Self-assembled organogels formed by mono-chain L-alanine derivatives

Article abstract of DOI:10.1039/b104428c

The mono-chain L-alanine derivatives self-assemble into bilayer aggregates in a number of organic liquids and gelatinize the liquids.

Full text of DOI:10.1039/b104428c

Self-assembled organogels formed by mono-chain -alanine derivatives
L
Xuzhong Luo, Bin Liu and Yingqiu Liang*
Lab of Mesoscopic Materials and Chemistry and State Key Lab of Coordination Chemistry, Nanjing
University, Nanjing 210093, P. R. China. E-mail: yqliang@netra.nju.edu.cn
Received (in Cambridge, UK) 18th May 2001, Accepted 26th June 2001
First published as an Advance Article on the web 1st August 2001
The mono-chain
bilayer aggregates in a number of organic liquids and
gelatinize the liquids.
L
-alanine derivatives self-assemble into
compounds 1–3 can form a stable physical organogel and
gelatinize a number of organic fluids even at a very low
concentration. For example, the amounts of 2 necessary to gel 1
L of CCl4, PhH, PhMe, o-xylene and mesitylene are 7, 4, 5, 5
and 4 g, respectively. Weiss and co-workers’ studies reveal that
the size of organogel colloids depends upon the nature of the
liquid.1c The amphiphiles 1–3, which form transparent or
translucent organogels in different liquids, may be due to the
different size of their colloids.
Fig. 1 shows the FT-IR spectra for the CCl4 gel and the
CHCl3 solution of 2. The FT-IR spectrum of the translucent
CCl4 gel of 2 is characterized by bands attributed to inter-
molecular hydrogen bonding, i.e. 3349 (nN–H of amide), 1704
(nCNO of 2COOH), and 1640 cm21 (nCNO of amide), whereas
the isotropic solution of 2 in CHCl3 affords bands at 3450 (nN–
H), 1740 (nCNO of carboxylic acid), and 1660 cm21 (nCNO of
amide), indicative of non-hydrogen bonding stretching vibra-
tions. The FT-IR spectrum of the KBr pellet of 2 is similar to
that of the CCl4 gel of 2, suggesting that the pattern of hydrogen
bonding in the gel is close to that in the crystal. The sharp peak
near 1704 cm21 is ascribed to the CNO stretching mode of
CO2H groups, characteristic of the formation of carboxylic acid
dimers.10 These results imply that in the gel the intermolecular
hydrogen bonds have been formed between the neighboring
molecules of 2 and thus they form a hydrogen bond network.
The TEM image of a CCl4 gel of 2 is shown in Fig. 2. It
reveals a number of fibers, juxtaposed and intertwined by
several long slender aggregations with width of ca. 30–100 nm.
The X-ray diffraction pattern (Fig. 3) of the gel cast film from
the CCl4 gel of 2 show periodical diffraction peaks, indicating
Recent discoveries of low molecular weight gelators have
stimulated considerable interest not only in academic investiga-
tions but also technological applications.1–9 Although dozens of
different categories of gelators have so far been identified, it is
difficult to select a molecule that will definitely gel a selected
liquid. Previous reports reveal that some oligomers of a-amino
acids2a,b,8 and bolaform amides derived from amino acids2c can
gel a number of organic liquids, while the reports relating mono-
amino acid derivatives are relatively few.2d,e Bhattacharya and
co-workers reported recently that mono-amino acid derivatives
can gelate selectively oil from oil–water mixtures.9 Herein, we
report simple compounds, mono-chain -alanine derivatives,
L
which form bilayer aggregates in a number of organic liquids
and gelate the liquids.
The -alanine derivatives, 1–8,† used here were prepared in
L
our laboratory. A typical procedure for studying gel formation
ability is as follows: a weighed sample was mixed with an
organic liquid (1 mL) in a sealed test tube and the mixture was
heated until the solid dissolved. The resulting solution was
cooled at 20 °C for 2 h and then the gelation was studied. Upon
formation the organogel is stable and the tube can be inverted
without any change of shape of the organogel.
The results of gelation tests of 1–3 are summarized in Table
1,‡ in which the values denote the minimum gel concentrations
(g L21) necessary for gelation. From Table 1, it is clear that
Table 1 Gelation test of 1–3 and minimum gel concentration necessary for
gelation at 20 °C. Values refer to the minimum gel concentration in
g L21 (gelator/liquid)
Gel concentration/g L21
Organic liquids
1
2
3
Carbon tetrachloride
Benzene
Toluene
o-Xylene
Mesitylene
6 (translucent) 7 (translucent) 6 (translucent)
5 (transparent) 4 (transparent) 4 (transparent)
5 (transparent) 5 (transparent) 4 (transparent)
6 (transparent) 5 (transparent) 5 (transparent)
5 (transparent) 4 (transparent) 4 (transparent)
1,1,2,2-Tetrachloroethane 6 (translucent) 6 (translucent) 5 (translucent)
Tetrachloroethylene
Tetralin
n-Octane
1,1,2-Trichloroethane
Cyclohexane
6 (transparent) 6 (transparent) 5 (transparent)
6 (translucent) 5 (translucent) 5 (translucent)
4 (translucent) 4 (translucent) 4 (translucent)
7 (translucent) 7 (translucent) 6 (translucent)
5 (transparent) 5 (transparent) 5 (transparent)
Fig. 1 FTIR spectra of 2 a) as a CCl4 gel, b) in CHCl3 solution (dashed
line).
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Chem. Commun., 2001, 1556–1557
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104428c

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molecular structure of 4–6 with 1–3, it can be inferred that
suitable length of the tail chain is essential for gelation since
organogels fail to form when the tail chain is lengthened to 18
C atoms (4) or is substituted by tert-butoxycarbonylcarbonyl (5)
and benzoyl (6). In experiment, we also find that the racemes of
N-dodecanoylalanine do not induce any gelation, which is
similar to that for N-dodecanoylglycine (7). The findings imply
that the homochiral effect plays an important role in the gel
formation. N-Dodecanoyl- -phenylalanine (8), a compound
L
containing a chiral center, does not exhibit gelling ability, which
may be due to the relatively large steric hindrance effect of the
benzyl group of the
L
-phenylalanine which weakens the
intermolecular hydrogen bonding between neighboring mole-
cules. Therefore the intermolecular hydrogen bonding is unable
to meet the need for gelation. From these observations and
analyses, it can be concluded that (i) gelling ability strongly
depends on the -alanine group, (ii) the hydrophile–lypophile
L
Fig. 2 TEM image of a CCl4 gel of 2 (7 g L21). The sample was prepared
by picking up the gel on a carbon grid and post-stained by uranyl acetate.
balance is a significant factor for gelation, the suitable length of
the tail chain is in the range of 8 to 14 C atoms, (iii) the
formation of the bilayer aggregates and the homochiral effect
play important roles in gel-forming.
In conclusion, this paper has shown that simple mono-chain
-alanine derivatives can self-assemble into bilayer aggregates
L
through intermolecular hydrogen bonding and the homochiral
effect in a number of organic liquids, which are juxtaposed and
interlocked by van der Waals interaction, and finally gelate the
organic liquids.
This work was financially supported by the National Natural
Science Foundation of China (NSFC, NO. 20073019).
Notes and references
† Amino acid (0.05 mol) is placed in a three-necked round-bottomed flask,
filled with an efficient stirrer, and dissolved in 50 mL of 1 M NaOH. The
flask is then almost completely immersed in a bath of ice and water (T ca.
+3 °C) and the stirrer is now set in very rapid motion. To the solution are
added, dropwise, 0.10 mol of pure acyl chloride and 80 mL of 1 M NaOH
at the same rate. When the reaction is complete, the suspension is acidified
carefully with 2 M HCl. The residue obtained after filtration is extracted
with petroleum ether to remove the fatty acid. The final amino acid
derivative is chromatographed on a silica gel column empolying appropriate
Fig. 3 X-ray diffraction patterns of a cast film from CCl4 gel of 2.
that 2 indeed assembles into an ordered structure. The long
spacing (D) of the aggregate obtained by the XRD method is
about 3.43 nm, which is much smaller than twice the evaluated
molecular length of 2 (2.14 nm, by the CPK model), but much
larger than the length of one molecule of 2. From the XRD and
FT-IR results, it can be deduced that that the gel aggregates
consist of a repeating bilayer unit, which bears the head-to-head
packing model with highly tilted alkyl chains relative to the
bilayer normal. Within the bilayer unit, the amphiphiles are
connected by intra- and inter-layer hydrogen bonds to form a
hydrogen bond network and then develop the superstructure that
is schematically shown in Fig. 4.
1
solvent. Satisfactory H NMR, IR, element analysis and optically activity
data were obtained for the corresponding amino acid derivatives.
‡ The minimum gel concentration was calculated as described in the
literature.8 The gels were stable in a sealed tube for over two weeks. On re-
heating, the physical gelation is reversible.
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Fig. 4 Local microstructure of the bilayer aggregates of 2 in organogel.
Chem. Commun., 2001, 1556–1557
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