Switching chirality in the assemblies of bio-based amphiphiles solely by varying their alkyl chain length

Article abstract of DOI:10.1039/c6cc10122d

Here we show that chirality inversion can be realized solely by changing the alkyl chain length of sorbitol-alkylamine surfactants. The chirality switch phenomenon is attributed to twisting of a headgroup, which depends on the balance between hydrophobic interaction and torsional stress, resulting in various orientational orders in assemblies and chirality inversion.

Full text of DOI:10.1039/c6cc10122d

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DOI: 10.1039/C6CC10122D
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Switching Chirality in the Assemblies of Bio-based Amphiphiles
Solely by Varying their Alkyl Chain Length
Received 00th January 20xx,
Accepted 00th January 20xx
Pei Zhanga, Jun Maa, Xinchen Kanga,b, Huizhen Liua,b, Chunjun Chena,b, Zhanrong Zhanga, Jianling
Zhanga,b, and Buxing Han*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here we show that chirality inversion can be realized solely by gelators, stabilizers, and emulsifiers have also been studied.14
changing the alkyl chain length of sorbitol-alkylamine surfactants. Furthermore, Song et al.15 reported that the chirality of
The chirality switch phenomenon is attributed to twist of assemblies formed by chiral gelator dibenzylidene˗D˗sorbitol
headgroup, which depends on the balance between hydrophobic could be tuned by solvent effects, which depends on the
interaction and torsional stress, resulting in various orientational stacking interaction or hydrogen bonding interactions.
orders in assemblies and chirality inversion.
In this work, we designed and synthesized a series of
sorbitol˗alkylamine surfactants denoted as SAAS˗Cm, in which
two alkylamine groups are attached to sorbitol and Cm
represents the carbon number of the alkyl chains. It was
demonstrated that self˗assembled aggregates with
supramolecular chirality could be obtained in aqueous solution.
More interestingly, the chirality inversion of the assemblies
could be realized simply by changing the length of alkyl chains.
As far as we know, this is the first report on the chirality
inversion switched solely by varying alkyl chain length of a
surfactant.
The bio˗based amphiphiles were synthesized by two steps,
which included bromination of D˗sorbitol to prepare
intermediate compound 1,6˗dibromosorbitol (1) using the
method reported16 and amine alkylation of (1) to obtain
SAAS˗Cm, as shown in Fig. 1. Steady˗state fluorescence spectra
showed that the intensity ratio I1/I3 decreased quickly with
SAAS˗Cm concentration after the critical micelle concentration
(cmc), indicating the formation of hydrophobic microdomains
(Fig. S1).
Chiral surfactants can self˗assemble to form different kinds
of nanoscale aggregates ranging from simple micelles to highly
organized fibers and even supramolecular chiral
nanostructures such as helices, helical tubes,1,2 which are
among the most elegant architectures. The modulation of
helicity is crucial for their possible applications in material
science, sensing or recognition and enantioselective catalysis.3
Moreover, modulation of chirality including chirality induction,
amplification and inversion phenomena are important for
understanding the asymmetry of various systems.4 The
chirality inversion in helical structures is a very interesting
phenomenon. Different methods have been reported on
tuning the helicity of supramolecular aggregates, such as
changing the structures of amphiphiles,5 adding chiral or
achiral additives,6,7 and using an external stimulus such as
light,8 changing solvent, etc.9
Synthesis of surfactants using carbohydrates derived from
biomass has attracted much attention, especially in recent
years.10 Carbohydrates have stereogenic centers, providing
important natural source of chiral building blocks and some
chiral architectures formed from carbohydrate˗based
surfactants have also been reported.11 Sorbitol is a kind of
sugar alcohol with four chiral centers in the skeleton and
sorbitol derivative surfactants of different properties have
been reported.12,13 Applications of this kind of surfactants as
a. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid and
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China.
E-mail: hanbx@iccas.ac.cn
Fig. 1. The synthesis procedures of sorbitol-alkylamides
surfactants SAAS˗Cm (m = 8, 10, 12, 16).
b. University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Electronic Supplementary Information (ESI) available: Synthetic procedures,
experimental
details,
characterizations
and
supporting
data. See
DOI: 10.1039/x0xx00000x
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images heterocyclic ring in the molecular aggregates. However, no
Transmission
electron
microscopy
(TEM)
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demonstrated that the assemblies underwent a transition absorption was observed in the sameDrOeIg: 1io0.n103fo9/rC6SCACA1S0˗1C2212D,
from nanosphere to nanotubes with the increase of alkyl chain suggesting different conformations of the molecules in the
length (Fig. 2). Sphere˗like micelles were observed in the assemblies.
solution of SAAS˗C8, and nanotubes with diameter of 50 ˗ 80
nm and hundreds of nanometers in length were formed in
systems of SAAS˗C12 and SAAS˗C16. The structure of assemblies
was studied by small angle X˗ray scattering (SAXS) technique,
and the information on the scattering objects was obtained by
generating a pair distance distribution function p(r) of the
scattering particles (Fig. S2).17 The curves of SAAS˗C8 were
nearly symmetric as showed in Fig. S2a, indicative of the
spherical micelles formation. However, the p(r) curves of
SAAS˗C10, SAAS˗C12 and SAAS˗C16 were asymmetric, and the
length˗width ratio L/Dc increased with the increase of alkyl
chain length, indicating the formation of 1˗D assemblies.18
Fig. 3. CD (A) and U˗Vvis ( B) spectra of SAAS˗Cm of the
hierarchical sel˗fassembly at 25
°C. The concentration of
samples: D˗sorbitol, 5 .0 mM; SAAS˗C 8, 5.0 mM; SAAS˗C10, 5.0
mM; SAAS˗C 12, 0.5 mM; SAAS˗C 16, 0.3 mM; Inset is the
enlargement of signals for SA˗ACS
and D˗sorbitol, both of
8
which exhibit no Cotton effect.
Nuclear magnetic resonance (NMR) spectroscopy is a unique
and powerful tool for studying intrinsic atomic˗level structural
information, which has been used to investigate the chirality of
nanostructures.21 The proton chemical shift supplies
information about the local chemical environment at the
aggregate surface and in the hydrophobic domains.22
Conformations of SAAS˗Cm in the assemblies were further
studied by NMR. In the assembly process, the SAAS˗Cm
monomers interact with each other via different interactions
such as hydrogen bonding and hydrophobic interaction. The
1HNMR spectra of SAAS˗Cm solutions with different
concentrations are presented in Fig. S5. The resonance peaks
of methylene in hydrophobic tails and methyne protons in the
headgroup merged together in the case of SAAS˗C12, and
shifted to a lower field as the concentration increased (Fig.
S5A). In contrast, for SAAS˗C16, the resonance peaks of the
protons in hydrophobic tails shifted to high field slightly,
whereas the protons in the headgroup shifted to low field and
splited into single resonance peaks (Fig. S5B and Table S1).
New emerging peaks in low field provided information of
strong hydrogen bond between the headgroups, which was
attributed to the shielding effect of hydrogen bond. It can be
deduced from the NMR data that there were different
geometry configurations in SAAS˗C12 and SAAS˗C16, which
exhibited two kinds of chiral environments. The results provide
evidences to clarify the conformations of SAAS˗C12 and
SAAS˗C16 in the assemblies. In addition, variations in chemical
shifts for protons of SAAS˗C8 and SAAS˗C10 (Fig. S5C) were
consistent with that of SAAS˗C12, suggesting that the
Fig. 2. Typical TEM images of the nanostructures assembled in
SAAS˗Cm aqueous solutions (m= 8, 10, 12, 16): (A) SAAS˗C , 5.0
8
mmol/L; (B) SAAS˗C 10, 5.0 mmol/L; (C) SAAS˗C 12, 1.0 mmol/L;
(D) SAAS˗C 16, 0.5 mmol/L. Concentrations of SAA˗SC chosen
m
were all above the critical micelle concentration (cmc), which
were discussed in Fig. S1.
Atomic force microscopy (AFM) measurements were carried
out to study the nanostructures formed at different
concentrations. The AFM images of the nanotubes formed by
SAAS˗C12 showed necklace-like structure with a pitch of 30 ˗ 40
nm (Fig. S3A, S3B), and the nanotubes formed by SAAS˗C16
showed an irregular pitch of 120 nm (Fig. S3C, S3D).19
The chirality of the assemblies was analyzed by circular
dichroism (CD) spectroscopy, which is a commonly used
method to probe the handedness of self˗assembled
nanostructures.20 As shown in Fig. 3A, there was no CD signal
observed in the spherical micelles assembled by SAAS˗C8. A
weak Cotton effect with negative signal at 200 nm and positive
signal at 206 nm was detected in SAAS˗C10 solution, displaying
the formation of chiral assemblies. Very interestingly, a strong
negative Cotton effect at 204 nm was observed for nanotubes
of SAAS˗C12, while positive signals appeared for that of
SAAS˗C16, indicating that a chirality inversion took place when
the hydrophobic chain increased from C12 to C16. It can be
known from UV˗vis spectra (Fig. 3B and Fig.S4) that there was
weak absorption (
ε = 1000 ˗ 2000) in 220 ˗ 300 nm range in the
case of SAAS C16, which was indicative of existence of
˗
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conformations of headgroups in SAAS˗Cm with short alkyl L˗TA or D˗TA at various concentrations: (a), SAAS˗C16; (b) L˗TA,
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DOI: 10.1039/C6CC10122D
0.05 mM; (c) D˗TA, 0.05 mM; (d) L˗TA, 0.1 mM; (e) D˗TA, 0.1
chains could keep stretch without twisting.
Chiral recognition is important for both scientific community mM; (f) L˗TA, 0.5 mM; (g) D˗TA, 0.5 mM; (D) CD spectra of
and industry, especially in chiral separation and asymmetric SAAS˗C16 solution after adding 0.05 mM TA enantiomeric
catalysis.23 Sugars are attractive natural sources of chirality for mixtures with various ee value; The CD spectra of tartaric acid
constructing artificial receptors for chiral recognition.24 Using (TA) with concentration of 0.1 M and UV/vis spectra was
tartaric acid (TA) as substrate, the enantioselective recognition provided in Fig. S4 (The concentration of SAAS˗C12 and
effect of SAAS˗C12 or SAAS˗C16 can be studied by CD SAAS˗C16 used is 1.0 mM and 0.5 mM respectively; ee % =
spectroscopy, and the results are given in Fig. 4. Upon the ([L˗TA]˗[D˗TA])/([L˗TA]+[D˗TA])
addition of individual enantiomers of L˗TA and D˗TA with
×100%).
various concentrations in SAAS˗C12 solution, corresponding
The chirality inversion phenomenon of SAAS˗Cm assemblies
new signals different from the enantiomer appeared (Fig. 4A), is fascinating, which results from transition of molecule
and the phenomenon was also embodied in the UV˗vis spectra conformation. Briefly, the headgroup with sorbitol skeleton
(Fig. S6). The enantiomers of tartaric acid under the same forms two five˗membered rings conformation by the
conditions provided an easy means of determining the intermolecular hydrogen bonds. In this case, the hydrophobic
enantiomeric excess. Over the introduction of various interaction of SAAS˗C12 was not strong enough to break the
enantiomeric composition of D˗ and L˗ tartaric acid, intense balance of torsional stress in headgroup, and thus the two
signals in the predicted positions corresponding to the alkyl chains choose to arrange in two orientations without
enantiomers were detected as indicated in Fig. 4B. In the case disrupting the conformation of the five
of SAAS˗C16, more remarkable signals were observed after headgroup, which is similar to the conformation of sorbitol.
adding the enantiomers of tartaric acid under the same Therefore, the SA˗ACS molecules stacked neatly into
˗membered rings in the
12
conditions, i.e., the positive signal of SAAS˗C16 was enhanced one˗dimensional network. Meanwhile, the chiral headgroup
by adding D˗TA, while the signal was weakened and divided could not pack in parallel but rather packed at a non˗zero twist
into two peaks by introducing L˗TA enantiomer (Fig. 4C). angle with respect to their neighbors, leading the whole
Similar CD spectra were detected with 10 mol % of various ee’s nanostructure to twist into a left˗handed helical nanotube
enantiomeric mixtures (Fig. 4D), showing different recognition (Scheme 1A). In contrast, SAAS˗C16 has enhanced hydrophobic
of enantiomers from SAAS˗C12. In a word, the nanotubes with interaction to destroy the balance of framework, which
various handedness exhibited different recognition induced chiral twist in the headgroup and the alkyl chains
phenomena of chiral tartaric acid enantiomer. Meanwhile, the orienting towards one side, leading to an interdigitating
results demonstrated a high sensitivity of enantiomeric excess. arrangement with opposite chiralities as shown in Scheme 1B.
Thus, the helical nanostructures assembled by sugar˗based Therefore, the orientation of headgroup resulted in
amphiphiles SAAS˗Cm with tunable helicity can be used as anticlockwise twist and right˗handed nanotubes were
effective sensor for chiral recognition. At the same time, the formed.25 The surfactants SAAS˗C12 and SAAS˗C16 formed
results in turn provided further evidences for the chirality one˗dimensional nanostructures is consistent with that
inversion of the assemblies as the carbon number of SAAS˗Cm obtained from SAXS curves as discussed above (Fig. S2).
changed.
Scheme 1. Schematic demonstration of configurations of
Fig. 4. (A) CD spectra of SAAS˗C12 solution upon the addition of SAAS˗C12 and SAAS˗C16 in the assemblies and the mechanism
individual enantiomers of L˗TA or D˗TA at various of chirality inversion.
concentrations: (a), SAAS˗C12; (b) L˗TA, 0.1 mM; (c) D˗TA, 0.1
mM; (d) L˗TA, 0.5 mM; (e) D˗TA, 0.5 mM; (f) L˗TA, 1.0 mM; (g)
D˗TA, 1.0 mM; (B) CD spectra of SAAS˗C12 solution upon the
addition of 0.1 mM TA enantiomeric mixtures with various ee
value; (C) CD spectra of SAAS-C16 solution upon the addition of
As discussed above, sorbitol˗based framework in SAAS˗Cm
has suitable torsional stress for varying its configuration by
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14. (a) F. W. Lichtenthaler and S. C. Peters, R. Chim., 2004, 7, 65;
changing the length of hydrocarbon chains. The anisotropic
configurations of the molecules can be tuned by controlling
the balance between hydrophobic interaction and the strength
of torsional stress in headgroup, and thus the chirality can be
switched by changing the length of hydrocarbon chains.
In conclusion, SAAS˗Cm can form chiral nanostructures by
self˗assembly in aqueous solution. Chirality inversion of the
nanostructures can be realized simply by changing the length
of alkyl chains. Left˗handed helical nanotubes are formed in
SAAS˗C12, while SAAS˗C16 generated right˗handed nanotubes.
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