Unexpected right-handed helical nanostructures co-assembled from l-phenylalanine derivatives and achiral bipyridines

Article abstract of DOI:10.1039/c6sc04808k

The construction of chiral supramolecular systems with desirable handedness is of great importance in materials science, chemistry, and biology since chiral nanostructures exhibit fascinating photophysical properties and unique biological effects. Herein,

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Unexpected Right-Handed Helical Nanostructures Co-Assembled
from L-Phenylalanine Derivatives and Achiral Bipyridines
Guofeng Liu,ab Linyi Bai,b Jinying Liu,a Chuanliang Feng*a and Yanli Zhao*bc
Received 00th January 20xx,
Accepted 00th January 20xx
The construction of chiral supramolecular systems with desirable handedness is of great importance in materials science,
chemistry and biology, since chiral nanostructures exhibit fascinating photophysical properties and unique biological
effects. Herein, we report that achiral bipyridines can co-assemble with L-phenylalanine derivatives into unexpected right-
handed helical nanostructures rather than left-handed helix by utilizing intermolecular hydrogen bonding interactions
formed between the pyridyl and carboxylic groups. This work opens up a route to develop chiral nanostructures with
desirable handedness through the co-assembly of simple molecular building blocks and provides a straightforward insight
into the chirality control of nanostructures in supramolecular systems.
DOI: 10.1039/x0xx00000x
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twist or helix is normally aggregated from D-type counterparts.9
There are some reports of right-handed twist or helix co-assembled
from L-type amino acid based building blocks.8c, 10 Thus, how to fine
tailor the building blocks aggregating into a desirable specific motif
remains in its infancy.
It is important to gain further insights into the fundamentals of
chiral transfer and expression in the co-assembled hydrogel systems,
which will enable us to obtain a comprehensive understanding of
the design of new chiral materials and to fine tune the chirality of
the co-assemblies. Herein, uniform right-handed helical
nanostructures were obtained from the co-assembly of various
achiral bipyridine derivatives with two chiral gelators (LPF and LCHF)
derived from L-phenylalanine through strong intermolecular
hydrogen bonding formed between achiral bipyridines and L-type
enantiomers (Fig. 1).
Introduction
Chiral supramolecular architectures assembled from small
molecular building blocks have been attracting extensive interests
owing to their controllable structural features, their relationship to
biological structures, and potential applications in chiral recognition
and separation.1 Although numerous helical or twisted
nanostructures and ordered ensembles have been successfully
produced by molecular self-assembly from either single or multiple
molecular components for utilizations in chemistry,2 biology3 and
materials science,4 it has still remained challenging to construct
chiral nanostructures with desirable conformation (i.e., right-
handed, P; left-handed, M) from specific chiral building blocks at
will. On the other hand, in comparison with the rich knowledge that
has been gathered with regard to establishing chiral nanostructures
from either chiral or achiral building blocks,5 rare studies have
reflected explicit relationship between the chirality of
nanoarchitectures and enantiomeric monomers.6 Obviously, gel-
phase materials are a key test-bed for understanding the impact of
molecular chirality on nanoscale self-assembly or co-assembly,
since supramolecular chirality of gels can be finely tuned by both
the chirality of component molecules7 and special spatial
arrangements of building blocks.8 After a detailed survey of
previous literature reports, it was wondrously found that left-
handed twist or helix is often co- or self-assembled from L-form
amino acid based molecular building blocks, while right-handed
Fig. 1 Schematic illustration of chiral nanostructures co-assembled
from L-type enantiomeric monomers (LPF and LCHF) with achiral
bipyridines (BPy, DPT, NDPT, and NPI). M and P denote left- and
right-handed helical nanostructures, respectively.
Results and discussion
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Unexpected right-handed helical nanostructures of hydrogels
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DOI: 10.1039/C6SC04808K
Molecules LPF and LCHF, based on 1,4-phenyldicarboxamide and
1,4-cyclohexanedicarboxamide respectively, contain a helicogenic L-
phenylalanine motif, and carry a COOH group at each terminus of
the two phenylalanine arms. Hence, they are bistopic ligands. To co-
assemble them into hydrogels, we rationally designed four bistopic
bipyridine ligands (BPy, DPT, NDPT, and NPI) by utilizing hydrogen-
bonding interactions between the pyridyl nitrogen atom and H-O
group of carboxylic acid.11 The synthesis of LCHF, DPT, and NPI is
outlined in the experimental section, and BPy is commercially
available. LPF and NDPT were synthesized according to a previous
report.8c All the newly synthesized compounds were fully
characterized by NMR spectroscopy and high-resolution mass
spectrometry (Fig. S1-S14).
The ability of four achiral bipyridines (BPy, DPT, NDPT, and NPI)
to co-assemble with equimolar LPF or LCHF was first determined by
the formation of hydrogels by means of heating-to-cooling and
inversion tests (Fig. S15). LPF+DPT, LPF+NPI, LPF+BPy and LPF+NDPT
formed stable homogeneous hydrogels in the vials. Scanning
electronic microscopy (SEM) images of diluted samples of the gels
on silicon wafer showed enantiomerically enriched, helical ribbon
fibers (Fig. 2 and S16-S23). All the co-assembled hydrogels (Fig. 2a-d)
are organized into ropelike fibers with the helicity pitches around
hundreds of nanometers. In Fig. 2a, the fibers from the LPF+BPy gel
exhibited exclusively left-handed (M-type) helicity with a diameter
in hundreds of nanometers. Surprisingly, fibers from LPF+DPT,
LPF+NPI, and LPF+NDPT (Fig. 2b-d) all displayed beautiful uniform
right-handed (P-type) helix with a diameter in tens of nanometers,
which are absolutely opposite with the chirality of the LPF+BPy gel.
According to previous reports,3a,9a-c specific one-dimensional
nanofibers self-assembled from L-phenylalanine derived monomers
usually exhibit exact left-handedness.
To explore if this unexpected phenomenon is also applicable to
other gel systems, microscopic nanostructures assembled from
achiral bipyridines with LCHF were investigated in detail.
Intriguingly, uncommon right-handed helical nanofibers with nearly
the same helicity pitches around hundreds of nanometers were
observed in the SEM images of all hydrogels (Fig. 2e-h). This is a bit
different with LPF based gel systems, where LPF+BPy exhibited
exclusively left-handed (M-type) helicity. But, right-handed
nanofibers were observed from LCHF+BPy. On the basis that the
only difference in LCHF+BPy is the central benzene ring of LPF
instead of the cyclohexyl core of LCHF, it was anticipated that the
chirality of supramolecular aggregates could be tuned by only
changing some functional groups rather than altering inherent
chirality of enantiomeric monomers. In addition, these studies
indicate how a slight change in molecular structure of the building
blocks dramatically influences the overall chirality of
supramolecular aggregates in two-component hydrogels. The
chirality of assemblies shown here is not only strongly determined
by the chiral center of phenylalanine units in LPF and LCHF, but also
highly affected by molecular structure of achiral bipyridines, both of
which collectively play vital roles in rigidifying the aggregates and
guiding them to form unique chiral hydrogels.
Fig. 2 SEM images of supramolecular hydrogels based on LPF or
LCHF co-assembled with achiral bipyridines. a) LPF+BPy, b) LPF+DPT,
c) LPF+NDPT, d) LPF+NPI, e) LCHF +BPy, f) LCHF +DPT, g) LCHF
+NDPT, and h) LCHF +NPI. Scale bars: 5 μm for a), 500 nm for b),
and 200 nm for c-h).
CD activity of co-assembled hydrogels
To gain further insight into these helical supramolecualr structures,
circular dichroism (CD) spectra of the co-assembled hydrogels were
measured at room temperature (Fig. 3 and S24). The CD spectra of
hydrogels LPF+NPI, LPF+DPT and LPF+BPy all exhibited a negative
dichroic signal at around 205 nm, assigning to intramolecular π-π*
transitions in the peripheral phenyl group of LPF.8c Interestingly, for
hydrogels of LPF+BPy, its CD spectrum also showed a negative
Cotton effect at 268 nm (Fig. 3A), whereas for LPF+DPT, LPF+NDPT,
and LPF+NPI hydrogels, a positive dichroic signal was shown at 235
nm, 293 nm, and 265 nm respectively, which was assigned to
intramolecular transitions from the amide linkage to the central aryl
group according to our previous calculations on LPF.8c Compared
with hydrogels of LPF+BPy, a chiral transition into opposite optically
active hydrogels (LPF+DPT, LPF+NDPT, and LPF+NPI) was obtained
only by changing achiral components (BPy, DPT, NDPT, and NPI),
which was in good correlation with the helical microscopic
structures observed in the SEM images. In addition, the
contribution of linear dichroism (LD) on the CD signals was
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investigated. As shown in Fig. S25, the intensity of LD signals was measurements (Fig.
ARTICLE
4
and S29-S35) can provide valuable
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DOI: 10.1039/C6SC04808K
much lower than that of corresponding CD signals (except for information about the interaction of supramolecular aggregates at
LPF+NPI), indicating that the LD contribution could be negligible in molecular level. The as-prepared xerogel of LPF was first
these gels. In the case of LPF+NPI hydrogel, in order to get rid of the characterized by FTIR, showing well-defined amide I bands centered
LD influence, the hydrogel film was placed in different angles and at 1621 cm-1, amide II bands centered at 1551 cm-1 and stretching
an average CD signal (Fig. S26) was taken as reported in literature.12 vibration bands of C＝O from carboxyl groups at 1738 cm-1 (Fig. 4
Thus, the relationship between the handedness of the helical and S31). These bands shifted in homogeneous dichloromethane
fibers and CD signals could be obtained. The M-type LPF+BPy co- solution (Fig. S31 and Table S1). These observations suggest well-
assembly exhibits a negative Cotton effect at 268 nm, whereas P- developed hydrogen bonding networks formed through the amide
type supramolecular structures in LPF+DPT, LPF+NDPT, and LPF+NPI and carboxylic acid units in the self-assembled nanofibers. FTIR
show a positive dichroic signal (Fig. 3A). It is worth to note that all spectrum (Fig. S32) of LPF+BPy gels clearly displayed well-defined
of the hydrogels possess the same S-type stereocenter within amide I and II bands centered at 1636 cm-1 and 1543 cm-1
peripheral L-phenylalanine units. Thus, their CD spectra should not respectively, indicating that the amide groups participate in strong
be exact mirror images. Nevertheless, the chirality of these hydrogen bonds. In comparison with LPF xerogel, new bands at
supramolecular assemblies and their chiroptical activities could be 2453 and 1951 cm-1 and the band decrease at 1713 cm-1 clearly
inversed by altering achiral bipyridines. For LCHF systems, all the CD suggest the formation of carboxylic acid-pyridyl hydrogen bonds in
spectra (Fig. 3B) of hydrogels (LCHF+BPy, LCHF+DPT, LCHF+NDPT, LPT+BPy. Moreover, the appearance of a peak due to N-H
and LCHF+NPI) exhibited positive dichroic signals around 200 nm stretching vibration at 3308 cm-1 further evidenced the complicated
and among 250-283 nm from the amide linkage, which are in good nature of hydrogen bonds. Similarly, FTIR spectra of LPF+DPT,
agreement with the right-handed helical nanofibers observed in LPF+NPI, and LPF+NDPT xerogels all showed well-defined amide I
SEM images (Fig. 2e-h). The enantiomer of LCHF, i.e., DCHF was also and II bands centered around 1635 and 1540 cm-1 respectively, and
synthesized as a control. Corresponding spectra of LCHF and DCHF the stretching vibration bands of C＝O from carboxylic groups at
were determined (Fig. S27 and S28), showing perfect mirror 1735 cm-1 disappeared coupled with a new peak at ~1695 cm-1. In
imaging profiles.
addition, two peaks assigned to O-H stretching vibrations at 2495
and 1950 cm-1 were observed. FTIR studies confirm that these co-
assembled hydrogel frameworks are stabilized by intermolecular
hydrogen bonding interactions between amide/pyridine units and
carboxylic acid groups.
Fig. 4 FTIR spectra of xerogels self- or co-assembled from LPF (A)
and LCHF (B).
Fig. 3 CD spectra of hydrogels for (A) LPF+BPy, LPF+DPT, LPF+NDPT,
and LPF+NPI, and (B) LCHF+BPy, LCHF+DPT, LCHF+NDPT, and
LCHF+NPI.
For LCHF based hydrogel systems (Fig. S33 and S34), compared
with LCHF gel exhibiting carboxylic band at 1726 cm-1 (νC=O of
COOH), the carboxylic band of LCHF+BPy gel was observed at 1722
cm-1, and a new peak occurred at 2527 cm-1 due to O-H stretching
vibrations was inferred from newly formed hydrogen bonding
interaction between pyridinyl and carboxylic acid groups. For
xerogels of LCHF+DPT, LCHF+NDPT, and LCHF+NPI, amide I bands
shifted to 1691, 1694 and 1697 cm-1, respectively. Well-defined
amide II bands (δN-H of CONH) at 1537 cm-1 in LCHF powder shifted
to 1535 cm-1 for LCHF+BPy, 1537 cm-1 for LCHF+DPT, 1506 cm-1 for
LCHF+NDPT, and 1510 cm-1 for LCHF+NPI. In addition, new peaks
due to O-H stretching vibrations were observed at 2546 and 1951
cm-1 for LPF+DPT, 2566 and 1945 cm-1 for LPF+NDPT, and 2503 and
1953 cm-1 for LPF+NPI. Thus, the co-assembled hydrogels based on
LCHF were also driven by intermolecular hydrogen bonds between
amide/pyridine units and carboxylic acid groups.
On account of the same L-type phenylalanine stereocenter within
the LPF and LCHF components and achiral bipyridines used, it was
reasonably inferred that the changes of CD signals and unexpected
enantiomerically enriched helical nanostructures observed from
SEM images could be attributed to specific stacking modes of co-
assembling building blocks, which bring vital effects on chiroptical
behavior and chiral morphology of hydrogels by strong and
extensive intermolecular hydrogen bonding between L-
phenylalanine derivatives and achiral bipyridines.
Co-assembly mechanism of hydrogels
The co-assembling mechanism of these hydrogels was further
investigated by Fourier transform infrared (FTIR), because FTIR
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two-component supramolecular hydrogel system, unexpected
DOI: 10.1039/C6SC04808K
right-handed nanostructures (except for LPF+BPy) were successfully
On the basis of all the results discussed above, it can be
concluded that the main driving forces for the co-assemblies are
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two types of intermolecular hydrogen bonds. Firstly, strong constructed by the co-assembly of L-phenylalanine derivatives with
hydrogen bond is formed between pyridyl nitrogen and the hydroxy
group of a carboxylic acid, which drives the building blocks to co-
assemble in a head-to-tail fashion. Second, three-dimensional fiber
networks are obtained through the formation of hydrogen bonds
between amide groups. These two kinds of intermolecular
hydrogen bonds stabilize the co-assembled hydrogel frameworks.
achiral bipyridines, strongly revealing that the supramolecular
chirality of nanostructures is not only determined by the chirality of
monomers (LPF and LCHF), but also highly influenced by the
stacking mode of building blocks through strong and extensive
intermolecular hydrogen bonding during the co-assembling process.
On the basis of these results, it can be confirmed that the
intermolecular hydrogen bonding from COOH-pyridine and amide-
amide leads to different interaction modes of LPF and LCHF with
achiral bipyridines. These two types of intermolecular hydrogen
bonding interactions could possibly enable the building blocks to
assemble into uniform helical aggregates.
VCD activity of co-assembled hydrogels
The chiroptical activities of these hydrogels were also studied by
vibrational circular dichroism (VCD).13 We conducted the VCD
measurements by coating hydrogels on CaF2 wafer and then dried
under infrared lamp. For LPF based hydrogel systems (Fig. 5A),
LPF+BPy exhibited a (-/+) VCD signal of the C=O stretching band
among 1750-1600 cm-1, whereas the VCD signal of the band
switched to a significant (+/-) pattern for LPF+DPT, LPF+NDPT, and
Conclusions
In conclusion, unexpected right-handed helical nanostructures have
LPF+NPI hydrogels. Thus,
a strong and extensive C=O···H-N been successfully constructed by the co-assembly of L-
hydrogen-bonding network significantly stabilizing the co-
assembled supramolecular hydrogels is inferred from vibrational
amide I stretching band at around 1636 cm-1. This amide I VCD band
in LPF+BPy gives a (-/+) pattern, and that in LPF+DPT, LPF+NDPT and
LPF+NPI shows an opposite (+/-) signal. The VCD patterns imply the
inversion of the chirality from LPF+BPy to LPF+DPT/LPF+NDPT
/LPF+NPI at room temperature. Surprisingly, all of these VCD bands
revealed a (-/+) pattern in LCHF based co-assembled hydrogels
among 1750-1600 cm-1, which may be ascribed to different central
cyclohexyl unit in LCHF (Fig. 5B).
phenylalanine derivatives with achiral bipyridines in
a two-
component supramolecular approach. In this way, we were able to
demonstrate that the chirality of supramolecular architectures
could be determined by both the molecular chirality and the
stacking mode of building blocks in the co-assembly process.
Studying a series of right-handed helical nanofibers containing
building blocks with the same L-type chiral stereocenter and achiral
bipyridines with different molecular conformation has enabled us
to gain insight into the conveyance of configurational information
during helical nanostructure formation. With the generality of this
approach demonstrated for a number of different building block
combinations, we expect that this approach should be applicable to
a broad variety of building blocks for promoting the establishment
of chiral nanomaterials with desirable topologies. Further
investigations using this strategy to define chiral relationship
between enantiomeric monomers and supramolecular assemblies
would bring new insights into deeper understanding of chiral
assembly process and regulating supramolecular aggregation.
Experimental
General
Fig. 5 VCD spectra of (A) LPF+BPy, LPF+DPT, LPF+NDPT, and LPF+NPI
hydrogels, and (B) LCHF+BPy, LCHF+DPT, LCHF+NDPT, and LCHF+NPI
hydrogels.
The NMR spectra were recorded on a Bruker Advance III 300
Instrument (300 MHz). HRMS were determined on a Water Q-Tof
Mass Instrument. Amino-4-pyridine, 1,4-benzene-dicarbonyl
dichloride, 4,4’-bipyridine, 4-carboxylicpyridine, 1,4-cyclohexane
dicarboxylic acid, 4-dimethylaminopyridine (DMAP), 1-ethyl-3-(3-
dimethylamino propyl)carbodiimide hydrochloride (EDCI), 4-
hydroxypyridine, L-phenylalaninemethylester hydrochloride, thionyl
chloride, and triethylamine (Et3N) were purchased from Aladdin
Chemicals.
Since all of the samples have the same S-type stereocenter within
the L-phenylalanine units, dissimilar VCD behavior between
LPF+BPy gel and LPF+DPT/LPF+NDPT/LPF+NPI gels suggests that
their supramolecular self-assembly could result in the formation of
distinct aggregates with opposite handedness by using different
achiral counterparts. The present results already indicate that LPF
can assemble into different enantiomerically enriched helical
supramolecular structures with achiral bipyridines. In this very rare
Synthesis of LCHF
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1,4-Cyclohexanedicarboxylic acid (1.73 g, 10.00 mmol) was added evaporated under vacuum. The residue was washed with deionized
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to dry dichloromethane containing thionyl chloride (20 mL), and the water, and finally dried in the vacuum oveDnOtIo: 1g0i.v1e03c9l/aCy6bSaCn0k48so08liKd
mixture was stirred at 100 °C for 4 h. All the solvents were NPI (0.23 g, 1.15 mmol, 23.3 %). 1H NMR (400 MHz, DMSO-d6, ppm):
evaporated under vacuum and the residue liquid was collected to δ = 7.78-7.80 (d, 2H, Ar-H), 7.87-7.89 (d, 2H, Ar-H), 8.52-8.53 (d, 2H,
give 1,4-cyclohexanedicarbonyl dichloride. 1,4-Cyclohexane Ar-H), 8.82-8.84 (d, 2H, Ar-H), 10.84 (s, 1H, CO-NH). 13C NMR (100
dicarbonyldichloride (2.0 g, 9.66 mmol) in dry dichloromethane MHz, DMSO-d6, ppm): δ = 166.64, 150.70-150.92, 147.60, 143.60,
(100 mL) was added dropwise to a dichloromethane solution (100 122.93, 115.65. EI-MS (m/z) for C11H9N3O calcd. 199.0746; found
mL) containing L-phenylalaninemethyl ester hydrochloride (5.0 g, 200.0811 [M+H]+.
23.18 mmol) and triethylamine (3.6 mL, 26.00 mmol) in an ice-
water bath. After completing the addition, the solution was stirred
at room temperature overnight. All the solvents were evaporated
Hydrogel preparation
The LPF+DPT hydrogel with 0.2 wt % LPF+DPT is used as an example
to describe the preparation procedure. LPF+DPT (2.0 mg/mL,
equimolar mixture of LPF and DPT) was suspended in a septum-
capped 5.0 mL glass vial and heated until a homogeneous solution
was obtained. The solution solidified into a hydrogel after standing
for a half-hour at room temperature.
under vacuum and the residue was subsequently dissolved in
dichloromethane (100 mL). After extraction by water, the organic
phase was dried by anhydrous MgSO4 and collected to give the
1
dimethyl ester of LCHF (LCHF-OMe, 4.60 g, 9.30 mmol, 84%). H
NMR (300 MHz, DMSO-d6, ppm): δ = 1.43 (t, 4H, CH2), 1.86 (m, 4H,
CH2), 2.05 (s, 2H, CH), 3.11 (dd, 4H, CH2), 3.71 (s, 6H, CH3), 4.87 (d,
2H, CH), 5.92 (d, 2H, CO-NH), 7.07 (m, 4H, Ar-H), 7.27 (d, 6H, Ar-H).
13C NMR (101 MHz, DMSO-d6, ppm): δ = 174.96, 172.34, 136.03,
128.76, 127.36, 125.47, 52.96, 52.54, 44.49, 38.04, 28.78, 28.49.
For the hydrolysis, aqueous NaOH (10 mL, 2.0 M) was added to a
cooled suspension of LCHF-OMe (5.43 g, 6.14 mmol) in MeOH (20
mL). The mixture was slowly warmed up to room temperature and
stirred for 24 h, and a clear solution was obtained. The solution was
then acidified with 3.0 M HCl until pH value was no more than 3.0,
and gel-like precipitate was formed. The gel phase was filtered,
washed with deionized water, and finally dried in the vacuum oven
to give LCHF (3.0 g, 6.38 mmol, 69.2%). Overall yield of LCHF: 66.6%.
1H NMR (300 MHz, DMSO-d6, ppm): δ = 12.66 (s, 2H, COOH), 8.06
(d, 2H, CONH), 7.30 (m, 10H, Ar-H), 4.45 (s, 2H, CH), 2.98 (m, 4H,
CH2), 2.10 (s, 2H, CH), 1.63 (d, 4H, CH2), 1.24 (d, 4H, CH2). 13C NMR
(75 MHz, DMSO-d6, ppm): δ = 174.31, 172.70, 137.52, 128.90,
127.89, 126.16, 53.42, 43.33, 37.20, 28.66. EI-MS for C26H30O6N2
calcd. 466.2104; found 467.2180 [M+H] +.
Scanning electron microscopy (SEM)
SEM was performed on a JEOL JSM-7600F microscope with an
accelerating voltage of 5 kV. Before SEM measurements, samples
were prepared by depositing dilute solutions of gels on silicon
wafers, followed by drying and coating them with a thin layer of Pt
to increase the contrast.
Circular dichroism (CD) spectra
CD spectra were obtained using JASCO J-1500 CD spectrometer with
bandwidth of 1.0 nm. CD spectra of hydrogels were recorded in the
UV region (190-400 nm) using a 0.1 mm quartz cuvette with the
total gelator concentration at 0.2 wt %.
Fourier transform infrared (FTIR) spectra
FTIR spectra of xerogels were taken using a Shimadzu FT-IR
Instrument. The KBr disk technique was used for the solid-state
measurements. Solution spectra were measured by dropping
dichloromethane solution on KBr wafers and were corrected for
solvent and cell absorption. The samples were scanned between
the wavelengths of 4000 and 400 cm−1 at an interval of 1.9285 cm−1.
Synthesis of DPT
1,4-Bnzenedicarbonyl dichloride (1.01 g, 4.98 mmol) in dry
dichloromethane (10 mL) was added dropwise to
a
dichloromethane solution (20 mL) containing 4-hydroxypyridine
(1.42 g, 14.93 mmol), EDCI (2.24 g, 11.94 mmol) and DMAP (0.06 g,
0.50 mmol). The solution was stirred at room temperature for 12 h.
After filtration, all the solvents were evaporated under vacuum. The
residue was washed with deionized water, and finally dried in the
vacuum oven to give solid DPT (0.96 g, 3.00 mmol, 60.2 %). 1H NMR
(400 MHz, DMSO-d6, ppm): δ = 8.71-8.72 (d, 4H, Ar-H), 8.35 (s, 4H,
Ar-H), 7.26-7.29 (dd, 4H, Ar-H). 13C NMR (100 MHz, DMSO-d6, ppm):
δ = 176.77, 167.36, 140.24, 135.28, 129.82, 116.49. EI-MS (m/z) for
C18H14N2O4 calcd. 320.0797; found 321.0869 [M+H]+.
Vibrational circular dichroism (VCD) spectra
VCD spectra were measured at BioTools, using a ChiralIR-2X Fourier
transform VCD (FT-VCD) spectrometer equipped with an MCT
detector and the Dual PEM option for enhanced VCD baseline
stability. VCD spectra were recorded at a resolution of 4 cm−1 by co-
adding 1000 scans. The gel samples (at a concentration of 2.0
mg/mL) were dried under infrared lamp after coating on CaF2 wafer
and held in a variable path length cell with CaF2 windows.
Synthesis of NPI
4-Carboxylicpyridine (0.61 g, 4.95 mmol) in dry dichloromethane
(10 mL) was added dropwise to a dichloromethane solution (20 mL)
containing amino-4-pyridine (0.75 g, 7.97 mmol), EDCI (0.99 g, 5.16
mmol) and DMAP (0.04 g, 0.33 mmol). The solution was stirred at
room temperature for 12 h. After filtration, all the solvents were
Acknowledgements
This research is jointly supported by NSFC (51573092), the Program
for Professor of Special Appointment (Eastern Scholar) at the
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Chemical Science
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ARTICLE
Journal Name
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6 | J. Name., 2012, 00, 1-3
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Chemical Science
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DOI: 10.1039/C6SC04808K
TOC
Achiral bipyridines can co-assemble with L-phenylalanine
derivatives into unexpected right-handed helical nanostructures
rather than left-handed helix by utilizing intermolecular
hydrogen bonding formed between pyridyl and carboxylic
groups.
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