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Aggregation-induced emission of a novel conjugated phosphonium salt and its application in mitochondrial imaging
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 471–473
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Short Communication
Aggregation-induced emission of a novel conjugated phosphonium salt and its application in mitochondrial imaging Wen-dan Chen a,b,1, Da-wei Zhang c,1, Wei-tao Gong a,b,⇑, Yuan Lin a,b, Gui-ling Ning a,b,⇑ a
School of Chemical Engineering, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, PR China State Key Laboratory of Fine Chemicals, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, PR China c Department of Orthopeadic Surgery, Xijing Hospital, The Fourth Military Medical University, 127 West Changle Road, Xi’an, Shanxi 710032, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TTP exhibited interesting AIEE
properties in aqueous solution. TTP can self-assemble to fluorescent
nanoparticles in aqueous solution. TTP can be used as fluorescence
probes to image mitochondria in cells.
a r t i c l e
i n f o
Article history: Received 15 September 2012 Received in revised form 4 March 2013 Accepted 16 March 2013 Available online 1 April 2013
a b s t r a c t A new conjugated phosphonium salt TPP was synthesized readily from phosphine-triggered ring-opening of 2,4,5-triphenylpyrylium salt. In aqueous solution, it exhibited interesting AIEE properties and selfassembled into fluorescent nanoparticles, which can be used as a fluorescence probe to image mitochondria in cells. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Conjugated phosphonium salt Aggregation-induced emission Mitochondrial imaging
Introduction Mitochondria, generally called ‘‘power house’’ in cell, play a key role in energy production through oxidative phosphorylation and lipid oxidation. Since the early 1990s, it has been common sense that mitochondrial dysfunction contributes to a large variety of human diseases, such as Friedreich’s ataxia, Parkinson’s disease, diabetes, and Huntington’s disease [1]. Therefore, mitochondrial fluorescent probes have drawn increasing attention due to their capability of monitoring mitochondrial morphology and organelle functioning so as to provide useful information for prophase
⇑ Corresponding authors. Address: School of Chemical Engineering, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, PR China. Tel./fax: +86 411 84986067. E-mail address: [email protected] (W.-t. Gong). 1 Contributed equally to this work. 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.03.088
diagnosis [2]. However, a thorny problem for present mitochondrial fluorescent probes is that they tend to aggregate when dispersed in aqueous media and the aggregation usually quenches the fluorescence [3]. Recently, organic dyes exhibiting strong fluorescent emission in aggregate state have attracted increasing attention, especially in the field of bio-imaging, which resolves primarily the problem of fluorescence quenching resulting from aggregation [4]. On the other hand, phosphonium salts have recently been found potential applicantions in anticancer agents, drug and probe carriers [5] due to their easy permeation through cells and accumulation in the mitochondria. However, these molecules require artful design and lengthy-step synthesis [6]. Especially for those conjugated analogues, some extreme reaction conditions and transition metals as catalysts are required [7]. In this paper, a new conjugated phosphonium salt was designed and synthesized readily from phosphine-triggered ring-opening of
472
W.-d. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 471–473
Ph
Ph Ph
Ph
O
Ph
PMe3 Ph
O
PMe3
TPP Scheme 1. Synthetic route of phosphonium salt TPP.
2,4,5-triphenylpyrylium ring. It exhibited novel aggregation-induced emission enhancement (AIEE) in aqueous solution and can be used as new a fluorescence probe to image mitochondria in living cells. Experimental section The synthetic route of conjugated phosphonium salt TPP is shown in Scheme 1. 2,4,5-triphenylpyrylium cation was prepared according to our previously reported work [8]. Reaction of this pyrylium cation with trimethyl phospine under room temperature gave the corresponding conjugated phosphonium salt TPP in good yield. The structure of TPP was further confirmed by NMR, ESI-MS and elemental analysis [9]. Results and discussion
400
(a)
5.0×10-4
600
300 200
3.9×10 -6
100 0
energy in CH3CN (10 lM) and accordingly make the fluorescence weak through the formation of the nonradiation decay channel [10]. As a contrast, the internal rotations are limited owing to the space limitation in aggregation state. In this case, the nonradiation decay channel is suppressed, and excited state molecules could only go back to ground state through the radiative decay, resulting in the impressive enhancement of the fluorescence. The fluorescence lifetime changed from 6.6617 ns in dilute solution to 11.7364 ns in aggregation state, further confirming this hypothesis and indicating the aggregation truly occurred [11]. Furthermore, the solid particles of TPP were prepared according to a simple precipitation method described in the literature [12]. Herein, water was used as a nonsolvent of TPP in acetonitrile. When 25% fractions of water was added, compound 1 (20 lM) in mixture solution started to aggregate into nanosized particles. These nanoparticles’ suspension was very transparent without precipitates and was observed to be stable even after 4 weeks. The morphology of these nanoparticles was observed by field emission scanning electron microscopy (FE-SEM), and it shows that most of the spheres have a diameter ranging from 100 to 150 nm (Fig. 2). It should be pointed out that the AIEE properties and selfassembly behavior endow phosphonium salt TPP with the potential to be used as a fluorescent probe to image mitochondria in cells. As a proof-of-concept, the confocal fiuorescence cellular imaging experiment of TPP for mitochondria in HEK-293 cells was performed. After incubation for 0.5 h at 37 °C with 5 lM of phosphonium salt TPP, HEK-293 cells were substantially marked by strongly blue intracellular fluorescence. Microscopic observation of the cells demonstrated that TPP yielded high intracellular selectivity toward mitochondria by comparison with the commer-
Fluorescence Intensity
Fluorescence Intensity
Owing to its structural characteristics, TPP was readily soluble in polar solvents, such as CH3CN, DMF, DMSO, methanol and ethanol, but only partially soluble in water. Furthermore, it exhibited interesting photophysical behaviors in solutions. When TPP was dissolved in CH3CN to dilute solution (e.g. 10 lM), almost no fluorescence was observed. Increasing the concentration of the solution dramatically enhanced the fluorescent intensity correspondingly, which indicates the aggregation-induced emission (AIE) phenomenon (Fig. 1a). This AIE feature was further confirmed by the fluorescence of TPP in water/CH3CN mixture (the concentration was kept at 10 lM). Upon addition of water, a poor solvent for TPP, into their molecularly dispersed dilute solutions, impressive AIEE phenomenon (see Fig. 1b and inset) was observed. This might be due to the rapid formation of a super-saturated solution and the subsequent consecutive growth of particles. In fact, when water fraction was over 40%, the solution became a little turbid and the quantum yield (Uf) increases from 0.1% to 5%. It is suggested that the aromatic groups around the single bond consume excited state
Fig. 2. FE-SEM images of nanoparticles of TPP obtained from nanoparticles’ suspension.
(b)
Water vol. fractions
500
40%
400 300 200
0%
100 0
400
450 500 Wavelength/nm
550
600
400
450
500
550
600
650
Wavelength/nm
Fig. 1. (a) Fluorescence emission of TPP at different concentrations in CH3CN. (b) The intensity plots of emissions of 0–40% of H2O/CH3CN mixed solutions containing TPP. Insets show the photographs obtained with the corresponding mixed solutions (left: pure CH3CN; right: 40%. of H2O/CH3CN mixed solutions).
W.-d. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 471–473
473
Fig. 3. Confocal fluorescence images of live HEK-293 cells incubated with Mito Tracker Deep Red and phosphonium salt TPP (5 lM). Images shown that HEK-293 cells treated with TPP (left), Mitotracker Deep Red (middle) and their merged image (right). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
cial MitoTracker Red (Fig. 3). Although the fluorescence is amongst UV–Vis region, to the best of our knowledge, this is the first fluorescent labeling agent to mitochondria in living cells with AIEE feature. Additionally, the phsophonium salts are highly conjugated and have a carbonyl functional group, which allow their easy modification via further transformations, such as Michael addition and Schiff-base reaction. Therefore, it is expected that the modified phosphonium salts might provide a way to design a new platform for drug delivery systems. Relevant experiments are still undergoing in our lab. Conclusion In summary, a novel phosphonium salt TPP with high conjugation and functional group was synthesized from phosphine-triggered ring-opening of 2,4,5-triphenylpyrylium ring. In aqueous solution, it exhibited interesting AIEE properties and self-assembled into fluorescent nanoparticles, which can be used as a fluorescence probe to image mitochondria in cells. Acknowledgments This research has been supported by the Specialized Research Fund for the Doctoral Program of Higher Education (201000 41120020) and the Fundamental Research Funds for the Central Universities (DUT11LK13). References [1] (a) A. Sharma, G.M. Soliman, N. Al-Hajaj, R. Sharma, D. Maysinger, A. Kakkar, Biomacromolecules 13 (2012) 239–252; [b] P.I. Moreira, X. Zhu, X. Wang, H.G. Lee, A. Nunomura, R.B. Petersen, G. Perry, M.A. Smith, Biochim. Biophys. Acta 1802 (2010) 212–220; (c) Y. Yamada, H. Harashima, Adv. Drug Delivery Rev. 60 (2008) 1439–1462. [2] (a) S.C. Dodani, S.C. Leary, P.A. Cobine, D.R. Winge, C.J. Chang, J. Am. Chem. Soc. 133 (2011) 8606–8616; (b) Y. Koide, Y. Urano, S. Kenmoku, H. Kojima, T. Nagano, J. Am. Chem. Soc. 129 (2007) 10324–10325; (c) C.S. Lim, G. Masanta, H.J. Kim, J.H. Han, H.M. Kim, B.R. Cho, J. Am. Chem. Soc. 133 (2011) 11132–11135.
[3] (a) L. Yuan, W. Lin, Y. Yang, H. Chen, J. Am. Chem. Soc. 134 (2012) 1200–1211; (b) Z. Yang, M. She, B. Yin, J. Cui, Y. Zhang, W. Sun, J. Li, Z. Shi, J. Org. Chem. 77 (2012) 1143–1147. [4] (a) Y. Hong, J.W.Y. Lam, B.Z. Tang, Chem. Commun. 29 (2009) 4332–4353; (b) Z. Zhao, S. Chen, X. Shen, F. Mahtab, Y. Yu, P. Lu, J.W.Y. Lam, H.S. Kwok, B.Z. Tang, Chem. Commun. 46 (2010) 686–688; (c) M. Wang, G. Zhang, D. Zhang, D. Zhu, B.Z. Tang, J. Mater. Chem. 20 (2010) 1858–1867; (d) J.-P. Xu, Y. Fang, Z.-G. Song, J. Mei, L. Jia, A.J. Qin, J.Z. Sun, J. Ji, B.Z. Tang, Analyst 136 (2011) 2315–2321. [5] (a) C.M. Porteous, A. Logan, C. Evans, E.C. Ledgerwood, D.K. Menon, F. Aigbirhio, R.A.J. Smith, M.P. Murphy, Biochim. Biophys. Acta 1800 (2010) 1009–1017; (b) M.P. Murphy, R.A.J. Smith, Ann. Rev. Pharmacol. Toxicol. 47 (2007) 629– 656; (c) M.F. Ross, G.F. Kelso, F.H. Blaikie, A.M. James, H.M. Cochemé, A. Filipovska, T.D. Ros, T.R. Hurd, R.A.J. Smith, M.P. Murphy, Biochem. (Mosc.) 70 (2005) 222– 230; (d) M.P. Murphy, Trends Biotechnol. 15 (1997) 326–330. [6] (a) M.L. Jauslin, T. Meier, R.A.J. Smith, FASEB J. 17 (2003) 1972–1974; (b) A. Dhanasekaran, S. Kotamraju, S.V. Kalivendi, T. Matsunaga, T. Shang, A. Keszler, J. Joseph, B. Kalyanaraman, J. Biol. Chem. 279 (2004) 37575–37587; (c) J. Asin-Cayuela, A.R. Manas, A.M. James, R.A. Smith, M.P. Murphy, FEBS Lett. 571 (2004) 9–16. [7] M. David, B.C. Andre, J. Org. Chem. 73 (2008) 590–593. [8] W.T. Gong, G.L. Ning, X.C. Li, L. Wang, Y. Lin, J. Org. Chem. 70 (2005) 5768– 5770. [9] Synthesis of TPP: A solution of 2,4,5-triphenylpyrylium perchlorate (0.3 g, 0.735 mmol) in 20 mL of CH3CN was treated dropwise with 0.056 g PMe3 (0.750 mmol) in 0.5 mL of toluene. The reaction mixture was stirred at room temperature for 1 h under Ar atmosphere. When TLC showed complete consumption of the starting pyrylium salt, the solvent was removed in vacuo to yield an organic solid. Recrystalization from mixed solvents of CH2Cl2 and hexane gave product TPP (0.310 g, 87%) as light yellow crystals. 1H NMR (acetonitrile-d3, 400 MHz) d: 8.07–7.38(m, 16H), 6.64(d, 1H, 2JH-P = 16.4 Hz), 1.73(d, 9H, 2 J HP = 14.4 Hz). 13C NMR (acetonitrile-d3, 100 MHz) d: 10.2, 108.1, 124.4, 127.4, 128.1, 128.7, 129.1, 129.7, 131.1, 131.5, 133.9, 136.7, 137.3, 137.5, 137.8, 149.3, 162.2, 189.2. 31P NMR (acetonitrile-d3, 100 MHz) d: 15.4. MS (API-ES) m/z: 385.1(M+). Anal. Calcd. for (C26H26OP+) ðClO4 Þ: C, 64.46; H, 5.37; P, 6.40. Found: C, 64.53; H, 5.34; P, 6.36. [10] (a) K.-S. Wong, H. Wang, G. Lanzani, Chem. Phys. Lett. 288 (1998) 59–64; (b) J.-W. Barr, T.-W. Bell, V.-J. Catalano, J.-I. Cline, J.-D. Philips, R.-J. Procupez, J. Phys. Chem. A 109 (2005) 11650–11654. [11] Y.-H. Jiang, Y.-C. Wang, B. Wang, J.-B. Yang, N.-N. He, S.-X. Qian, J.-L. Hua, Chem. – Asia J. 6 (2011) 157–165. [12] (a) M. Kumar, S.J. George, Nanoscale 3 (2011) 2130–2133; (b) Y.-Y. Sun, J.-H. Liao, J.-M. Fang, P.-T. Chou, C.-H. Shen, C.-W. Hsu, L.-C. Chen, Org. Lett. 8 (2006) 3713–3716; (c) H.-H. Lin, Y.-C. Chan, J.-W. Chen, C.-C. Chang, J. Mater. Chem. 21 (2011) 3170–3177.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Short Communication
Aggregation-induced emission of a novel conjugated phosphonium salt and its application in mitochondrial imaging Wen-dan Chen a,b,1, Da-wei Zhang c,1, Wei-tao Gong a,b,⇑, Yuan Lin a,b, Gui-ling Ning a,b,⇑ a
School of Chemical Engineering, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, PR China State Key Laboratory of Fine Chemicals, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, PR China c Department of Orthopeadic Surgery, Xijing Hospital, The Fourth Military Medical University, 127 West Changle Road, Xi’an, Shanxi 710032, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
TTP exhibited interesting AIEE
properties in aqueous solution. TTP can self-assemble to fluorescent
nanoparticles in aqueous solution. TTP can be used as fluorescence
probes to image mitochondria in cells.
a r t i c l e
i n f o
Article history: Received 15 September 2012 Received in revised form 4 March 2013 Accepted 16 March 2013 Available online 1 April 2013
a b s t r a c t A new conjugated phosphonium salt TPP was synthesized readily from phosphine-triggered ring-opening of 2,4,5-triphenylpyrylium salt. In aqueous solution, it exhibited interesting AIEE properties and selfassembled into fluorescent nanoparticles, which can be used as a fluorescence probe to image mitochondria in cells. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Conjugated phosphonium salt Aggregation-induced emission Mitochondrial imaging
Introduction Mitochondria, generally called ‘‘power house’’ in cell, play a key role in energy production through oxidative phosphorylation and lipid oxidation. Since the early 1990s, it has been common sense that mitochondrial dysfunction contributes to a large variety of human diseases, such as Friedreich’s ataxia, Parkinson’s disease, diabetes, and Huntington’s disease [1]. Therefore, mitochondrial fluorescent probes have drawn increasing attention due to their capability of monitoring mitochondrial morphology and organelle functioning so as to provide useful information for prophase
⇑ Corresponding authors. Address: School of Chemical Engineering, Dalian University of Technology, No. 2 Ling Gong Road, High Technology Zone, Dalian 116024, PR China. Tel./fax: +86 411 84986067. E-mail address: [email protected] (W.-t. Gong). 1 Contributed equally to this work. 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.03.088
diagnosis [2]. However, a thorny problem for present mitochondrial fluorescent probes is that they tend to aggregate when dispersed in aqueous media and the aggregation usually quenches the fluorescence [3]. Recently, organic dyes exhibiting strong fluorescent emission in aggregate state have attracted increasing attention, especially in the field of bio-imaging, which resolves primarily the problem of fluorescence quenching resulting from aggregation [4]. On the other hand, phosphonium salts have recently been found potential applicantions in anticancer agents, drug and probe carriers [5] due to their easy permeation through cells and accumulation in the mitochondria. However, these molecules require artful design and lengthy-step synthesis [6]. Especially for those conjugated analogues, some extreme reaction conditions and transition metals as catalysts are required [7]. In this paper, a new conjugated phosphonium salt was designed and synthesized readily from phosphine-triggered ring-opening of
472
W.-d. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 471–473
Ph
Ph Ph
Ph
O
Ph
PMe3 Ph
O
PMe3
TPP Scheme 1. Synthetic route of phosphonium salt TPP.
2,4,5-triphenylpyrylium ring. It exhibited novel aggregation-induced emission enhancement (AIEE) in aqueous solution and can be used as new a fluorescence probe to image mitochondria in living cells. Experimental section The synthetic route of conjugated phosphonium salt TPP is shown in Scheme 1. 2,4,5-triphenylpyrylium cation was prepared according to our previously reported work [8]. Reaction of this pyrylium cation with trimethyl phospine under room temperature gave the corresponding conjugated phosphonium salt TPP in good yield. The structure of TPP was further confirmed by NMR, ESI-MS and elemental analysis [9]. Results and discussion
400
(a)
5.0×10-4
600
300 200
3.9×10 -6
100 0
energy in CH3CN (10 lM) and accordingly make the fluorescence weak through the formation of the nonradiation decay channel [10]. As a contrast, the internal rotations are limited owing to the space limitation in aggregation state. In this case, the nonradiation decay channel is suppressed, and excited state molecules could only go back to ground state through the radiative decay, resulting in the impressive enhancement of the fluorescence. The fluorescence lifetime changed from 6.6617 ns in dilute solution to 11.7364 ns in aggregation state, further confirming this hypothesis and indicating the aggregation truly occurred [11]. Furthermore, the solid particles of TPP were prepared according to a simple precipitation method described in the literature [12]. Herein, water was used as a nonsolvent of TPP in acetonitrile. When 25% fractions of water was added, compound 1 (20 lM) in mixture solution started to aggregate into nanosized particles. These nanoparticles’ suspension was very transparent without precipitates and was observed to be stable even after 4 weeks. The morphology of these nanoparticles was observed by field emission scanning electron microscopy (FE-SEM), and it shows that most of the spheres have a diameter ranging from 100 to 150 nm (Fig. 2). It should be pointed out that the AIEE properties and selfassembly behavior endow phosphonium salt TPP with the potential to be used as a fluorescent probe to image mitochondria in cells. As a proof-of-concept, the confocal fiuorescence cellular imaging experiment of TPP for mitochondria in HEK-293 cells was performed. After incubation for 0.5 h at 37 °C with 5 lM of phosphonium salt TPP, HEK-293 cells were substantially marked by strongly blue intracellular fluorescence. Microscopic observation of the cells demonstrated that TPP yielded high intracellular selectivity toward mitochondria by comparison with the commer-
Fluorescence Intensity
Fluorescence Intensity
Owing to its structural characteristics, TPP was readily soluble in polar solvents, such as CH3CN, DMF, DMSO, methanol and ethanol, but only partially soluble in water. Furthermore, it exhibited interesting photophysical behaviors in solutions. When TPP was dissolved in CH3CN to dilute solution (e.g. 10 lM), almost no fluorescence was observed. Increasing the concentration of the solution dramatically enhanced the fluorescent intensity correspondingly, which indicates the aggregation-induced emission (AIE) phenomenon (Fig. 1a). This AIE feature was further confirmed by the fluorescence of TPP in water/CH3CN mixture (the concentration was kept at 10 lM). Upon addition of water, a poor solvent for TPP, into their molecularly dispersed dilute solutions, impressive AIEE phenomenon (see Fig. 1b and inset) was observed. This might be due to the rapid formation of a super-saturated solution and the subsequent consecutive growth of particles. In fact, when water fraction was over 40%, the solution became a little turbid and the quantum yield (Uf) increases from 0.1% to 5%. It is suggested that the aromatic groups around the single bond consume excited state
Fig. 2. FE-SEM images of nanoparticles of TPP obtained from nanoparticles’ suspension.
(b)
Water vol. fractions
500
40%
400 300 200
0%
100 0
400
450 500 Wavelength/nm
550
600
400
450
500
550
600
650
Wavelength/nm
Fig. 1. (a) Fluorescence emission of TPP at different concentrations in CH3CN. (b) The intensity plots of emissions of 0–40% of H2O/CH3CN mixed solutions containing TPP. Insets show the photographs obtained with the corresponding mixed solutions (left: pure CH3CN; right: 40%. of H2O/CH3CN mixed solutions).
W.-d. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 471–473
473
Fig. 3. Confocal fluorescence images of live HEK-293 cells incubated with Mito Tracker Deep Red and phosphonium salt TPP (5 lM). Images shown that HEK-293 cells treated with TPP (left), Mitotracker Deep Red (middle) and their merged image (right). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
cial MitoTracker Red (Fig. 3). Although the fluorescence is amongst UV–Vis region, to the best of our knowledge, this is the first fluorescent labeling agent to mitochondria in living cells with AIEE feature. Additionally, the phsophonium salts are highly conjugated and have a carbonyl functional group, which allow their easy modification via further transformations, such as Michael addition and Schiff-base reaction. Therefore, it is expected that the modified phosphonium salts might provide a way to design a new platform for drug delivery systems. Relevant experiments are still undergoing in our lab. Conclusion In summary, a novel phosphonium salt TPP with high conjugation and functional group was synthesized from phosphine-triggered ring-opening of 2,4,5-triphenylpyrylium ring. In aqueous solution, it exhibited interesting AIEE properties and self-assembled into fluorescent nanoparticles, which can be used as a fluorescence probe to image mitochondria in cells. Acknowledgments This research has been supported by the Specialized Research Fund for the Doctoral Program of Higher Education (201000 41120020) and the Fundamental Research Funds for the Central Universities (DUT11LK13). References [1] (a) A. Sharma, G.M. Soliman, N. Al-Hajaj, R. Sharma, D. Maysinger, A. Kakkar, Biomacromolecules 13 (2012) 239–252; [b] P.I. Moreira, X. Zhu, X. Wang, H.G. Lee, A. Nunomura, R.B. Petersen, G. Perry, M.A. Smith, Biochim. Biophys. Acta 1802 (2010) 212–220; (c) Y. Yamada, H. Harashima, Adv. Drug Delivery Rev. 60 (2008) 1439–1462. [2] (a) S.C. Dodani, S.C. Leary, P.A. Cobine, D.R. Winge, C.J. Chang, J. Am. Chem. Soc. 133 (2011) 8606–8616; (b) Y. Koide, Y. Urano, S. Kenmoku, H. Kojima, T. Nagano, J. Am. Chem. Soc. 129 (2007) 10324–10325; (c) C.S. Lim, G. Masanta, H.J. Kim, J.H. Han, H.M. Kim, B.R. Cho, J. Am. Chem. Soc. 133 (2011) 11132–11135.
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