Dynamic self-assembled polymer: HCl responsive inversion of supramolecular polymer handedness

Article abstract of DOI:10.1039/c7ra08035b

A series of discotic tricarboxyamides with varied amino acid side arm functionality and their HCl responsive diverse self-assembly behavior and formation of dynamic polymer is studied. Discotic trisamide 1 obtained from Boc protected l-Lys self-assembled into a long fibrillar aggregate-like supramolecular P helical polymer. However, on addition of HCl, the supramolecular helical handedness of the assemblies is completely inverted. Irrespective of the chiral centres configuration, supramolecular chiral bias is arising from molecular conformations. FE-SEM reveals the formation of right handed entangled polymeric fibers. However, left handed entangled fibers have appeared on addition of 0.12 M HCl. The trisamide 2 containing Boc protected l-Trp exhibits disk like morphology both in the presence and absence of 0.12 M HCl and does not show a change of supramolecular handedness. This demonstrates how remarkably distinct morphologies originate from stimuli responsive building blocks assembled in a subtly different manner.

Full text of DOI:10.1039/c7ra08035b

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Dynamic self-assembled polymer: HCl responsive
inversion of supramolecular polymer handedness†
Cite this: RSC Adv., 2017, 7, 47170
Arpita Paikar and Debasish Haldar
*
A series of discotic tricarboxyamides with varied amino acid side arm functionality and their HCl responsive
diverse self-assembly behavior and formation of dynamic polymer is studied. Discotic trisamide 1 obtained
from Boc protected L-Lys self-assembled into a long fibrillar aggregate-like supramolecular P helical
polymer. However, on addition of HCl, the supramolecular helical handedness of the assemblies is
completely inverted. Irrespective of the chiral centres configuration, supramolecular chiral bias is arising
from molecular conformations. FE-SEM reveals the formation of right handed entangled polymeric
fibers. However, left handed entangled fibers have appeared on addition of 0.12 M HCl. The trisamide 2
containing Boc protected L-Trp exhibits disk like morphology both in the presence and absence of
Received 20th July 2017
Accepted 3rd October 2017
0
.12 M HCl and does not show a change of supramolecular handedness. This demonstrates how
DOI: 10.1039/c7ra08035b
remarkably distinct morphologies originate from stimuli responsive building blocks assembled in a subtly
rsc.li/rsc-advances
different manner.
chiral nanostructures by simply changing the molecular confor-
mation are relatively rare.
Herein, we report the conformational preferences in the
Introduction
39
Supramolecular polymers are highly interesting for their
dynamic character. Tuning the nature, directionality and spatial trisamide system over the conguration of the chiral center to
arrangement of stimuli responsive building blocks in self- determine the helical handedness of supramolecular polymers.
assembled systems provides chiral supramolecular polymers We have designed and synthesized a series of trisamides con-
1
with new features compared to the bulk materials. Under- taining Boc protected L-lysine (Lys), L-tryptophan (Trp) and
2–4
standing the chirality of supramolecular systems is highly benzene-1,3,5-triamine and investigated their structures and
important for biological sciences as well as material sciences stimuli responsive behavior. Interestingly the trisamide 1 con-
5
mainly in the elds of chiral recognition and sensing, opto- taining (Boc)
-L-Lys formed one-dimensional supramolecular
2
6
7,8
electronic materials, chiroptical switches,
asymmetric helical polymer. On protonation, the supramolecular helical
and separation processes, metal coordination,
9,10
11
12
catalysts
hendedness of the polymeric assemblies has inverted. However,
13
14
15,16
redox reactions, host–guest interactions, anions,
solvent the analogue 2 containing (Boc)-L-Trp exhibits disk like
20,21
1
7
18,19
polarity, concentrations variations,
light irradiation,
high morphology both in presence and absence of 0.12 M HCl and
25,26
22
23
24
pressure and temperature on large discotic, dendritic,
does not show change of supramolecular handedness.
27
and gelator molecules. Feringa, van Esch and coworkers have
reported the cyclohexane based C
symmetric discotic hydro- (picric acid), 1,3,5-triaminobenzene was synthesized in single
gelator containing three amino acid side groups.
Starting from commercially available 1,3,5-trinitrophenol
3
28–31
Meijer and step in 86% yield, by reuxing with ammonium formate and Zn
ꢀ
coworkers have discussed analogous systems with 1,3,5-benzene dust in methanol at 60 C for 1 h (Scheme 1, ESI†). The
32–36
tricarboxylic acid core.
For these compounds, tuning experi-
mental conditions promotes the selection of a predominant
37
assembly. Changing the stereochemistry of the building blocks
as well as experimental condition can control the supramolecular
38
chiral amplication. However, the examples of designer
supramolecular assemblies that can adopt diverse well-dened
Department of Chemical Sciences, Indian Institute of Science Education and Research
Kolkata, Mohanpur, West Bengal 741252, India. E-mail: deba_h76@yahoo.com;
deba_h76@iiserkol.ac.in; Fax: +91 3325873020; Tel: +91 3325873119
†
Electronic supplementary information (ESI) available: Synthesis and
1
13
characterization of compounds, H NMR, C NMR, solid state FTIR spectra,
Fig. S1–S17. See DOI: 10.1039/c7ra08035b
Scheme 1 The trisamides 1 and 2.
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40
trisamides 1 and 2 (Scheme 1) were synthesized by conventional 2 : 1 conformations and formed M helix (Fig. 1a). However,
solution phase DCC (dicyclohexylcarbodiimide) mediated trisamide containing achiral g-aminobutyric acid (Fig. 1b) or
coupling of 1,3,5-triaminobenzene with the (Boc) -L-Lys and chiral methionine (Fig. 1c) adopts 3 : 0 conformations and
2
1
41
(
(
Boc)-L-Trp following a high purity, as conrmed by H-NMR formed P helix.
1
3
nuclear magnetic resonance),
C-NMR, FT-IR (Fourier-
transform infrared) and mass spectrometry (MS) analysis
Results and discussion
3
(ESI†). For C -symmetric trisamides 1 and 2 the design principle
explored was to incorporate chiral amino acid side chains that The supramolecular propensities of the reported trisamides 1
are responsive to externational stimuli like protonation. The and 2 containing chiral (Boc) -L-Lys and (Boc)-L-Trp have been
2
bulky Boc protective groups at side chains may force the amides studied using a wide variety of different spectroscopic tech-
groups out of the plane of the central aromatic core and facili- niques in methanol solution. The typical UV-Vis absorption
tates the formation of three intermolecular hydrogen bonds for spectra of trisamide 1 in methanol (0.13232 mM) show an
stack or dimer structure.
absorption band at 208 nm, responsible for p to p* transition
For trisamide, the hydrogen-bonded self-assembly promotes (Fig. S3, ESI†). However, the 10 nm bathochromic shi of the
mainly dimers or stacks, which can be controlled by the nature absorption band with increasing concentration indicates J-type
2
of the substituent or experimental conditions (concentration) or stacking between the trisamide 1 molecules. There is also
external stimuli such as temperature, magnetic eld, metal another band at 360 nm. Compairing with studies, we conclude
39
ions. Depending on the ratio of the number of pairs of that trisamide 1 formed one-dimensional supramolecular
hydrogen bond dipole vectors which are parallel or antiparallel columnar stacks. The absorption spectra of trisamide 2 in
to each other, the hydrogen-bonded stack of trisamide can methanol (0.06788 mM) also show an absorption bands at
39
exists in two conformations, 3 : 0 and 2 : 1. In 3 : 0 stacking, 208 nm and 280 nm but only intensity increase with increasing
all the amide oxygens attached to central core are oriented in concentration (Fig. S4, ESI†). Hence the trisamides 1 and 2 have
the same direction with respect to the central benzene plane different aggregation propensities.
(Fig. S1, ESI†). However, in 2 : 1 stacking, two amide oxygens
The methanol solution of trisamide 1 is light green in colour.
attached to central core are on one side and the third one is on With increasing trisamide 1 concentration in methanol solu-
39
other side of the central benzene plane (Fig. S2, ESI†). From tion the intensity of the colour increases. But we observed that
our previous reports, by X-ray crystallography it has proved that on addition of 0.12 M HCl the solution become colourless (inset
3
: 0 conformation stack in right handed helical manner (P of Fig. 2a). There is a possibility of Boc-NH groups deprotection
helix) and the 2 : 1 conformation stack in le handed helical in presence of acid. But in the solution phase synthesis proce-
manner (M helix) (Fig. 1). This is irrespective on the chirality of dure we have work up the trisamides with 2 M HCl and there is
the side chains. A trisamide containing achiral b-alanine adopts no Boc-NH groups de protection as it was conrmed by
1H-NMR, 13C-NMR, FT-IR and mass spectrometry analysis. So,
the self-assembly of the trisamide 1 is responsive towards HCl.
We have further explore this phenomenon using a wide variety
of different spectroscopic techniques. The UV-Vis titration of
ꢂ4
trisamide 1 methanol solution (1.984 ꢁ 10 M) with gradual
addition of 0.12 M HCl shows the gradual increase of the
absorption band intensity at 208 nm (Fig. 2a). However, the
absorption band at 360 nm disappeared (Fig. 2a). On the other
hand, very light green colour of trisamide 2 in methanol solu-
tion become intense green colour by addition of 0.12 M HCl
(
inset of Fig. 2b). The UV-Vis titration of trisamide 2 methanol
ꢂ5
solution (3.451 ꢁ 10 M) with gradual addition of HCl shows
the gradual increase of the absorption bands intensities at
208 nm and 280 nm (Fig. 2b). So, protonation can tuned self-
assembly of the trisamides.
FT-IR spectroscopy is an excellent technique to investigate
the HCl responsive self-assembly of trisamide 1. The region
ꢂ1
3
500–3200 cm is important for the N–H stretching vibrations
ꢂ1
and 1800–1500 cm corresponding to C]O stretching vibra-
tion. The intense band at 3382 cm is for six Boc-NH groups
which are hydrogen bonded (Fig. 3a). The band at 3201 cm is
ꢂ1
ꢂ1
responsible for three core amide NH groups which are hydrogen
Fig. 1 (a) M helix from a trisamide with 2 : 1 conformations containing
achiral b-alanine. (b) P helix by a trisamide with 3 : 0 conformations
containing achiral g-aminobutyric acid. (c) The trisamide containing
chiral methionine (pink balls) adopts 3 : 0 conformations and formed P
helix. Amino acids side chains are omitted for clarity.
bonded to form the columnar stack (Fig. 3a). The trisamide 1
ꢂ1
shows the amide I band at 1682 cm and amide II band at
ꢂ1
1
523 cm . On addition of HCl the N–H stretching vibrations
ꢂ1
ꢂ1
appears at 3406 cm (Fig. 3a). The 3201 cm brand is now very
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Fig. 2 The UV-Vis titration of (a) trisamide 1 and (b) trisamide 2
ꢂ4
ꢂ5
Fig. 3 (a) FT-IR spectra of trisamide 1 in absence of HCl (black) and in
presence of HCl (red). (b) The acid dependence of NH chemical shifts
of trisamide 1 at varying concentrations of HCl. Star: core CONH,
circle: core aromatic Hs, square: Lys side chain NHs, triangle: Lys NHs.
methanol solution (1.984 ꢁ 10 M) and (3.451 ꢁ 10 M) respectively
with gradual addition of 0.12 M HCl. Inset: colour change by addition
of HCl.
broad. The amide I bands are now split in 1 : 2 ratio and appear
ꢂ1
ꢂ5
at 1689 and 1640 cm (Fig. 3a). The intensity of amide II band (1.474 ꢁ 10 M) is suggesting a supramolecular right handed
ꢂ1
at 1523 cm is very low (Fig. 3a). Fig. S5, ESI† shows the FT-IR helix (Fig. 4a) which is consistent with the handedness of the
spectra of 2.
helices formed by the tricarboxyamide containing L-methionine
41
Moreover, to investigate the effect of HCl on trisamide 1, the methyl ester with 3 : 0 conformation in the crystalline state.
ꢂ5
NMR titration experiments were performed. To a solution of On addition of HCl, (4 ꢁ 10 M) the helical handedness of the
ꢂ2
compound 1 1.082 ꢁ 10 (M) in CD
3
OD the 0.57 (M) HCl supramolecular polymer reverses to negative cotton effect
solution was gradually added by an amount of 10 mL at room (Fig. 4a). The CD spectra is suggesting a le handed supramo-
temperature and the NMR spectra were recorded. The results of lecular polymer which is consistent with the handedness of the
the NMR titrations of trisamide 1 have been shown in the helices formed by the tricarboxyamide containing b-alanine
40
Fig. 3b. The NMR titrations data show that the NH protons get methyl ester with 2 : 1 conformation in crystal. This suggest
exposed to the acid and there are signicant shi of NH protons that only 2.713 equivalent HCl is enough for the chirality
for the trisamide 1 with gradually increase in HCl concentra- change of tricarboxamide 1. However the trisamide 2 in meth-
ꢂ5
tion. From the stack plot, the core amide NH exhibits upeld anol (1.474 ꢁ 10 M) has a positive band at 185 nm which does
ꢂ5
shi suggest the strong hydrogen bond formation. Whereas the not change aer addition of HCL (4 ꢁ 10 M) (Fig. 4b). There
protons of central aromatic core show no shi i.e. the stacking also a negative band at 200 nm and 217 nm which does not shi
structure is not changing. The Boc protected NHs of Lys exhibits in presence of acid (Fig. 4b). The CD experiments also per-
downeld shi as well as broadening suggest that these NHs are formed with trisamide containing achiral b-alanine and g-
open to acid. Fig. S6, ESI† shows the acid dependence NH aminobutyric acid in absence and presence of HCl. The g-
chemical shis of 2.
aminobutyric acid containing trisamide shows a positive band
In order to gain more information on the structural changes at 210 nm which does not shi in presence of acid (Fig. S7,
of these trisamides in presence of acid, we performed circular ESI†). Another trisamide containing b-alanine shows a negative
dichroism (CD) measurements. The positive cotton effect band at 200 nm and aer addition of HCl it remain in the same
observed (185 nm) in the CD spectra of trisamide 1 in methanol conformation (Fig. S8, ESI†). To examine the supramolecular
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Fig. 6 AFM images of helical supramolecular polymer of trisamide 1 (a)
without acid, (b) after addition of 0.12 M HCl.
The morphology of trisamide 1 was observed by eld emis-
sion scanning electron microscopy (FE-SEM). The analysis of
the morphology of trisamide 1 by FE-SEM reveals the formation
of right handed entangled polymeric bers (Fig. 5b). Aer
addition of 0.12 M HCl, le handed entangled bers have
appeared. The trisamide 2 exhibits disk like morphology
(
Fig. 5c) which is consistent with the morphology of the tri-
42
carboxyamides that formed dimer in crystal. On addition of
.12 M HCl, the trisamide 2 does not exhibit signicant change,
0
only the disks are talking to each other (Fig. 5d).
To investigate the topology of the supramolecular polymer,
atomic force microscopic (AFM) studies were performed. The
trisamide 1 solutions in methanol were drop-casted on
a microscopic glass cover slip and investigated by AFM. The
trisamide 1 shows right handed helical supramolecular polymer
Fig. 4 (a) CD spectra of trisamide 1 in methanol without HCl (black)
and in presence of HCl (red). (b) CD spectra of trisamide 2 in methanol
without HCl (black) and in presence of HCl (red).

bers (Fig. 6a). However, aer addition of 0.12 M HCl, it shows
le handed helical bers (Fig. 6b).
polymer formation, we have performed the DOSY NMR experi-
ment with trisamide 1. From DOSY experiment, trisamide 1 have
ꢂ14
2
ꢂ1
viscosity coefficient 3.047 ꢁ 10
m s , however aer addition
ꢂ14
2
ꢂ1
Experimental
General methods and materials
of HCl it shows viscosity coefficient 2.416 ꢁ 10
m s . This is
due to the supramolecular polymer formation (Fig. S9, ESI†).
All L-amino acids (L-lysine, L-tryptophan, and trinitrophenol)
were purchased from Sigma chemicals. HOBt (1-hydrox-
ybenzotriazole) and DCC (dicyclohexylcarbodiimide) were
purchased from SRL. Synthesis. The peptides were synthesized
by conventional solution-phase methodology by using a race-
mization free fragment condensation strategy. The Boc group
was used for N-terminal protection and. Couplings were
mediated by dicyclohexylcarbodiimide/1-hydroxybenzotriazole
(DCC/HOBt). The products were puried by column chroma-
tography using silica gel (100–200 mesh) as the stationary phase
and n-hexane–ethyl acetate mixture as eluent. All the interme-
1
diates were characterized by 500 MHz and 400 MHz H NMR
and mass spectrometry. The nal compound was fully charac-
1
13
terized by 500 MHz and 400 MHz H NMR spectroscopy,
C
NMR spectroscopy (125 MHz, 100 MHz), mass spectrometry,
and IR spectroscopy.
(
a) Boc
a mixture of dioxane (20 mL), water (10 mL) and 1 M NaOH
10 mL) was stirred and cooled in an ice-water bath. Di-tert-
2
-Lys-OH. A solution of L-lysine (1.46 g, 10 mmol) in
(
Fig. 5 FE-SEM images of trisamide 1 (a) without acid, (b) after addition
of HCl. (c) and (d) FE-SEM images of trisamide 2 without HCl and with
butylpyrocarbonate (4.8 g, 22 mmol) was added and stirring was
continued at room temperature for 6 h. Then the solution was
0
.12 M HCl respectively.
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concentrated under vacuum to about 20–30 mL, cooled in an by silica gel column (100–200 mesh size) using ethyl acetate and
1
ice-water bath, covered with a layer of ethyl acetate (about hexane (3 : 1) as eluent. Yield: 1.562 g (1.41 mmol, 53.4%). H
5
0 mL) and acidied with a dilute solution of KHSO to pH 2–3 NMR (CDCl , 500 MHz, d in ppm): 6.568 (s, 3H, triamine NH
4
3
(
Congo red). The aqueous phase was extracted with ethyl acetate protons), 6.0317 (s, 3H, TAB), 5.4141 (s, 3H, NH), 4.1120 (s, 3H,
a
and this operation was done repeatedly. The ethyl acetate NH Boc), 3.0823–3.0747 (d, 3H, J ¼ 1.92 C H Lys), 1.8325–1.7632
b+g+u
extracts were pooled, washed with water and dried over anhy- and 1.6800–1.6012 (2m, 18H, C
H of Lys), 1.3781 (s, 54H,
1
3
drous Na
2
SO
4
and evaporated under vacuum. The pure material Boc). C NMR (CDCl
3
, 125 MHz, d in ppm): 175.5228, 156.6967,
was obtained as a waxy solid. Yield: 2.98 g (8.64 mmol, 86.4%). 156.1335, 80.3308, 79.6014, 54.4205, 40.3461, 40.3461, 34.0851,
1
H NMR (DMSO-d
6
, 400 MHz, d in ppm): 12.32 (b, 1H, COOH), 32.3809, 30.0484, 22.9715.
.0172–6.9981 (d, 1H, NH, J ¼ 3.82), 6.7824–6.7576 (t, 1H, NH), (e) Trisamide 2. 325 mg (2.64 mmol) triaminobenzene was
.0496–3.9962 (m, 1H, Lys C H), 2.9179–2.8683 (m, 2H, Lys dissolved in 25 mL dry DMF. 2.41 g and (7.91 mmol) Boc-Trp-
7
4
a
u
b
C H), 1.6202–1.5076 (m, 2H, Lys C H), 1.3702 (s, 18H, BOC Hs), OH, 1.65 g (8 mmol) DCC and 1.08 g (8 mmol) HOBt were
g
13
1
.3397–1.2309 (m, 4H, Lys C H). C NMR (DMSO-d
d in ppm): 173.2634, 155.6395, 78.9982, 66.3907, 53.4861, solution was added to 200 mL water, taken in a separating
0.4427, 29.0924, 28.2330, 22.9325. funnel and shaken for 15 min and 150 mL ethyl acetate was
b) Boc-Trp-OH. A solution of L-tryptophan (2.04 g, 10 added and shaken for another 10 min. The organic layer was
6
, 125 MHz, added and stirred for 48 h at room temperature. Then the
3
(
mmol) in a mixture of dioxane (20 mL), water (10 mL), and 1 N collected and DCU was ltered off. The organic layer was
NaOH (10 mL) was stirred and cooled in an ice-water bath. Di- washed with 2 M HCl (3 ꢁ 50 mL), brine (2 ꢁ 50 mL), then 1 M
tert-butylpyrocarbonate (2.6 g, 12 mmol) was added and stirring sodium carbonate (3 ꢁ 50 mL) and brine (2 ꢁ 50 mL) and dried
was continued at room temperature for 6 h. Then, the solution over anhydrous sodium sulfate and evaporated under vacuum
was concentrated in vacuum to about 10–15 mL, cooled in an to yield trisamide 1 as a yellow solid. Purication was performed
ice-water bath, covered with a layer of ethyl acetate (about by silica gel column (100–200 mesh size) using ethyl acetate and
1
5
0 mL), and acidied with a dilute solution of KHSO
The aqueous phase was extracted with ethyl acetate and this NMR (DMSO-d
operation was done repeatedly. The ethyl acetate extracts were 7.6088–7.5897 (d, 3H, triamine NH protons, J ¼ 3.82), 7.3740 (s,
pooled, washed with water, and dried over anhydrous Na SO
3H, TAB), 7.3244–7.3053 (d, 3H, Trp aro protons, J ¼ 3.82)
and evaporated in a vacuum. The pure material was obtained as 7.1183 (s, 3H, Boc NH), 7.0687–6.9465 (m, 9H, Trp aro protons),
4
to pH 2–3. hexane (4: 1) as eluent. Yield: 1.936 g (1.97 mmol, 74.6%). H
6
, 400 MHz, d in ppm): 10.79 (s, 3H, indole NH),
2
4
1
a
a solid. Yield: 2.31 g (7.56 mmol, 75.6%). H NMR (500 MHz, 4.1565–4.1011 (m, 3H, C H Trp), 3.0859–3.0382 and 2.9122–
b
13
DMSO-d
6
, d in ppm): 12.53 [b, 1H, COOH], 10.89 [b, 1H, NH, 2.8531 (2m, 6H, C H of Trp), 1.3092 (s, 27H, Boc). C NMR
, 100 MHz, d in ppm): 174.8672, 156.2366, 136.7765,
.14 [d, 1H, CH, J ¼ 5], 7.06–7.0 [t, 2H, CH], 4.72 [m, 1H, C H], 127.5184, 123.7809, 122.7225, 120.2054, 119.3092, 111.7387,
indole], 7.5 [s, 1H, amide NH], 7.33–7.32 [d, 1H, CH, J ¼ 5], 7.15– (DMSO-d
6
a
7
3
b
b
13
.12 [m, 1H, C H], 2.99 [m, 1H, C H], 1.3 [s, 9H, BOC] C NMR 80.5606, 55.1986, 28.7687.
, d in ppm): 174.03, 155.45, 136.13, 127.22,
(125 MHz, DMSO-d
6
1
2
23.69, 120.94, 118.39, 118.19, 111.43, 110.2, 78.05, 54.56,
8.21, 28.1, 27.81, 26.85.
(
NMR experiments
All NMR studies were carried out on a Bruker AVANCE 500 MHz
and Jeol 400 MHz spectrometer at 278 K. Compound concen-
trations were in the range 1–10 mM in CDCl and DMSO-d .
3 6
c) 1,3,5-Triaminobenzene. 1.145 g (5 mmol) of picric acid
was dissolved in 50 mL MeOH taken in a 100 mL round bottom
ux. To this solution 3.78 g (60 mmol) ammonium formate and

1
.96 g (30 mmol) Zn dust were added and the mixture was
ꢀ
FT-IR spectroscopy
reuxed at 60 C for 1 h. The reaction mixture was cooled to
room temperature and ltered. The ltrate was evaporated
under vacuum to provide tri-aminobenzene as black solid.
All reported solution FT-IR spectra were obtained with a Perkin
Elmer Spectrum RX1 spectrophotometer.
1
Yield: 528.8 mg (4.29 mmol, 85.8%). H NMR (DMSO-d , 400
6
MHz, d in ppm): 8.300 (s, 3H, aromatic protons), 5.418 (b, 6H,
UV-Vis spectroscopy
1
3
NH
2
). C NMR (DMSO-d
d) Trisamide 1. 325 mg (2.64 mmol) triaminobenzene was
dissolved in 25 mL dry DMF. 2.73 g and (7.91 mmol) Boc -Lys-
OH, 1.65 g (8 mmol) DCC and 1.08 g (8 mmol) HOBt were
6
, 100 MHz, d in ppm): 166.35, 125.33.
UV-Vis absorption spectra were recorded on a UV-Vis spectro-
photometer (Hitachi).
(
2
added and stirred for 48 h at room temperature. Then the Mass spectra
solution was added to 200 mL water, taken in a separating
Mass spectra were recorded on a Q-Tof Micro YA263 high
resolution (Waters Corporation) mass spectrometer by positive-
mode electrospray ionization.
funnel and shaken for 15 min and 150 mL ethyl acetate was
added and shaken for another 10 min. The organic layer was
collected and DCU was ltered off. The organic layer was
washed with 2 M HCl (3 ꢁ 50 mL), brine (2 ꢁ 50 mL), then 1 M
sodium carbonate (3 ꢁ 50 mL) and brine (2 ꢁ 50 mL) and dried
CD spectroscopy
over anhydrous sodium sulfate and evaporated under vacuum Circular dichroism absorption spectra were recorded Jasco
to yield trisamide 1 as a yellow solid. Purication was performed J-815 CD spectrophotometer.
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Atomic force microscopy
The morphology of the reported compound was investigated by
atomic force microscopy (AFM). A drop of the sample solution
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ꢀ
to dry under vacuum at 30 C for two days. Images were taken 10 M. Raynal, F. Portier, P. W. N. M. van Leeuwen and
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1
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In summary, we have shown the addition of HCl reverses the
helical handedness of the supramolecular polymer obtained
from the self-assembly of chiral amino acid modied
3
, 68–73.
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3
C -symmetric trisamide. The intermolecular three fold
1
1
1
1
hydrogen bonds break and amide functional groups re-
organized in the opposite side of central aromatic core by
virtue of interaction with HCl. Irrespective of chiral centres
conguration, supramolecular chiral bias is arising from
molecular conformations. FE-SEM reveals the formation of
right handed entangled polymeric bers. However, le handed
entangled bers have appeared on addition of 0.12 M HCl. The
trisamide 2 containing Boc protected L-Trp exhibits disk like
morphology both in presence and absence of 0.12 M HCl and
does not show change of supramolecular handedness. The acid
responsive supramolecular chirality inversion strategy has
potential for other building blocks that form multiple
competing chiral supramolecular structures.
2
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3 Z. Huang, S.-K. Kang, M. Banno, T. Yamaguchi, D. Lee,
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We acknowledge Dr Rangeet Bhattacharyya, Department of
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Chemical Sciences, Indian Institute of Science education and
Research Kolkata for their help in DOSY experiments.
2
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