Microtubule formation using two-component gel system

Article abstract of DOI:10.1021/ja0676197

We report a two-component gel system in which an amphiphile group and aromatic cores act as organic gelators by the process of self-assembly. The shape and size of the aromatic cores have a pronounced effect on the gel properties and on structures such as fibers and microtubules. Copyright

Full text of DOI:10.1021/ja0676197

Published on Web 01/10/2007
Microtubule Formation Using Two-Component Gel System
Ho Yong Lee, Seong Ryong Nam, and Jong-In Hong*
Department of Chemistry, College of Natural Sciences, Seoul National UniVersity, Seoul 151-747, Korea
Received October 25, 2006; E-mail: jihong@snu.ac.kr
Self-assembly is a powerful tool for constructing various types
of nano- and microstructures. Uniform self-assembled tubular
structures can potentially be applied in the controlled generation
Scheme 1
1
of novel nano- or microscopic materials and devices. Moreover,
hollow tubular structures are abundant in nature and perform diverse
2
biological functions. Therefore, considerable attention is currently
being focused on the fabrication of uniform tubular structures. Since
3
the discovery of carbon nanotubes, there have been reports on a
variety of inorganic4 and organic tubules.5 Among them, self-
assembled organic tubular structures have a lot of advantages such
as a low-temperature fabrication process and easy modification.
Recently, there has been enormous interest in the development
of a low molecular weight gelator (LMWG) for use in organic
6
solvents. The gelation process is accompanied by one-dimensional
growth of the gelator in various types of fibers, strands, tapes, and
tubes. There are an increasing number of reports about nanotubular
such as toluene and xylene which disrupt the further stacking of
the complex between AC and AG. However, the complexes between
AG2 and the aromatic cores showed a high gelation behavior in
nonaromatic hydrocarbon solvents such as cyclohexane and decalin.
The complexes between AG1 and the aromatic cores showed a poor
gelation ability only in decalin. This indicates that an amide alkyl
chain plays an important role in the gelation ability of a two-
component system. Gelation was not observed when only one of
the two components was present. Thus, the complex between the
AC and AG groups is essential for gel formation.
7
structures being developed in the gelation process. However,
8
uniform microtubular structures are still rare. In contrast with single
LMWG gel systems, new gelation systems using two components
9
have been developed. In these systems, two components that serve
as gelators are held together by noncovalent interactions. The most
important feature of the two-component systems is the ease with
which the properties and structures of the gel can be modulated by
changing the molar ratio of the two components or by changing
one of the two components. Excellent examples have been reported
The gelation ability of the two-component system in cyclohexane
was investigated under various concentrations (Figure 1). All the
gels were thermoreversible. The temperature (gel-to-sol temperature,
by Smith et al.9
a-g
They used a dendritic building block based on
an amino acid in combination with an aliphatic diamine and
examined the properties and structures of the resulting gel. In this
study, we report a two-component gel system in which an
amphiphile group and aromatic cores act as organic gelators by
the process of self-assembly. The shape and size of the aromatic
cores have a pronounced effect on the gel properties and on
structures such as fibers and microtubules.
T
gel) at which the gel vanishes was investigated under various
concentrations. The Tgel value increased with the molar concentra-
tion. As reported elsewhere, Tgel reaches a plateau region above a
certain concentration. All mixtures have a similar threshold
concentration of approximately 25 mM, although they have different
shapes and sizes. Another interesting feature is that Tgel changes
according to the type of the aromatic core. In all the investigated
concentrations, Tgel increased with the aromatic size of the AC
group. The Tgel value of the complex between AC1 and AG2 is 42
A two-component gel system is composed of aromatic cores
(
AC1, AC2) and amphiphile groups (AG1, AG2) (See Scheme 1).
We have selected phenyl- and naphthyl-based aromatic groups
AC1, AC2). These two aromatic molecules are different in shape
°
5
C at 10 mM, while that of the complex between AC2 and AG2 is
5 °C. The average change in Tgel is approximately 15 °C. A
(
and size and are expected to show different packing patterns and
strength when combined with the amphiphile groups. The aromatic
core has amine groups, which are able to interact with the benzoic
acid of the amphiphile group using hydrogen bonds. The amphiphile
group is composed of four parts: a carboxylic acid, amide group,
long alkyl chain (dodecyl), and phenyl ring. Benzoic acid forms a
hydrogen bond with the amine group of the aromatic core. The
amide groups act as additional hydrogen-bonding sites to enable
further assembly of the complex between AC and AG. AG1 has
one alkyl group, while AG2 has two alkyl groups.
The gelation behavior of the complexes between AC and AG
were tested in various organic solvents. Gelation did not occur in
polar protic and aprotic solvents because the hydrogen-bonded
complexes between AC and AG might be disturbed in the polar
solvents. Furthermore, gelation did not occur in aromatic solvents
Figure 1. Effect of concentration on the gel-sol transition temperature of
the gel in cyclohexane. AC and AG were mixed in a ratio of 1:2 (b, AC1
+ AG2; 9, AC2 + AG2).
1040
9
J. AM. CHEM. SOC. 2007, 129, 1040-1041
10.1021/ja0676197 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
have smaller external diameters and wall thicknesses than those in
cyclohexane. This implies that the solvent affects the microscopic
structure of the gelators.
In conclusion, we have developed a new gelation system for
organic solvents by the self-assembly of aromatic amines and
amphiphile groups. The thermal and morphological properties
dramatically changed according to the solvent and the size and shape
of the aromatic groups. The microtubular structure formed during
gel formation is unique and reproducible. We expect that our results
can be applied to the preparation of various types of microstructures
by using different aromatic cores and amphiphile groups.
Figure 2. Gel images of the AC2 and AG2 mixture in an inversed vial
(
left). The image was captured by a digital camera ([AC2 + 2AG2] ) 20
Acknowledgment. We thank the MOCIE (Grant No. 10022945)
for financial support. Partial support from the Seoul R&BD is also
acknowledged. H.Y.L. and S.R.N. are grateful to the Ministry of
Education for a BK 21 fellowship.
mM). SEM image of dried gel (right). (bar ) 100 µm.) AC2 and AG2
were mixed in a ratio of 1:2 in cyclohexane.
Supporting Information Available: Experimental details, ad-
ditional SEM images, and photomicroscope images. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(
1) (a) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew.Chem., Int.
Ed. 2003, 42, 980-999. (b) Reches, M.; Gazit, E. Nature 2003, 300, 625-
627. (c) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.;
Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228-232.
2) (a) Eisenberg, B. Acc. Chem. Res. 1998, 31, 117-123. (b) Sigler, P. B.;
Xu, Z.; Rye, H. S.; Burston, S. G.; Fenton, W. A.; Horwich, A. L. Annu.
ReV. Biochem. 1998, 67, 581-608. (c) Horwich, A. L.; Weber-Ban, E.
U.; Finley, D. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 11033-11040. (d)
Zwickl, P.; Voges, D.; Baumeister, W. Philos. Trans. R. Soc. London,
Ser. B 1999, 354, 1501-1511.
Figure 3. SEM images of the dried AC2 and AG2 mixture in a ratio of
1
:2 in decalin (left, bar ) 50 µm; right, bar ) 5 µm).
(
dramatic change in Tgel was observed by the addition of one
aromatic ring. Therefore, we can deduce that the π-π interactions
between the aromatic cores may play an important role in stabilizing
the gel.
The microstructures of the two-component gel aggregates were
significantly changed by the replacement of the AC groups. The
scanning electron microscopy (SEM) image of the mixture of AC1
and AG2 in cyclohexane and decalin showed fiberlike structures.10
A 1:2 mixture of AC1 and AG2 results in a clear gel.
However, in the case of the mixture of AC2 and AG2 in
cyclohexane, fibrous structures in the gel medium, which were
revealed to be microtubules, were observed with the naked eye
(3) (a) Iijima, S. Nature 1991, 354, 56-58. (b) Ajayan, P. M. Chem. ReV.
999, 99, 1787-1800.
(
1
4) (a) Suenaga, K.; Colliex, C.; Demoncy, N.; Loiseau, A.; Pascard, H.;
Willaime, F. Science 1997, 278, 653-655. (b) Chopra, N. G.; Luyken,
R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A.
Science 1995, 269, 966-967. (c) Stephan, O.; Ajayan, P. M.; Colliex,
C.; Redlich, P.; Lambert, J. M.; Bernier, P.; Lefin, P. Science 1994, 266,
1
683-1685. (d) Remskar, M.; Mrzel, A.; Skraba, Z.; Jesih, A.; Ceh, M.;
Dem sˇ ar, J.; Stadelmann, P.; Le’vy, F.; Mihailovic, D. Science 2001, 292,
479-481. (e) Hacohen, Y. R.; Grunbaum, E.; Tenne, R.; Sloan, J.;
Hutchison, J. L. Nature 1998, 395, 336-337.
(
5) (a) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem.,
Int. Ed. 2001, 40, 988-1011. (b) Kim, Y.; Mayer, M. F.; Zimmerman, S.
C. Angew. Chem., Int. Ed. 2003, 42, 1121-1126. (c) Vauthey, S.; Santoso,
S.; Gong, H.; Watson, N.; Zhang, S. Proc. Natl. Acad. Sci. U.S.A. 2002,
(Figure 2, left). The SEM image of a dried gel shows two distinct
microstructures (fibrous and tubular structures). The fibrous
structures contribute to the gelation of cyclohexane and tubular
structures pass through them. Microtubules can be separated by
filtration after vigorously stirring the diluted gel. In most cases,
the microtubular structures were collapsed or were deprived of their
tubular function without a solvent. However, the separated micro-
tubules retained their hollow tubular structure without the solvent
and were stable in air. The photomicroscope and SEM images
clearly show hollow tubular structures with uniform external
diameters of 30-40 µm and wall thicknesses of 2-3 µm. The
length of the longest tubule is 5 mm. We performed the same tests
more than twenty times and obtained the same reproducible results.
The reproducible formation of microtubules implies that the tubular
structures have a definite composition of AC2 and AG2. The flat
structure and hydrogen-bonding sites in the amphiphile group (AG2)
are likely to facilitate the π-π interactions of the aromatic cores.
We could also observe microtubules in the gel of AC2 and AG2
in delcalin, which were smaller than those in the cyclohexane gel.
The SEM images clearly show a hollow space (Figure 3).
Microtubules have external diameters of 5-10 µm, lengths of 20-
99, 5355-5360.
(6) Reviews: (a) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237-
1
247. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000,
3
9, 2263-2267. (c) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-
3160. (d) Shimizu, T. Macromol. Rapid Commun. 2002, 23, 311-331.
(e) Gronwald, O.; Shinkai, S. Chem.sEur. J. 2001, 7, 4329-4334.
(7) (a) John, G.; Masuda, M.; Okada, Y.; Yase, K.; Shimizu, T. AdV. Mater.
2
001, 13, 715-718. (b) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J.
M. J. Org. Chem. 1999, 64, 412-426. (c) Fuhrhop, J.-H.; Spiroski, D.;
Boettcher, C. J. Am. Chem. Soc. 1993, 115, 1600-1601. (d) Masuda,
M.; Shimizu, T. Langmuir 2004, 20, 5969-5977.
(8) Self-assembled microtubes: (a) Wong, G. C. L.; Tang, J. X.; Lin, A.; Li,
Y.; Janmey, P. A.; Safinya, C. R. Science 2000, 288, 2035-2039. (b)
Yan, D.; Zhou, Y.; Hou, J. Science 2004, 303, 65-67. (c) Shimizu, T.;
Kosigo, M.; Masuda, M. Nature 1996, 383, 487-488. (d) Shimizu, T.;
Kosigo, M.; Masuda, M. J. Am. Chem. Soc. 1997, 119, 6209-6210. (e)
Douliez, J.-P.; Gaillard, C.; Navailles, L.; Nallet, F. Langmuir 2006, 22,
2942-2945.
(
9) (a) Smith, D. K. Chem. Commun. 2006, 34-44. (b) Patridge, K. S.; Smith,
D. K.; Dykes, G. M.; McGrail, T. P. Chem. Commun. 2001, 319-320.
(
c) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M.; Wright,
A. C. J. Am. Chem. Soc. 2003, 125, 9010-9011. (d) Hardy, J. G.; Hirst,
A. R.; Smith, D. K.; Brennan, C.; Ashworth, I. Chem. Commun. 2005,
3
85-387. (e) Hirst, A. R.; Smith, D. K.; Feiters, M. C.; Geurts, H. P. M.
Chem. Eur. J. 2004, 10, 5901-5910. (f) Hirst, A. R.; Smith, D. K. Chem.s
Eur. J. 2005, 11, 5496-5508. (g) Hirst, A. R.; Smith, D. K.; Harrington,
J. P. Chem.sEur. J. 2005, 11, 6552-6559. (h) Yagai, S.; Nakajima, T.;
Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. J. Am. Chem.
Soc. 2005, 127, 11134-11139. (i) Oda, R.; Huc, I.; Candau, S. J. Angew.
Chem., Int. Ed. 1998, 37, 2689-2691. (j) Zemb, T.; Dubois, M.; Dem e´ ,
B.; Gulik-Krzywicki, T. Science 1999, 283, 816-819.
70 µm, and wall thicknesses of 0.5-1 µm. The microtubules formed
in decalin are faceted, while those formed in cyclohexane show a
round shape. In the case of the delcalin gel, we failed to separate
the microtubules in the gel medium. The microtubules in decalin
(
10) See the Supporting Information.
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