (b) Directional fluorescence switching from green (HBR) to red (GA-Rho) to yellow (mCA-Rho) (top), from yellow (mCA-Rho) to red (GA-Rho) to green (HBR) (middle), and from red (GA-Rho) to yellow (mCA-Rho) to green (HBR) (bottom). Cfg indicates “centrifugation”. (c) Five continuous cycles of fluorescent emission shifts. (d) Fluorescence images of A549 cells expressing Y-FAST: HBR (left), mCA-Rho (middle), and GA-Rho (right). Scale bar = 20 μm.
demonstrates potentially useful fluorescent labeling and switching in both prokaryotes and eukaryotes. In summary, we introduced a color switchable reporter system with a novel approach for generating spectrally tuned fluorogens for the Y-FAST system. Compared to the original fluorogen, HBR, the newly synthesized fluorogens, mCA-Rho and GA-Rho, show red-shifted fluorescence due to modification
of the chemical structure resulting in electron delocalization. The fluorogens reversibly bound to Y-FAST both in the form of a free protein molecule and when expressed in bacteria. Sequential addition of each fluorogen to E. coli expressing YFAST resulted in spectral shifts to the corresponding colors over multiple cycles. Considering the photostability of the YFAST protein upon binding with the original or spectrally 1469
DOI: 10.1021/acs.chemmater.7b05136 Chem. Mater. 2018, 30, 1467−1471
Communication
Chemistry of Materials tuned fluorogen, this system can be widely applied as a reporter system for rapid, temporal control of fluorescence in multiple colors. This system provides significant advantages over the conventional procedures such as immunocytochemistry or DNA/RNA hybridization, which are only applicable to cells that are fixed, permeabilized, or lysed. Furthermore, there are applications of this switchable multicolor fluorescence system that can be adjusted to the mammalian cells. We suggest their use for monitoring and intracellular trafficking of target proteins, as a reporter tag for molecular selection of potential therapeutic cells, and as an assay reporter for genetic selection. We anticipate that our multiple color switching fluorescence reporter system will enable fine-tuning and dynamic characterization of molecular mechanisms within cells as well as systemic responses in living organisms.
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(6) Nie, S.; Xie, M.; Zhou, P.; Cao, S. In vitro antioxidative and anticancer activities of tea glycoprotein in green tea. Eur. Food Res. Technol. 2007, 224, 437−442. (7) Landis-Piwowar, K. R.; Huo, C.; Chen, D.; Milacic, V.; Shi, G.; Chan, T. H.; Dou, Q. P. A novel prodrug of the green tea polyphenol (−)-epigallocatechin-3-gallate as a potential anticancer agent. Cancer Res. 2007, 67, 4303−4310. (8) Sileika, T. S.; Barrett, D. G.; Zhang, R.; Lau, K. H. A.; Messersmith, P. B. Colorless multifunctional coatings inspired by polyphenols found in tea, chocolate, and wine. Angew. Chem., Int. Ed. 2013, 52, 10766−10770. (9) Hong, S.; Yeom, J.; Song, I. T.; Kang, S. M.; Lee, H.; Lee, H. Pyrogallol 2-Aminoethane: A Plant Flavonoid-Inspired Molecule for Material-Independent Surface Chemistry. Adv. Mater. Interfaces 2014, 1, 1400113. (10) Ejima, H.; Richardson, J. J.; Liang, K.; Best, J. P.; van Koeverden, M. P.; Such, G. K.; Cui, J.; Caruso, F. One-step assembly of coordination complexes for versatile film and particle engineering. Science 2013, 341, 154−157. (11) Shin, M.; Lee, H. Gallol-Rich Hyaluronic Acid Hydrogels: Shear-Thinning, Protein Accumulation against Concentration Gradients, and Degradation-Resistant Properties. Chem. Mater. 2017, 29, 8211−8220. (12) Bae, K. H.; Tan, S.; Yamashita, A.; Ang, W. X.; Gao, S. J.; Wang, S.; Chung, J. E.; Kurisawa, M. Hyaluronic acid-green tea catechin micellar nanocomplexes: Fail-safe cisplatin nanomedicine for the treatment of ovarian cancer without off-target toxicity. Biomaterials 2017, 148, 41−53. (13) Shin, M.; Ryu, J. H.; Park, J. P.; Kim, K.; Yang, J. W.; Lee, H. DNA/tannic acid hybrid gel exhibiting biodegradability, extensibility, tissue adhesiveness, and hemostatic ability. Adv. Funct. Mater. 2015, 25, 1270−1278. (14) Xiong, K.; Qi, P.; Yang, Y.; Li, X.; Qiu, H.; Li, X.; Shen, R.; Tu, Q.; Yang, Z.; Huang, N. Facile immobilization of vascular endothelial growth factor on a tannic acid-functionalized plasma-polymerized allylamine coating rich in quinone groups. RSC Adv. 2016, 6, 17188− 17195. (15) Kim, K.; Shin, M.; Koh, M. − Y.; Ryu, J. H.; Lee, M. S.; Hong, S.; Lee, H. TAPE: A medical adhesive inspired by a ubiquitous compound in plants. Adv. Funct. Mater. 2015, 25, 2402−2410. (16) Shin, M.; Kim, K.; Shim, W.; Yang, J. W.; Lee, H. Tannic acid as a degradable mucoadhesive compound. ACS Biomater. Sci. Eng. 2016, 2, 687−696. (17) Chung, J. E.; Tan, S.; Gao, S. J.; Yongvongsoontorn, N.; Kim, S. H.; Lee, J. H.; Choi, H. S.; Yano, H.; Zhuo, L.; Kurisawa, M.; Ying, J. Y. Self-assembled micellar nanocomplexes comprising green tea catechin derivatives and protein drugs for cancer therapy. Nat. Nanotechnol. 2014, 9, 907−912. (18) Riedinger, A.; Leal, M. P.; Deka, S. R.; George, C.; Franchini, I. R.; Falqui, A.; Cingolani, R.; Pellegrino, T. Nanohybrids” based on pHresponsive hydrogels and inorganic nanoparticles for drug delivery and sensor applications. Nano Lett. 2011, 11, 3136−3141. (19) Wang, Y.; Park, J. P.; Hong, S. H.; Lee, H. Biologically Inspired Materials Exhibiting Repeatable Regeneration with Self-Sealing Capabilities without External Stimuli or Catalysts. Adv. Mater. 2016, 28, 9961−9968. (20) Krogsgaard, M.; Andersen, A.; Birkedal, H. Gels and threads: mussel-inspired one-pot route to advanced responsive materials. Chem. Commun. 2014, 50, 13278−13281. (21) Rahim, Md. A.; Kempe, K.; Müllner, M.; Ejima, H.; Ju, Y.; van Koeverden, M. P.; Suma, T.; Braunger, J. A.; Leeming, M. G.; Abrahams, B. F.; Caruso, F. Surface-Confined Amorphous Film from Metal-Coordinated Simple Phenolic Ligands. Chem. Mater. 2015, 27, 5825−5832. (22) Maerten, C.; Lopez, L.; Lupattelli, P.; Rydzek, G.; Pronkin, S.; Schaaf, P.; Jierry, L.; Boulmedais, F. Electrotriggering Confiend Selfassembly of Metal-Polyphenol Nanocoatings Using a Morphogenic Approach. Chem. Mater. 2017, 29, 9668−9679.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05136. Experimental section (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*
[email protected] (H.J.C.). *
[email protected] (H.L.). ORCID
Ju-Kang Kim: 0000-0002-9945-9107 Haesung A. Lee: 0000-0002-1035-8036 Haeshin Lee: 0000-0003-3961-9727 Hyun Jung Chung: 0000-0001-5055-902X Author Contributions §
These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The study was supported by grants from the Ministry of Health and Welfare of Korea (HI14C2270 and HI15C1946 to H.J.C.; 1631060 to H.L.), the National R&D Program for Cancer Control, the National Research Foundation of Korea (2015R1C1A1A02036647 to H.J.C.; 2017R1A2A1A05001047, 2017M3C1B7014223 to H.L.), and the Ministry of Trade, Industry and Energy of Korea (N0002463).
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REFERENCES
(1) Romeo, J. T. Phytochemicals in Human Health Protection, Nutrition, and Plant Defense; Kluwer Academic/Plenum Publishers: Norwell, MA, 1999. (2) Pizzi, A. In Monomers, Polymers, and Composites from Renewable Resources; Belgacem, M. N., Gandini, A., Eds.; Elsevier: Amsterdam, 2008; Chapter 8, pp 179−200. (3) Khan, N.; Afaq, F.; Saleem, M.; Ahmad, N.; Mukhtar, H. Targeting Multiple Signaling Pathways by Green Tea Polyphenol (−)-Epigallocatechin-3-Gallate. Cancer Res. 2006, 66, 2500−2505. (4) Goldstein, J. L.; Swain, T. Changes in tannins in ripening fruits. Phytochemistry 1963, 2, 371−383. (5) Tachibana, H.; Koga, K.; Fujimura, Y.; Yamada, K. A receptor for green tea polyphenol EGCG. Nat. Struct. Mol. Biol. 2004, 11, 380− 381. 1470
DOI: 10.1021/acs.chemmater.7b05136 Chem. Mater. 2018, 30, 1467−1471
Communication
Chemistry of Materials (23) Rahim, Md. A.; Bjornmalm, M.; Suma, T.; Faria, M.; Ju, Y.; Kempe, K.; Müllner, M.; Ejima, H.; Stickland, A. D.; Caruso, F. MetalPhenolic Supramolecular Gelation. Angew. Chem., Int. Ed. 2016, 55, 13803−13807. (24) Rahim, Md. A.; Björnmalm, M.; Bertleff-Zieschang, N.; Besford, Q.; Mettu, S.; Suma, T.; Faria, M.; Caruso, F. Rust-Mediated Continuous Assembly of Metal-Phenolic Networks. Adv. Mater. 2017, 29, 1606717. (25) Kawamura, A.; Kohri, M.; Morimoto, G.; Nannichi, Y.; Taniguchi, T.; Kishikawa, K. Full-Color Biomimetic Photonic Materials with Iridescent and Non-iridescent Structural Colors. Sci. Rep. 2016, 6, 33984. (26) Kohri, M.; Nannichi, Y.; Taniguchi, T.; Kishikawa, K. Biominetic non-iridescent structural color materils from polydopamine black particles that mimic melanin granules. J. Mater. Chem. C 2015, 3, 720− 724. (27) Guo, J.; Ping, Y.; Ejima, H.; Alt, K.; Meissner, M.; Richardson, J. J.; Yan, Y.; Peter, K.; von Elverfeldt, D.; Hagemeyer, C. E.; Caruso, F. Engineering Multifunctional Capsules through the Assembly of MetalPhenolic Networks. Angew. Chem., Int. Ed. 2014, 53, 5546−5551. (28) Guo, J.; Tardy, B. L.; Christofferson, A. J.; Dai, Y.; Richardson, J. J.; Zhu, W.; Hu, M.; Ju, Y.; Cui, J.; Dagastine, R. R.; Yarovsky, I.; Caruso, F. Modular assembly of superstructures from polyphenolfunctionalized building blocks. Nat. Nanotechnol. 2016, 11, 1105− 1112. (29) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-molecule mechanics of mussel adhesion. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 12999−13003. (30) Maier, G. P.; Rapp, M. V.; Waite, J. H.; Israelachvili, J. N.; Butler, A. Adaptive synergy between catechol and lysine promotes wet adhesion by surface salt displacement. Science 2015, 349, 628−632. (31) Gebbie, M. A.; Wei, W.; Schrader, A. M.; Cristiani, T. R.; Dobbs, H. A.; Idso, M.; Chmelka, B. F.; Waite, J. H.; Israelachvili, J. N. Tuning underwater adhesion with cation−π interactions. Nat. Chem. 2017, 9, 473−479. (32) Lim, C.; Huang, J.; Kim, S.; Lee, H.; Zeng, H.; Hwang, D. S. Nanomechanics of Poly(catecholamine) Coatings in Aqueous Solution. Angew. Chem., Int. Ed. 2016, 55, 3342−3346. (33) Hoff, W. D.; Dux, P.; Hard, K.; Devreese, B.; NugterenRoodzant, I. M.; Crielaard, W.; Boelens, R.; Kaptein, R.; Van beeumen, J.; Hellingwerf, K. J. Thiol Ester-Linked p-coumaric Acid as a New Photoactive Prosthetic Group in a Protein with Rhodopsin-Like Photochemistry. Biochemistry 1994, 33, 13959−13962. (34) Kroon, A. R.; Hoff, W. D.; Fennema, H. P. M.; Gijzen, J.; Koomen, G.; Verhoeven, J. W.; Crielaard, W.; Hellingwerf, K. J. Spectral tuning, fluorescence, and photoactivity in hybrids of photoactive yellow protein, reconstituted with native or modified chromophores. J. Biol. Chem. 1996, 271, 31949−31956. (35) Mataga, N.; Chosrowjan, H.; Taniguchi, S.; Hamada, N.; Tokunaga, F.; Imamoto, Y.; Kataoka, M. Ultrafast photoreactions in protein nanospaces as revealed by fs fluorescence dynamics measurements on photoactive yellow protein and related systems. Phys. Chem. Chem. Phys. 2003, 5, 2454−2460. (36) Wu, D.; Deepankumar, K.; Ping, Y.; Tang, G.; Saravananprabhu, N.; Miserez, A.; Fang, W. Catechol-modified green fluorescent protein as a specific biosensor for Al ions. Sens. Actuators, B 2017, 251, 326− 333. (37) Plamont, M.-A.; Billon-Denis, E.; Maurin, S.; Gauron, C.; Pimenta, F. M.; Specht, C. G.; Shi, J.; Querard, J.; Pan, B.; Rossignol, J.; Moncoq, K.; Morellet, N.; Volovitch, M.; Lescop, E.; Chen, Y.; Triller, A.; Vriz, S.; Le Saux, T.; Jullien, L.; Gautier, A. Small fluorescence-activating and absorption-shifting tag for tunable protein imaging in vivo. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 497−502. (38) Gross, L. A.; Baird, G. S.; Hoffman, R. C.; Baldridge, K. K.; Tsien, R. Y. The structure of the chromophore within DsRed, a red fluorescent protein from coral. Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 11990−11995. (39) Li, C.; Plamont, M.-A.; Aujard, I.; Le Saux, T.; Jullien, L.; Gautier, A. Gautier, Design and characterization of red fluorogenic
push−pull chromophores holding great potential for bioimaging and biosensing. Org. Biomol. Chem. 2016, 14, 9253−9261. (40) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt, J. C.; Kannan, R.; Yuan, L.; He, G. S.; Prasad, P. N. Highly active twophoton dyes: design, synthesis, and characterization toward application. Chem. Mater. 1998, 10, 1863−1874. (41) Critchlow, A.; Haslam, E.; Haworth, R. D.; Tinker, P. B.; Waldron, N. M. The oxidation of some pyrogallol and purpurogallin derivatives. Tetrahedron 1967, 23, 2829−2847.
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