College of Engineering start-up fund and the University of
Michigan Rackham grant from the Horace H. Rackham School
of Graduate Studies. J. K. and K. L. also acknowledge the Ilju
Foundation for the Ilju scholarship.
Notes and references
1 For a comprehensive review of water-soluble conjugated polymers, see:
B. Liu and G. C. Bazan, Chem. Mater., 2004, 16, 4467; M. Pinto and
K. S. Schanze, Synthesis, 2002, 9, 1293. For a comprehensive review of
conjugated polymer-based chemical sensors, see: D. T. McQuade,
A. E. Pullen and T. M. Swager, Chem. Rev., 2000, 100, 2537.
2 J. H. Wosnick, C. M. Mello and T. M. Swager, J. Am. Chem. Soc.,
2005, 127, 3400; H. A. Ho, K. Dore´, M. Boissinot, M. G. Bergeron,
R. M. Tanguay, D. Boudreau and M. Leclerc, J. Am. Chem. Soc., 2005,
127, 12673.
3 B. Liu and G. C. Bazan, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 589;
K. P. R. Nilsson and O. Ingana¨s, Nat. Mater., 2003, 2, 419; S. Zhang
and T. M. Swager, J. Am. Chem. Soc., 2003, 125, 3420; H. A. Ho,
M. Boissinot, M. G. Bergeron, G. Corbeil, K. Dore´, D. Boudreau and
M. Leclerc, Angew. Chem., Int. Ed., 2002, 41, 1548–1551.
Scheme 2 Peptide–PPE coupling reaction.
4 (a) J. J. Lavigne, D. L. Broughton, J. N. Wilson, B. Erdogan and
U. H. F. Bunz, Macromolecules, 2003, 36, 7409; (b) C. Tan, E. Atas,
J. G. Mu¨ller, M. R. Pinto, V. D. Kleiman and K. S. Schanze, J. Am.
Chem. Soc., 2004, 126, 13685; (c) L. Chen, D. W. McBranch, H. Wang,
R. Helgeson, F. Wudl and D. G. Whitten, Proc. Natl. Acad. Sci. U. S.
A., 1999, 96, 12287.
5 A. Khan, S. Mu¨ller and S. Hecht, Chem. Commun., 2005, 584.
6 U. Lauter, W. H. Meyer, V. Enkelmann and G. Wegner, Macromol.
Chem. Phys., 1998, 199, 2129.
7 C. Tan, M. R. Pinto and K. S. Schanze, Chem. Commun., 2002, 446;
W. Chen, A. G. Joly, J. Malm, J. Bovin and S. Wang, J. Phys. Chem. B,
2003, 107, 6544.
8 P. Samor´ı, V. Francke, K. Mu¨llen and J. P. Rabe, Chem.–Eur. J., 1999,
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Fig. 3 (a) 1H NMR spectrum of pentatyrosine–PPE in DMSO (b) A
confocal image of pentatyrosine–PPE (scale bar: 20 mm).
9 S. A. Jenekhe, Adv. Mater., 1995, 7, 309; J. Kim and T. M. Swager,
Nature, 2001, 411, 1030; J. Kim, I. A. Levitsky, T. McQuade and
T. M. Swager, J. Am. Chem. Soc., 2002, 124, 7710.
confirmed by NMR. New aromatic proton peaks at 7.8–8.8 ppm,
corresponding to pentatyrosine, are shown in Fig. 3 (a). It was
confirmed that pentatyrosine units were coupled at both ends of
the PPE-R1–COOH by end-group analysis.14 Fig. 3 (b) shows a
confocal microscope image of photoluminescent 4-chloro-trityl
resin reacted with PPE-R1–COOH. The image was taken after
3 stringent rinses of the resin with methanol, DMF, water and
dichloromethane to remove any unreacted copolymers. The filtrate
of the washing step to remove unbound polymers hardly showed
any fluorescence, confirming that almost every polymer chain end
has a carboxyl group that had reacted with the PS resin. After
cleaving the pentatyrosine from the resin, the resulting peptide-
conjugated PPE does not have any carboxylic acid directly bound
to the conjugated backbone .15
10 Absolute quantum yield was measured with excitation at 365 nm in de-
ionized water using PTI QuantaMaster2 spectrofluorometers with an
integrating sphere. Fully dried PPE-R1 completely dissolves in pure
water with a solubility exceeding approximately 1 mg ml21. Until this
paper, the best quantum yield of PPE-based polyelectrolyte was 57%,
according to the results of Jiang et al: D.-L. Jiang, C.-K. Choi,
K. Honda, W.-S. Li, T. Yuzawa and T. Aida, J. Am. Chem. Soc., 2004,
126, 12084. While they introduced dendritic side chains into PPEs by a
complicated synthetic route in order to overcome aggregation, we
synthesized PPEs using a simple method. To our knowledge, ours is the
highest quantum yield ever reported of water-soluble conjugated
polymers prepared through a simple synthetic route.
11 There is another method to overcome the aggregation of conjugated
polyelectrolytes in water using a surfactant. See ref. 4a; L. Chen, S. Xu,
D. McBranch and D. Whitten, J. Am. Chem. Soc., 2000, 122, 9302;
H. D. Burrows, V. M. M. Lobo, J. Pina, M. L. Ramos, J. Seixas de
Melo, A. J. M. Valente, M. J. Tapia, S. Pradhan and U. Scherf,
Macromolecules, 2004, 37, 7425; M. J. Tapia, H. D. Burrows,
A. J. M. Valente, S. Pradhan, U. Scherf, V. M. M. Lobo, J. Pina and
J. Seixas de Melo, J. Phys. Chem. B, 2005, 109, 19108.
In conclusion, we have established a simple and practical
approach for the bioconjugation of a conjugated polyelectrolyte
and a pentatyrosine, a model biological molecule. We designed
and synthesized completely water-soluble and highly fluorescent
sulfonated PPE with bifurcated ethylene oxide side chains. End-
functionalized PPE, prepared by in situ chemical modification
during polymerization, was successfully attached to a model
peptide, pentatyrosine on a 4-chloro-trityl PS resin. This study
provides a design principle for the preparation of functionalized,
water-soluble, fluorescent, conjugated polymers for bioconjuga-
tion. Bio/synthetic hybrid conjugated polymers have a large
potential as molecular biosensors to detect biological analytes
quickly and selectively.
12 For comprehensive reading, see: G. Hermanson, Bioconjugate
Techniques, Academic Press, San Diego, CA, 1996.
13 K. Lee, T. Yucel, D. J. Pochan and J. Kim, manuscript in preparation.
14 The ratio of the integration values in the 1H NMR corresponding to the
phenyl rings in the PPE backbone and the tert-butyl group in
pentatyrosine was in accordance with the calculated value. It is assumed
that the polymer chains lay down on the large PS resin surface, such that
every carboxyl group of the polymer has reacted with an amine group.
For papers on NMR end-analysis, see: S. Jo, H. Shin and A. G. Mikos,
Biomacromolecules, 2001, 2, 255; X. Kong and S. A. Jenekhe,
Macromolecules, 2004, 37, 8180.
15 Due to the hydrophobic tyrosine unit, solubility of the PPE–
pentatyrosine bioconjugate in water significantly decreased. However,
the PPE–peptide is completely soluble in DMSO and other organic
solvents.
J. K. gratefully acknowledges support from the National
Science Foundation (BES 0428010), the University of Michigan
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Chem. Commun., 2006, 1983–1985 | 1985