Synthesis and solvent-controlled self-assembly of diketopiperazine-based polyamides from aspartame

Article abstract of DOI:10.1039/d0ra10086b

An aspartame-based AB-type diketopiperazine monomer, cyclo(l-aspartyl-4-amino-l-phenylalanyl) (ADKP), was synthesized and subsequently utilized in the polycondensation of homo-polyamides with high molecular weights. By using various amino acids, dicarboxylic acids, and diamines, random DKP-based copolymers were also synthesized. The self-assembly properties of ADKP and poly(cyclo(l-aspartyl-4-amino-l-phenylalanyl)) (PA1) were studiedviathe solvent displacement method. Notably, PA1 self-assembled into particles with various morphologies in different solvent systems, such as irregular networks, ellipsoids, and hollow particles. The morphological transformation was also confirmed by dropping acetone and toluene onto the PA1 particles. Furthermore, infrared spectra and Hansen solubility parameters of PA1 and different solvents revealed the particle formation mechanism, which provided more insights into the relationship between the morphology and strength of the hydrogen bonding of each solvent.

Full text of DOI:10.1039/d0ra10086b

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Synthesis and solvent-controlled self-assembly of
diketopiperazine-based polyamides from
aspartame†
Cite this: RSC Adv., 2021, 11, 5938
a
a
a
Hongrong Yin,
Kenji Takada,
Amit Kumar,
Thawinda Hirayamab
a
*
and Tatsuo Kaneko
An aspartame-based AB-type diketopiperazine monomer, cyclo(L-aspartyl-4-amino-L-phenylalanyl)
(ADKP), was synthesized and subsequently utilized in the polycondensation of homo-polyamides with
high molecular weights. By using various amino acids, dicarboxylic acids, and diamines, random DKP-
based copolymers were also synthesized. The self-assembly properties of ADKP and poly(cyclo(L-
aspartyl-4-amino-L-phenylalanyl)) (PA1) were studied via the solvent displacement method. Notably, PA1
self-assembled into particles with various morphologies in different solvent systems, such as irregular
networks, ellipsoids, and hollow particles. The morphological transformation was also confirmed by
dropping acetone and toluene onto the PA1 particles. Furthermore, infrared spectra and Hansen
solubility parameters of PA1 and different solvents revealed the particle formation mechanism, which
provided more insights into the relationship between the morphology and strength of the hydrogen
bonding of each solvent.
Received 30th November 2020
Accepted 28th January 2021
DOI: 10.1039/d0ra10086b
rsc.li/rsc-advances
DKP units in polymer chains may lead to unique self-
assembly.22 However, only a few DKP-based polymers have been
Introduction
reported, and their self-assembly properties have not been
adequately studied.20–24 The main problem in the development
of DKP-based polymers is the low solubility, which induces
lower molecular weight and complicates property evaluation.
Thus, the design of DKP monomers with high solubility is
important for obtaining self-assembled DKP polymers. Almost
all DKP monomers used in the literature have been obtained
from the homo-coupling of amino acids, which exhibit high
symmetry and tend to increase the crystallinity and decrease the
solubility in many solvents. The use of asymmetric AB-type
monomers of two different amino acids is envisaged to be one
of the efficient methods to control solubility.16,25,26 Masuda et al.
reported various synthesis approaches of DKP-based polymers
and conrmed that the hydrogen bonding of DKP moieties
affected polymer properties.23 We recently reported DKP-based
polyimides whose particles showed high heat resistance and
polymorphism properties.24 However, the self-assembly mech-
anism of the DKP moiety in polymers remains unclear, while
self-assembly is a bottom-up formation of ordered structures
under thermodynamic and kinetic conditions.5,27–30 The self-
assembly behavior of DKPs with low molecular weight has
been reported to be deeply affected by conditions and envi-
ronment. In a solution system, DKP–DKP interactions always
compete with DKP-solvent interactions; therefore, investigating
the interaction nature of both DKP-based polymers and solvents
is important.31,32
Amino acids are important renewable resources for food,
medical, and biological studies, as well as synthetic materials.1–5
Amino acids and their derivatives have been used as monomers
for functional and high-performance polymers.6–8 Notably, 2,5-
diketopiperazines (DKPs), which are cyclic a-amino acid
dimers, contain two centrosymmetric s-cis-amide bonds in a six-
membered ring. DKPs have multiple hydrogen bonding
acceptor and donor sites in a rigid ring, which enable the
formation of regular molecular assembly through continuous
intermolecular hydrogen bonding.9–12 This remarkable DKP
self-assembly behavior has been attracting the attention of
scientists in crystal engineering, gelators, and supramolecular
architectures.13–19 As described above, DKPs exhibit a unique
particle morphology, even when they are used as monomers; it
can be expected that they still exhibit such morphologies when
they adopt a polymer structure. In addition, the rigid structure
of DKP may impart high thermal and mechanical properties to
polymeric materials.20,21 The continuous hydrogen bonding of
aEnergy and Environment Area, Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan. E-mail: kaneko@jaist.ac.jp
bDepartment of Chemistry, Faculty of Science, Chulalongkorn University, 254
Phayathai Road, Pathumwan, Bangkok 10330, Thailand
† Electronic supplementary information (ESI) available: 1H NMR spectra, 13C
NMR spectrum and FTIR spectra of obtained monomer. 1H NMR spectrum,
FTIR spectrum, TGA curves XRD spectrum and table of solubilities of PA1 or
PAs. See DOI: 10.1039/d0ra10086b
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The L-aspartyl-L-phenylalanine methyl ester is a synthetic an acceleration voltage of 10 kV and an emission current of 10
sweetener that is commercially known as aspartame; it is one of mA. Hansen solubility parameters of polymer and solvents are
the most highly produced peptides worldwide.33,34 Further, it is calculated by Hansen Solubility Parameters in Practice (HSPiP)
known that intramolecular cyclocondensation of aspartame 5th Edition soware.
occurs through heating, and aspartame diketopiperazine (APM-
DKP) with an asymmetric structure is produced.35,36 Therefore,
Synthesis of cyclo(L-aspartyl-L-phenylalanyl) (AMP–DKP)
aspartame can be a promising candidate for synthesizing the
L-Aspartyl-L-phenylalanine methyl ester (5.86 g, 20.0 mmol) was
above-mentioned highly soluble DKP and its polymer.
dissolved in DMSO (20 mL) and stirred at 80 ꢁC for 8 h. The
The present article deals with the synthesis of an AB-type
resulting solution was added into a mixture of acetone (300 mL)
DKP with high solubility using a simple and commercially
and hexane (300 mL) to obtain a precipitate. The white precip-
available method. The obtained DKP monomer was applied to
itateꢁwas obtained by ltration and dried in a vacuum oven at
the polymerization reactions of polyamides with or without
100 C with a yield of 4.61 g (88%).
using other monomers. Moreover, the morphological control
and self-assembly mechanism of DKP-based polyamides were
investigated.
Synthesis of cyclo(L-aspartyl-4-nitro-L-phenylalanyl) (NDKP)
Here, APM-DKP (1.30 g, 4.96 mmol) was dissolved in concen-
trated sulfuric acid (30 mL) in an ice bath. A prepared solution
mixture of nitric acid (1 mL) and sulfuric acid (4 mL) was added
dropwise to the APM-DKP sulfuric acid solution at 0 ꢁC.
Experimental section
Material
Subsequently, the solution was stirred for 5 min. The resulting
L-Aspartyl-L-phenylalanine methyl ester was supplied by Ajino-
solution was poured into ice water (300 mL) dropwise and the
pH was adjusted to approximately 5 by adding sodium
hydroxide solution. Thereaer, the solution was extracted with
moto Co., Inc. Triphenyl phosphite (P(OPh)3), L-phenylalanine
(Phe), succinic acid, ethylene diamine and p-phenylenediamine
were purchased from Tokyo Chemical Industry Co., Ltd. 5%
Palladium on activated carbon (Pd/C) and N-methylpyrrolidone
ethyl acetate (200 mL) three times and vapor was removed using
a vacuum evaporator and vacuum oven at 100 ꢁC. The objective
(NMP) were purchased from FUJIFILM Wako Pure Chemical
product was obtained as a white powder with a 73% yield(1.10
Corporation. Dimethyl sulfoxide (DMSO), sulfuric acid, nitric
acid, pyridine, ethanol, tetrahydrofuran, ethyl acetate, acetone
and toluene were purchased from Kanto Chemical Co., Inc. All
the chemicals were directly used as purchased.
g). 1H NMR (400 MHz, DMSO-d6, d, ppm): 2.07 (dd, 1H, J ¼ 5.9,
16.6 Hz), 2.28 (dd, 1H, J ¼ 5.4, 16.6 Hz), 3.10 (dd, 1H, J ¼ 5.3,
14.0 Hz), 3.24 (dd, 1H, J ¼ 4.8, 13.9 Hz), 4.11 (t, 1H, J ¼ 5.6 Hz),
4.34 (t, 1H, J ¼ 5.1 Hz), 7.49 (d, 2H, J ¼ 8.7 Hz), 8.1 (s, 1H), 8.17
(d, 1H, J ¼ 8.7 Hz), 8.23 (s, 1H).
Measurements
1H NMR and 13C NMR spectra were performed by a Bruker
Synthesis of cyclo(L-aspartyl-4-amino-L-phenylalanyl) (ADKP)
Biospin AG 400 MHz, 54 mm spectrometer using DMSO-d6 as
NDKP (0.50 g, 1.63 mmol) not subjected to purication was
the solvent. The FT-IR spectra were recorded with a Perkin-
dissolved in pure water, and Pd/C (0.11 g, 22 wt%) was added.
Elmer Spectrum One spectrometer between 4000 and
The mixture was connected with a hydrogen generator and
400 cmꢀ1 using a diamond-attenuated total reection (ATR)
reacted with hydrogen at a feed rate of 500 mL minꢀ1 at 50 C
ꢁ
accessory. The mass spectra were measured using a FT-ICR MS
(Solarix) equipped with a Nanospray source operating in the
nebulizer-assisted ESI mode used in the negative ion mode and
scanned from m/z 50 to m/z 1000. The number-average molec-
ular weight (Mn), weight-average molecular weight (Mw) and the
molecular weight distribution (Mw/Mn) were determined by gel
for 6 h. The resulting mixture was ltered and evaporated under
vacuum. The resulting solid was recrystallized in water and
dried in a vacuum oven at 100 ꢁC with an 88% yield (0.40 g). 1H
NMR (400 MHz, DMSO-d6, d, ppm): 1.63 (dd, 1H, J ¼ 7.4, 11.9
Hz), 2.12 (dd, 1H, J ¼ 5.1, 16.4 Hz), 2.72 (dd, 1H, J ¼ 4.6, 13.8
Hz), 2.91 (dd, 1H, J ¼ 4.2, 13.7 Hz), 4.01 (t, 1H, J ¼ 6.0 Hz), 4.05
(t, 1H, J ¼ 4.7 Hz), 6.46 (d, 2H, J ¼ 8.3 Hz), 6.80 (d, 2H, J ¼ 8.3
Hz), 7.83 (s, 1H), 7.98 (s, 1H). FT-ICR MS (ESI): calcd for
[C13H14N3O4]ꢀ, 276.10626, found 276.09867.
permeation chromatography (GPC, concentration 1 g Lꢀ1
,
10 mM LiBr/DMF eluent) aer calibration with polystyrene
standards using two of OHpack SB-806M HQ column (Shodex).
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out by Seiko Instruments SII,
SSC/5200 and Seiko Instruments SII, X-DSC7000T, respectively,
Syntheses of ADKP-based polyamides (PAs)
ꢀ1
ꢁ
at a heating rate of 5 C min under a nitrogen atmosphere. A typical polymerization procedure for DKP-based polyamides
Remaining solvent and absorbed moisture in polymer samples is shown below. ADKP (0.055 g, 0.2 mmol) was mixed with
were removed at 200 oC for 1 hour before TGA and DSC P(OPh)3 (40 mL, 0.15 mmol), pyridine (50 mL, 0.6 mmolꢁ), and
measurement. Monomer and polymer particles morphology NMP (100 mL). The reaction solution was stirred at 100 C for
were characterized with scanning electron microscope (JCM- 48 h under nitrogen atmosphere. Aer the reaction nished, the
6000Plus Versatile Benchtop SEM). Samples were coated with resulting solution was precipitated in acetone, and the resulting
a layer of gold in thickness of 15 nm by a sputter coater solid was washed with acetone and dried in a vacuum oven at
(Magnetron sputter MSP-1S). SEM instrument was operated at 100 ꢁC to give PA1 as a yellow powder with an 84% yield.
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Following the same protocol, ADKP (0.027 g, 0.1 mmol) was bond, although monomer solubility is a key factor for efficient
treated with other monomers such as L-phenylalanine (0.016 g, polymerization. Almost all of the reported AA-type DKP mono-
0.1 mmol) to synthesize PA2 with a 79% yield; further, succinic mers exhibiting low solubility were converted into polymers
acid (0.012 g, 0.1 mmol) was treated with ethylene diamine (67 with low molecular weights and poor performances. To solve
mL, 0.1 mmol) to synthesize PA3 with a 81% yield, and succinic this problem, we designed a new AB-type DKP monomer that
acid (0.012 g, 0.1 mmol) was treated with p-phenylenediamine was expected to demonstrate high solubility. The AB-type DKP-
(0.011 g, 0.1 mmol) to synthesize PA4 with a 84% yield.
based monomer ADKP was obtained from the L-aspartyl-L-
phenylalanine methyl ester, following intramolecular conden-
sation, nitration, and reduction (Scheme 1). Notably, aer the
nitration of APM-DKP, both para-substituted and ortho-
substituted products were conrmed (Fig. S1†). The ratio of the
para-substituted product to the ortho-substituted product was
9 : 1, as calculated by NMR. Recrystallization of the crude
product could not completely remove the ortho-substituted
product. However, the reduction product could be easily
recrystallized to give pure para-substituted ADKP. The structure
of ADKP was conrmed by 1H NMR, 13C NMR, FTIR and ESI-MS
(Fig. S1–S3†).
As a result, the obtained ADKP showed high solubility in
water and polar organic solvents such as methanol, NMP, DMF,
and DMSO (Table 1). The high solubility of ADKP is attributable
to the asymmetric structure, which increases the solvation
entropy of ADKP.37 Otherwise, the existence of ionic amine and
carboxylic groups can contribute to solubility, which disturbs
the formation of intermolecular hydrogen bonds between the
DKP moieties in solvated states.38
Preparation of particles of ADKP and ADKP-based polyamides
The obtained ADKP or PA1 (ca. 2.5 mg) were dissolved in 1 mL
DMSO as a stock solution. Next, a 0.1 mL stock solution was
dispersed into 3 mL of different poor solvents, such as water,
ethanol, THF, ethyl acetate, acetone, and toluene, under
vigorous magnetic stirring. A droplet (3 mL) of the dispersion
liquid was dropped on a glass slide and air-dried to assess the
SEM images.
In the secondary solvent treatment route 1, 0.1 mL PA1 stock
solution was dispersed into 3 mL of toluene under stirring. A
droplet (3 mL) of the dispersion liquid was dropped on a glass
slide and dried. Aer drying, a droplet (3 mL) of acetone was
dropped on the sample glass slide and dried for the SEM
observation. In route 2, similarly, 0.1 mL PA1 stock solution was
dispersed into 3 mL of acetone under stirring. A droplet (3 mL) of
the dispersion liquid was dropped on a glass slide and dried.
Aer drying, a droplet (3 mL) of toluene was dropped on the
sample glass slide and dried for the SEM observation. In route 3,
0.1 mL PA1 stock solution was dispersed into 3 mL of acetone
under stirring. A droplet (3 mL) of the dispersion liquid was
dropped on a glass slide and a droplet (3 mL) of toluene was
added immediately. Aer drying, a part of the sample was
observed by SEM. And a droplet (3 mL) of acetone was dropped
on another part of sample and dried for SEM observation.
The drying process of all samples was under air humidity of
Polymer syntheses
PAs were prepared by homo-polymerization or co-
polymerization of ADKP as an AB-type monomer (with or
without other comonomers) using P(OPh)3 and pyridine as
condensation reagents (Scheme 2). 1H NMR spectra of the
obtained PAs showed a new peak at 9.5 ppm due to the phenyl
amide proton, and amide protons in the DKP moiety remained
unchanged at 7.9 and 8.1 ppm which conrmed the presence
of the ADKP moiety in all PAs, including co-polymers PA2, PA3
and PA4 (see Fig. S4†). In addition, the Mw and Mn of PAs have
been determined by GPC (Fig. S5†), which are summarized in
ꢁ
about 40–45% and temperature of 25 C.
Results and discussion
Monomer syntheses
The solubility of DKP derivatives is low due to the rigid six- Table 2. Based on these results, the polymerizations of ADKP
membered ring and the strong intermolecular hydrogen successfully proceeded, and the molecular weights were
Scheme 1 Synthesis of AB-type DKP-based monomer from aspartame.
Table 1 Solubility of NDKP and ADKPa
Water
Methanol
Ethanol
Acetone
DMSO
DMF
DMAc
NMP
NDKP
ADKP
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
+
+
+
+
+
+
+
+
+
+
+
a
Conditions; samples, 10 mg; solvent, 2 mL; temperature, r.t.; (+), soluble; (ꢂ), partially soluble; (ꢀ), insoluble.
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Scheme 2 Synthesis of DKP-based polyamides from cyclo(L-aspartyl-4-amino-L-phenylalanyl).
observed to be in the range of 1.59–1.98 ꢃ 104 g molꢀ1; the Mw/ assembling behaviors that originate from small DKP deriva-
Mn ratio was 1.17–1.36. Moreover, all the GPC traces showed tives. In this study, the simple DKP-based polymer (PA1) was
monomodal distribution. These results supported that co- considered a suitable model for carrying out the self-assembly
polymerizations were progressed. Furthermore, the obtained study; subsequently, it was used for particle fabrication. The
PAs are soluble in NMP, DMSO, DMAc, and concentrated solvent displacement method was adopted for the self-assembly
sulfuric acid (Table S1†). The high molecular weight of PAs is studies of ADKP and PA1. Firstly, using ADKP as a monomer,
due to the high solubility of the monomers and polymers in the particle was prepared as follows: a DMSO solution of ADKP
organic solvents.
was slowly dropped into toluene to rapidly form particles.
Furthermore, SEM images revealed the ower-like morphology
of the particles (Fig. 1a). Flowers consist of two-dimensional
petals with a so and smooth surface. It is suggested that,
with continuous hydrogen bonding between DKPs, ADKP may
self-assemble into a planar tape-like structure, resulting in
a petal-like substructure. As shown in Fig. 1b, all the petals were
observed to bend inwards, and were suspended in toluene for 1
d. Therefore, petal bending can be regarded as a result of the
reduction of surface free energies.40 The same protocol was used
for the self-assembly of PA1. To evaluate the effects of poor
solvents on particulation, a DMSO solution of PA1 was dropped
into water, ethanol, tetrahydrofuran, ethyl acetate, acetone, and
toluene to precipitate particles. In cases of uses of water,
ethanol, and THF, irregular network-like morphologies were
observed (Fig. 1c–e). However, in ethyl acetate, ellipsoids were
formed (Fig. 1f). In case of acetone and toluene, particles with
hollow structures were observed (Fig. 1g and h). Notably, in
contrast to other hollow particles reported in the literature,41–44
the PA1 homopolymer (without any side chain and template)
spontaneously self-assembled into a hollow morphology, which
is a unique case for polymeric hollow particles. The particle
sizes from acetone and toluene were approximately 1 mm and 12
mm, respectively. Because of the thin shell, these hollow parti-
cles show a porous structure or tend to collapse into a bow-like
structure.
Thermal properties of PAs
TGA was utilized to evaluate the thermal decomposition of PAs
in a nitrogen atmosphere. As shown in Table 2 and Fig. S6,†
the 5% and 10% weight-loss temperatures (Td5 and Td10) of PAs
ranged from 272–326 ꢁC. Remarkably, although all PAs had
similar molecular weights, the homopolymer PA1 revealed the
highest Td5 and Td10 values, which was approximately 50 ꢁC
higher than the others. Moreover, the thermal physical
transformation of PAs was investigated via DSC in a nitrogen
atmosphere. However, between 50 ꢁC and 250 ꢁC, no glass
transition temperature (Tg), melting temperature (Tm), or
recrystallization temperature was detected. These results
indicate that PA1 has the simplest chemical structure among
all PAs, with the most abundant DKP moiety and benzene ring,
which is considered to contribute to its relatively superior
thermal properties.
Self-assembly behaviors of DKP-based monomer and PA1
DKP derivatives with low molecular weights are well known for
their orderly aggregation through parallel hydrogen bonds,
which are utilized to fabricate crystals or semi-crystals with
various morphologies. Long chains reduce the crystallinity of
polymeric systems,39 which may lead to different self-
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Table 2 Molecular weights and thermal properties of PAs derived from ADKP
Mwa (ꢃ104 g molꢀ1
)
Mna (ꢃ104 g molꢀ1
)
Mw/Mn
Td5 (ꢁC)
Td10 (ꢁC)
a
b
b
Polymers
PA1
PA2
PA3
PA4
1.93
1.59
1.76
1.98
1.64
1.36
1.36
1.46
1.18
1.17
1.29
1.36
326
276
272
280
348
296
289
302
a
Determined by GPC measurements based on poꢀly1styrene standards; eluent, 10 mmol Lꢀ1 of LiBr DMF solution. b Td5 and Td10 were observed from
TGA curve scanned at a heating rate of 5 ꢁC min under nitrogen atmosphere.
Fig. 1 SEM images of particles obtained by the dispersion of ADKP into toluene, (a) immediately after dispersion, (b) 1 d aging after dispersion, and
PA1 into (c) water, (d) ethanol, (e) THF, (f) ethyl acetate, (g) acetone, and (h) toluene.
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Fig. 2 (a) Process illustration of particle formation and transformation. SEM images of particles obtained in different processes related to the
process illustration: particles obtained by (b) adding acetone into particles formed from toluene, (c) adding toluene into particles formed from
acetone, (d) adding toluene into the suspension of the acetone sample, and (e) adding acetone into particles formed from mixture of acetone and
toluene.
energies of the systems. Therefore, if the environment changes,
a morphological transformation of PA1 particles may occur.24
The morphological transformation was further investigated
through the secondary solvent treatment of the formed parti-
cles. The treatment procedures are shown in Fig. 2a. By adding 3
Morphology transformation
The self-assembly of PA1 was highly sensitive to the solvent
environment as observed in the solvent displacement method,
which provided primitive thermodynamic conditions. Notably,
self-assembly is a thermodynamic process to reduce the free
Fig. 3 IR spectra of PA1 particles precipitated from water, acetone, toluene, and mixture of acetone/toluene.
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of the hydrogen bond for the solvent and PA.46 Thus, the FTIR
spectra were studied to evaluate the hydrogen bond behavior
in the self-assembled particles formed in different solvents.
Three types of morphologies were observed for the four
solvents: irregular networks of water, hollow particles of
acetone or toluene, and cubic particles of the solvent produced
by mixing acetone and toluene. Fig. 3 shows the IR absorption
in the N–H stretching vibration region. A shoulder peak for
N–H stretching appears in all particles, whereby the band
around 3191 cmꢀ1 indicates a moderate hydrogen bond, and
the band at 3240 cmꢀ1 indicates the free N–H group.47 In
particles formed from water, the shoulder peak is at, indi-
cating that the number of free N–H and the hydrogen bond-
interacted N–H was approximately equal. The distinct peak
around 3191 cmꢀ1 in the particles formed from acetone and
toluene indicated the strong hydrogen bond, thereby sug-
gesting a continuous hydrogen bond in the PA intra/inter
molecules. Specically, particles from toluene exhibits
stronger peak at 3191 cmꢀ1 and weaker peak at 3240 cmꢀ1
compared to particles from acetone. It suggests that more and
stronger hydrogen bonds were formed between DKP units in
particles from toluene, which presumably affected the size of
hollow spheres. On the other hand, for the particles formed
from the solvent mixture of acetone and toluene, the bands at
3191 cmꢀ1 and 3240 cmꢀ1 became weaker and stronger,
respectively. It is considered that acetone and toluene produce
particles with strong hydrogen bonds, while their solvent
mixture results in particles with weak hydrogen bonds. It is
suspected that in particles that are formed from a mixed
solvent, p-stacking becomes a dominant interaction rather
than hydrogen bonding, thereby resulting in different
morphologies.
Table 3 Hansen solubility parameters of PA1 and solvents
Molecules
dD (MPa0.5
)
dP (MPa0.5
)
dH (MPa0.5
)
PA1
Water
Ethanol
Tetrahydrofuran
Ethyl acetate
Acetone
22.2
15.5
15.8
16.8
15.8
15.5
18
13.1
16
7.9
42.3
19.4
8
7.2
7
8.8
5.7
5.3
10.4
1.4
Toluene
2
mL acetone to the particles formed over toluene and subsequent
drying in ambient air condition, hollow particles (same form of
Fig. 1f) were broken into peel-like particles in the size of about 2
mm (route 1, Fig. 2b). Similarly, by simply changing the
precipitate solvent to acetone and adding 3 mL toluene on the
particles formed, aer drying, small particles seemed to fuse
into larger particles (route 2, Fig. 2c). Nonetheless, when
toluene was added to the droplet of the acetone sample without
drying, cubic particles were formed (route 3, Fig. 2d). Cubic
particles are expected to have high crystallinity based on
hydrogen bonds. However, X-ray diffraction did not show any
distinct diffraction due to the small amount of cubic particles
used for measurement (Fig. S7†). A similar behavior to cube
formation has been previously reported for amphipathic poly-
peptides poly{g-glutamic acid-g-(L-phenylalanine ethyl)}, but
PA1 is not amphipathic.45 However, both results indicated that
molecular assembly through the p-stacking of phenylalanine
moiety was important to form cubic morphology. In addition,
based on such morphological changes that depend on the
solvent, the addition of 3 mL acetone on the cubic particles aer
drying transformed the cubic particles into
a spherical
morphology that appeared porous or hollow (route 3, Fig. 2e).
These hollow particles had a disordered shape because they
were not vigorously stirred. Furthermore, it was envisaged that
the shape would be the same as that shown in Fig. 1h, if the
same experimental operation as that used for the particle
preparation was performed.
Hansen solubility parameters
As discussed above, the self-assembly of PA1 was strongly
affected by solvent composition. Solvent parameter evaluation
has become a powerful tool to explain gelation, crystal growth,
polymer aggregation, and other colloidal phenomena.31,32,48,49
Here, we use Hansen solubility parameters (HSP), which
include dispersion (dD), polarity (dP), and hydrogen bond (dH), to
explain such a complicated polymorphism.50,51 The HSPs of PA1
and the solvents used for elucidating the morphologies are
Evaluation of the strength of hydrogen bond by FTIR
spectroscopy
In the previous section, particle formations were dependent summarized in Table 3. The dD and dP values for solvents seem
on the solvent, which in turn were dependent on the strength to have little relation with particle morphology. Compared to
Fig. 4 Schematic illustration of formation of (a) irregular gel-like morphologies, (b) sheet-like substructure and (c) cubic particles.
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the hydrogen bond, the dispersion interaction and dipole– have been treated by acetone, are in sizes of about 1 mm. When
dipole interaction are weak, which are considered to have less toluene is added to the acetone droplet sample, the thermody-
inuence on the self-assembly of PA1 with multiple hydrogen namic environment of the particles changes, thereby resulting
bonding sites. As for dH, when solvents have higher dH than PA1 in the unfolding of the sheet-like substructure. Notably, in this
(dH ¼ 7.9 MPa0.5), precipitates produce irregular aggregates, case, no stirring is performed. It is hypothesized that with the
which is in contrast to the results obtained for individual aid of toluene, p-stacking occurs in the benzene ring moiety of
particles. In water with the highest dH, the DKP moieties in PA1 PA1 units, which also weakens the hydrogen bond between the
tend to form hydrogen bonds much more easily with water DKP moieties because of the steric effect, resulting in the
molecules than with themselves, thereby resulting in the stacking of substructures and cubic morphology (Fig. 4c). When
formation of random hydrogen bonds (Fig. 1c). As for ethanol, acetone is added to the cubic particles, the cubic particles
with the decrease in the dH value for the solvent, the probability decompose again to form the sheet-like substructure. Along
of DKP–DKP interaction increases, leading to the emergence of with the relatively fast drying of acetone, the sheet-like
a two-dimensional substructure (Fig. 1d). As for THF, the dH substructures tend to bend to form hollow particles again.
value for the solvent approaches PA1, which implies that the However, acetone dries rapidly and cannot impart sufficient
DKP moieties have a similar chance to form hydrogen bonds bending energy as compared to that provided by vigorous stir-
with the solvent and themselves, resulting in a median ring; therefore, the hollow particles shown in Fig. 2e are not
morphology between the network and individual particles completely formed.
(Fig. 1e). When solvents exhibit lower dH values than PA1, the
DKP moieties have a greater chance to form hydrogen bonds
with themselves rather than with solvent molecules, resulting in
Conclusion
individual particles.
We have established an AB-type DKP-based monomer with high
solubility from a commercial dipeptide sweetener. Various DKP-
based PAs, with or without other monomers, were synthesized.
All the PAs showed high molecular weights and high thermal
Mechanism of self-assembly behavior for PAs
Based on the morphological study of PA1, the formation of both resistances. Morphologies of the homopolymer PA1 were
hollow and cubic particles exhibits unique characteristics elucidated using solvent displacement methods that employed
because they are not typical for a homopolymer without side a variety of solvents ranging from a DMSO solution to various
chains. It is important and challenging to analyze this self- poor solvents, thereby resulting in particles with various
assembly mechanism. Polymorphism phenomenon is also re- morphologies; for example, irregular networks as well as rugby-
ported in the self-assembly of amino acids and small peptides. like and hollow particles. In particular, the preparation of
The self-assembly process of these building blocks is driven by hollow particles was achieved without any additive or fabrica-
thermodynamics and kinetics, which could be easily inuenced tion. Furthermore, PA1 particles underwent a morphological
by solvent environment. For an example, diphenylalanine could transformation by secondary solvent treatment using the
self-assemble into crystal and brous gel, which was strongly acetone and toluene dropping technique. To investigate the role
impacted by solvents, even in a trace amount.52,53 Similarly, of hydrogen bonds in self-assembly, IR and HSP approaches
regarding to the above-mentioned IR and HSP studies, were applied to dene the mechanism governing the
hydrogen bond is considered as the dominating driving force of morphology formation of DKP-based PAs. The present study
self-assembly, which deeply affected by poor solvents. The provides a high potential as a self-assembly building block for
mechanism of the self-assembly of PA1 is shown in Fig. 4. When functional nanomaterials.
water is used as poor solvent, water molecules can form
hydrogen bond with DKP unit, which prevents the formation of
DKP–DKP hydrogen bond. Besides, water molecules also can
Conflicts of interest
bridge two polymer chains, as shown in Fig. 4a. As a result, PA1 There are no conicts to declare.
particles from water show irregular gel-like morphology. When
PA1 DMSO solution is added into poor solvents with low dH,
such as acetone and toluene, polymer chains aggregate regu-
Acknowledgements
larly due to the parallel hydrogen bond formed by DKP moieties This work has been nancially supported by Japan Science and
between polymer molecules, thereby resulting in a two- Technology Agency (JST)-Advanced Low Carbon Technology
dimensional sheet-like substructure (Fig. 4b). Following Research and Development Program (JPMJAL10100) (ALCA),
vigorous stirring, the sheet-like substructure eventually bends Center of Innovation (COI) program “Construction of next-
to form a hollow sphere. Because acetone has higher dH than generation infrastructure using innovative materials – realiza-
toluene, polymer chains still have chance to formed hydrogen tion of a safe and secure society that can coexist with the Earth
bond with acetone molecules. As a result, the packing of poly- for centuries-, Japan Society for the Promotion of Science (JSPS)
mer chains from acetone is not as stable as which from toluene. of Grant-in-Aid for Scientic Research (B) (15H03864), and
Therefore, the sheet-like substructure from acetone becomes Cross-ministerial Strategic Innovation Promotion Program
smaller, resulting in smaller hollow particles, compared to (SIP2), “Smart-bio” (Bio-oriented Technology Research
particles from toluene. Similarly, particles in Fig. 2b–e, which Advancement Institution, NARO, Japan).
© 2021 The Author(s). Published by the Royal Society of Chemistry
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28 Y. Li, L. Yan, K. Liu, J. Wang, A. Wang, S. Bai and X. Yan,
Small, 2016, 12, 2575–2579.
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5946 | RSC Adv., 2021, 11, 5938–5946
© 2021 The Author(s). Published by the Royal Society of Chemistry