Sol-gel polycondensation of tetraethoxysilane in a cholesterol-based organogel system results in chiral spiral silica

Article abstract of DOI:10.1002/(SICI)1521-3773(20000515)39:10<1862::AID-ANIE1862>3.0.CO;2-3

A right-handed helical motif is shown by the spiral silica shown in the picture. It is formed by sol-gel polycondensation of tetraethoxysilane in the gel phase supported by an azacrown-appended cholesterol gelator. This is rare example of 'chirality' bein

Full text of DOI:10.1002/(SICI)1521-3773(20000515)39:10<1862::AID-ANIE1862>3.0.CO;2-3

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[11] M. S. P. Sansom, Quart. Rev. Biophys. 1993, 26, 365 ± 321.
The increase of JA production in the Lima bean plants
prompted us to test ALA in tendril-coiling experiments.
Tendrils of Bryonia dioica respond to JA, MeJA, and 12-
OPDA with a coiling reaction comparable to the free coiling
reaction of mechanically stimulated tendrils.[19] Tendrils of
Bryonia dioica, Pisum sativum, and Lathyrus sp. were treated
with solutions of ALA (5 mm) through the transpiration
stream and, after 20 h, the degree of coiling was measured.
Alternatively, tendrils were subjected to the coiling assay
described in the literature.[20] All tested plant species exhib-
ited a rapid coiling response towards ALA. Surprisingly,
inhibition of the octadecanoid cascade with phenidone did not
hamper the coiling reaction, which indicates that signaling
systems independent of the lipid-based pathway, such as ion
fluxes, have to be considered as elements of mechanotrans-
duction.[20]
[12] Experimental details concerning plant cultivation, application of test
compounds, volatile compound collection, and mass spectrometric
analysis are described in: T. Koch, T. Krumm, V. Jung, J. Engelberth,
W. Boland, Plant Physiol. 1999, 121, 153 ± 162. Alamethicin, melittin,
and valinomycin were purchased from Sigma ± Aldrich. Bradykinin
and substance P were from Calbiochem. All test compounds were
applied at a concentration of 10 mgmLÀ1
.
[13] B. Beûler, S. Schmitgen, F. Kühnemann, R. Gäbler, W. Urban, Planta
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[14] J. Piel, R. Atzorn, R. Gäbler, F. Kühnemann, W. Boland, FEBS Lett.
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1602.
[16] J. Engelberth, T. Koch, N. Bachmann, J. Rechtenbach, W. Boland,
Plant Physiol. 1999, in press.
[17] C. Cucurou, J. P. Battioni, D. C. Thang, N. H. Nam, D. Mansuy,
Biochemistry 1991, 30, 8964 ± 8970.
[18] M. D. Rosenthal, B. S. Vishwanath, R. C. Franson, Biochim. Biophys.
Acta 1989, 1001, 1 ± 8 .
[19] E. W. Weiler, T. Albrecht, B. Groth, Z.-Q. Xia, M. Luxem, H. Liss, L.
Andert, P. Spengler, Phytochemistry 1993, 32, 591 ± 600.
[20] B. Klüsener, G. Boheim, H. Liss, J. Engelberth, E. W. Weiler, EMBO
J. 1995, 14, 2708± 2714.
[21] A. W. Bernheimer, B. Rudy, Biochim. Biophys. Acta 1986, 864, 123 ±
141.
Besides ALA, many other peptaibols and peptides endow-
ed with pore-forming or ion-transporting properties are
known. To establish whether pore formation within a
membrane or ion transport through a membrane is the
underlying principle for the induction of volatile compound
biosynthesis, other pore-forming peptaibols and some typical
ion transporters were tested. The results are compiled in
Table 1. The entire group of peptaibols induced the same
pattern of volatile compounds in Lima bean leaves, suggesting
a common mode of action. Although the basic peptide
melittin is known as a pore-forming compound,[21] it failed
to induce volatile biosynthesis in Lima bean leaves. A typical
[22] D. W. Urry, Top. Curr. Chem. 1985, 128, 175 ± 218.
[23] M. M. Klavdieva, Front. Neuroendocrinol. 1996, 17, 155 ± 179.
[24] A. Schaller, C. A. Ryan, Proc. Natl. Acad. Sci. USA 1994, 91, 11802 ±
11806.
[25] A. Berg, B. Schlegel, W. Ihn, U. Demuth, U. Gräfe, J. Antibiotics 1999,
52, 666 ± 669.
[26] D. Spiteller, V. Jung, W. Boland, unpublished results.
‡
ion transporter, K selective valinomycin[22] (9 mm), was also
found to be inactive. Other biologically active small peptides
which act through specific receptors, such as the undecapep-
tide ªsubstance Pº,[23] the nonapeptide bradykinin,[22] and
systemin, a signal peptide of tomato plants,[24] did not induce
volatile compound biosynthesis, supporting the pore-forming
capability of peptaibols as the essential property for the
elicitation process.
Channel-forming peptides, such as alamethicin, may be
used as valuable tools to unravel the early events of plant
defense under well defined conditions. First analyses of insect
salivary secretions have already demonstrated the presence of
pore-forming compounds,[26] suggesting that membrane de-
polarisation also has to be considered as an important element
of insect-induced plant defense.
Sol ± Gel Polycondensation of
Tetraethoxysilane in a Cholesterol-Based
Organogel System Results in Chiral Spiral
Silica
Jong Hwa Jung, Yoshiyuki Ono, and Seiji Shinkai*
Exploitation of new organic gelators that can gelate various
organic solvents has become an active area of research.[1±11]
These organogels are of particular interest because they are
different from polymer gels. Fibrous aggregates of low
molecular weight compounds formed by noncovalent inter-
actions are responsible for such gelation phenomena. Hence,
the xerogels exhibit various superstructures, reflecting the
monomeric structure of each gelator. This is why the study of
organogels is considered to be a new field of supramolecular
chemistry.[11]
Received: December 9, 1999 [Z14379]
[1] M. G. Hahn, Annu. Rev. Plant Pathol. 1996, 34, 387 ± 412.
Â
[2] P. W. Pare, J. H. Tumlinson, Plant Physiol. 1999, 121, 325 ± 331.
[3] T. Jabs, M. Tschöpe, C. Colling, K. Hahlbrock, D. Scheel, Proc. Natl.
Acad. Sci. USA 1997, 94, 4800 ± 4805.
[4] J. L. Dangl, K. D. Hauffe, S. Lipphardt, K. Hahlbrock, D. Scheel,
EMBO J. 1987, 6, 2551 ± 2556.
[5] Y. Matthieu, A. Kurkdjian, H. Xia, J. Guern, A. Koller, M. D. Spiro,
M. OꢁNeill, P. Albersheim, A. Darvill, Plant J. 1991, 1, 333 ± 343.
[6] B. Klüsener, E. W. Weiler, FEBS Lett. 1999, 459, 263 ± 266.
[7] D. S. Cafiso, Annu. Rev. Biophys. Biomol. Struct. 1994, 23, 141 ± 165.
[8] M. Ritzau, S. Heinze, K.-J. Dornberger, A. Berg, W. Fleck, B. Schlegel,
A. Härtl, U. Gräfe, J. Antibiot. 1997, 50, 722 ± 728.
Recently, it was found that certain cholesterol derivatives
can gelate even tetraethoxysilane (TEOS), which results in
[*] Prof. Dr. S. Shinkai, Dr. J. H. Jung, Y. Ono
Chemotransfiguration Project
Japan Science and Technology Corporation (JST)
2432 Aikawa, Kurume, Fukuoka 839-0861 (Japan)
Fax : (‡81)942-39-9012
[9] K.-J. Dornberger, W. Ihn, M. Ritzau, U. Gräfe, B. Schlegel, W. F.
Fleck, J. Antibiot. 1995, 48, 977 ± 989.
[10] R. C. Pandey, J. C. Meng, C. Cook, R. L. Rinehardt, J. Am. Chem. Soc.
1977, 99, 8469 ± 8483.
E-mail: seijitcm@mbox.nc.kyushu-u.ac.jp
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Table 1. The gelation ability[a] of 1 in organic solvents.
silica by sol ± gel polycondensation.[12, 13] Sol ± gel polyconden-
sation of gelated TEOS solutions affords novel silica with a
hollow fiber structure, because the organogel fibers act as a
template to create an inner tube in the TEOS polycondensa-
tion process.[12, 13] One may therefore consider that the unique
superstructures of organogel fibers can be transcribed in the
silica structure: Superstructures of organic aggregates tem-
porarily formed by noncovalent interactions can be perma-
nently fixed in inorganic silica. It was later proven that the
presence of the positive charge in the organogel fibers is
indispensable for transcribing the organogel superstructure
into the silica structure.[13] This prerequisite is rationalized in
terms of an electrostatic interaction between oligomeric
ªanionicº silica and ªcationicº organogel fibers during the
sol ± gel polycondensation process.
Solvent
Result[b]
Solvent
Result[b]
methanol
I
acetone
I
I
ethanol
I
acetonitrile
1-propanol
1-butanol
tert-butyl alcohol
isobutanol
1-octanol
dimethyl sulfoxide
1,2-diaminoethane
1,3-diaminopropane
G
G
G
G
G
G
PG
G
cyclohexane
PG
PG/R
G
S
S
methylcyclohexane
acetic anhydride
acetic acid
dichloromethane
[a] The concentration of the gelator was 5.0 wt%. [b] G ˆ stable gel
formed at room temperature; I ˆ insoluble; PG ˆ partially gelatinized;
R ˆ recrystallization; S ˆ solution.
To characterize the chirality of the organogel, the circular
dichroism (CD) spectrum of the organogel formed by 1 in the
presence and absence of AgNO3 or CsClO4 was measured
(Figure 1). Since the l values for q ˆ 0 (q ˆ ellipticity) in the
Organogel fibers formed from cholesterol-based gelators
frequently result in a helical structure.[3, 7] This fact tempted us
to transcribe the helical organogel fiber structure into the
inorganic silica. We have noticed, however, that the presence
of a cationic group tends to reduce the gelation ability and
that these systems rarely result in the formation of a helical
structure.[12] An idea to overcome this dilemma came to mind:
The positive charge can be readily incorporated by a metal ±
crown interaction. We therefore designed compound 1 (see
Scheme 1), which has a cholesterol skeleton as a chiral
aggregate-forming site and an azacrown moiety as a metal
binding site.[14] We found that 1 not only gelates various
organic solvents, but in the presence of metal cations also acts
as a template for sol ± gel polycondensation of TEOS to
produce the novel ªchiral spiral silicaº. To the best of our
knowledge, this is a very rare example of ªchiralityº being
induced in the inorganic silica by the organogel template
method.[15]
Compound 1 (m.p. 113.3 ± 115.28C) was synthesized ac-
cording to Scheme 1 and identified by IR and 1H NMR
spectroscopy, mass spectrometry, and elemental analysis. The
gelation ability was estimated for 17 different organic solvents
containing 1 (5.00 Â 10À2 molLÀ1).[16] As summarized in Ta-
ble 1, 1 can gelate 8out of the 17 solvents, and therefore acts
as a versatile gelator of organic solvents. The gelation ability
for alcohols in general is excellent.
Figure 1. CD spectra of 1 (5.00 Â 10À2 molLÀ1) in 1-butanol in the absence
and presence of CsClO4 (5.00 Â 10À2 molLÀ1
)
or AgNO3 (5.00 Â
10À2 molLÀ1; concentration in the gel phase). The concentration of 1
required to gelate the 1-butanol solution was so high that the CD spectra
could not be measured with conventional optical cells. They were obtained
by sandwiching the gel between two glass plates to make a thin layer.
Hence, the ellipticity q is given in arbitrary units.
CD spectra (365 and 358nm in the presence of CsClO 4
(3.15 Â 10À3 molLÀ1) and AgNO3 (3.15 Â 10À3 molLÀ1), re-
spectively) are very close to the lmax of the absorption
spectrum (360 nm), one can regard these bands as positive
exciton-coupling bands: Compound
1
forms aggregates and the dipoles in the
chromophoric azobenzene moieties are
orientated in a clockwise direction (i.e.,
with an R configuration). Although the
gel formed in the absence of a metal salt
was CD-active, the spectrum did not show
a clear exciton-coupling band.
The scanning electron microscopy
(SEM) pictures of the xerogels obtained
by a freeze-and-pump method from gel-
phase solutions of 1 in tert-butyl alcohol
below the sol ± gel phase-transition tem-
perature ([1] ˆ 5.00  10À2 molLÀ1) are
shown in Figure 2.[17] In the absence of a
metal salt, the major structures are film-
like lamella, some of which show a
pseudocylindrical structure (Figure 2A).
N
O
Br
N
COOH
2
3
cholesterol/DCC
CH2Cl2
N
O
Br
N
COO
N-monobenzyldiaza[18]crown-6
Na2CO3/nPrCN
O
O
O
O
N
N
N
O
N
COO
1
Scheme 1. Synthesis of 1. DCC ˆ dicyclohexyl carbodiimide.
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Figure 3. SEM pictures of the spiral silica obtained from 1 (5.00 Â
10À2 molLÀ1) in the presence of A) AgNO3 (5.00 Â 10À2 molLÀ1) and
B) CsClO4 (5.00 Â 10À2 molLÀ1) before calcination. A similar structure was
also observed after calcination, indicating that the organogel fibers are
surrounded by the silica.
In contrast, the silica obtained in the presence of AgNO3
shows a novel spiral structure (Figure 3A). As far as can be
recognized, all the spirals possess a right-handed helical motif.
Since the exciton-coupling band of the organogel also shows R
helicity in the presence of AgNO3, we consider that micro-
scopic helicity to be reflected by a macroscopic spiral helicity.
Figure 3B shows the SEM picture of the silica obtained in
the presence of CsClO4Ðagain the well-grown spiral struc-
ture is seen, which features a right-handed helical motif.
In this picture, one can also recognize a filmlike structure,
which is presumably constructed by transcription of the
lamellar structure of the organogel (which is also seen in
Figure 2C). The silica obtained in the presence of NaClO4 or
KClO4 showed a hollow fiber structure as observed previ-
ously,[12, 14] but the spiral structure was not recognized (not
shown).
To further corroborate that the organogel fibers really
acted as a template for the growth of spiral silica, transmission
electron microscopy (TEM) pictures were taken after remov-
al of 1 by calcination.[12, 14] We found that the spirals have a
tubular structure with an inner diameter of 25 ± 50 nm (Fig-
ure 4). These results support the view that anionic oligomeric
siloxanes are adsorbed onto metal-containing cationic orga-
nogel fibers, and the silica spirals thus grown reflect the chiral
factor present in the organogel fibers.
Figure 2. SEM pictures of the xerogels obtained from the gel-phase
solution of 1 (5.00 Â 10À2 molLÀ1) in tert-butyl alcohol A) in the absence of
and B) in the presence of AgNO3 (5.00 Â 10À2 molLÀ1) or C) CsClO4
(5.00 Â 10À2 molLÀ1). The gel was first cooled in liquid nitrogen and then
the solvent was removed under vacuum at 158C. In each case the bar
corresponds to 1.5 mm.
In the presence of AgNO3 or CsClO4, on the other hand, both
the fibrous structure and the lamellar structure can be
recognized (Figure 2B and C, respectively). These results,
together with the CD spectra, suggest that the exciton-
coupling band originates from the chirality present in the
fibrous structure.
Sol ± gel polycondensation of TEOS was carried out in the
gel phase of 1 in the absence and the presence of AgNO3 or
CsClO4. A typical mixture consists of 1-butanol (98mg),
TEOS (16 mg), water (5.7 mg), and benzylamine (5.6 mg) as a
catalyst. This mixture was left at room temperature for one
day. The SEM pictures taken at this stage are shown in
Figure 3. The silica obtained in the absence of metal salt
showed the nonimpressive granular structure, which is also
obtained from conventional TEOS polycondensation in
solution (not shown). This result indicates that 1) the presence
of the positive charge is indispensable for transcribing the
organogel structure into the silica structure[12, 14] and 2) as
shown by the CD spectrum in Figure 1, the aggregates formed
in the absence of a metal salts are not so highly orientated that
they affect the silica structure.
In conclusion, sol ± gel polycondensation of TEOS in the gel
phase supported by an azacrown-appended cholesterol gela-
tor results in a unique spiral silica with a right-handed helical
motif. This is a very rare example of chirality being created in
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[15] Although several inorganic materials with a helical structure have
been reported, none were prepared by the chiral template method
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Figure 4. TEM pictures of the spiral silica obtained from 1 in the presence
of A) AgNO3 (the dots are silver particles) and B) CsClO4 after calcination.
The final calcination was carried out at 5008C for 4 h under aerobic
conditions. The silica obtained from the experiment in the presence of
CsClO4 was confirmed with a electron probe microanalyzer (Si: 45.45, O:
44.34, Cs: 9.32 wt%).
the inorganic material. The results indicate the versatility of
the template method for the creation of various silica
structures.[15]
Received: October 1, 1999 [Z14089]
Revised: December 27, 1999
[16] For experimental details of the gelation test and the xerogel
preparation, see ref. [7].
[17] Because a freeze-and-pump method was used to prepare xerogels
suitable for SEM observation, the xerogel was obtained from a
solution in tert-butyl alcohol (m.p. 25.78C) instead of 1-butanol
(m.p. À 908C).
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