Self-assembling behaviour of a modified aromatic amino acid in competitive medium

Article abstract of DOI:10.1039/d0sm00584c

Aromatic amino acid, specifically phenylalanine (Phe), is one of the most studied building blocks in peptide synthesis due to its importance in biology. It is reported in the literature that Phe-containing peptides have a high tendency to form different self-assembled materials due to efficient aromatic-aromatic interactions. In this article, we have tuned the supramolecular interactions of phenylalanine by making it electron-deficient upon introduction of the nitro group in the ring. The presence of the nitro group has a profound influence on the self-assembly process. It has been observed that 4-nitrophenylalanine (4NP) is a highly efficient gelator compared with the native phenylalanine in DMSO solvent in terms of minimum gelation concentration and it forms hydrogen bonding mediated crystals in water. The change of self-assembling patterns of 4NP in these solvents was studied using X-ray diffraction, UV-Vis spectroscopy, FE-SEM and other techniques. With the help of different experimental data and density functional theory (DFT), we have simulated the theoretical structure of 4NP in DMSO. The theoretical structure of 4NP in DMSO is different compared with that of crystals in water. We then studied the self-assembly process of 4NP in the mixed solvent of DMSO (polar aprotic) and water (polar protic). Different competitive non-covalent interactions of solvents as well as the ratio of the solvent mixture guide the final self-assembly state of 4NP. This journal is

Full text of DOI:10.1039/d0sm00584c

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ISSN 1744-6848
PAPER
Karsten Baumgarten and Brian P. Tighe
Viscous forces and bulk viscoelasticity near jamming
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DOI: 10.1039/D0SM00584C
ARTICLE
Self-assembling behaviour of a modified aromatic amino acid in
competitive medium
Pijush Singha, Souvik Misraa, Nayim Sepayb, Sanjoy Mandalc, Debes Rayd, V. K. Aswald, Jayanta
Received 00th January 20xx,
Accepted 00th January 20xx
Nanda,*,a
DOI: 10.1039/x0xx00000x
Aromatic amino acid, specifically phenylalanine (Phe), is one of the most studied building blocks in the peptide synthesis
due to its importance in biology. It is reported in the literature that Phe containing peptides have a high tendency to form
different self-assembled materials due to efficient aromatic-aromatic interactions. In this article, we have tuned the
supramolecular interactions of phenylalanine by making it electron-deficient upon introduction of the nitro group in the
ring. The presence of the nitro group has a profound influence on the self-assembly process. It has been observed that 4-
nitrophenylalanine (4NP) is a highly efficient gelator than the native phenylalanine in DMSO solvent in terms of minimum
gelation concentration and forms hydrogen bonding mediated crystals in water. The change of self-assembling patterns of
4NP in different solvents was studied using X-rays diffraction, UV-Vis spectroscopy, FE-SEM and other techniques. With the
help of different experimental data and density functional theory (DFT), we have simulated the theoretical structure of 4NP
in DMSO. The theoretical structure of 4NP in DMSO is significantly different than that of crystal in water. We have then
studied the self-assembly process of 4NP in the mixed solvent of DMSO (polar aprotic) and water (polar protic). Different
competitive non-covalent interactions of solvents as well as the ratio of solvent mixture guide the final self-assembly state
of 4NP.
through diet. Another source of Phe is artificial sweetener11
aspartame (Asp-Phe-OMe), which are frequently used in the
current time. Due to metabolic disorder, the presence of an
excess concentration of phenylalanine causes many diseases
like phenylketonuria (PKU)12.
Introduction
Phenylalanine (Phe) is one of the crucial building blocks used in
peptide synthesis among all the naturally occurring amino acids.
Gazit and co-workers highlighted the extensive role of aromatic
amino acids1 in the amyloid fibril formation. They marked the
‘Phe-Phe’ residue as the core residue for the amyloid fibril
formation2. Later, several other research groups also described
the role of diphenylalanine residue and analogues in the
formation of different functional nanostructures3-8 in water,
different organic and mixed solvents. With the improvement of
understanding of self-assembly mechanism, scientists recently
showed that simple Phe is capable of forming amyloid-like fiber
in aqueous solution9,10. Phe is an important amino acid as it is
the precursor of the essential amino acid (e.g. tyrosine), a
neurotransmitter (e.g. dopamine), several hormones (e.g.
adrenaline) and skin pigment (melanin). It is found in the breast
milk of mammals. In the human body, it is also accumulated
In 2002, Myerson and co-workers13 reported thermo-reversible
hydrogel (containing crystal) formation based on Phe. They
termed the mixture as gel-crystal. Lloyd and co-workers
recently demonstrated that simply Phe itself able to form
organogel in DMSO and gel-crystal14 in water. In recent years,
scientists have attempted to increase the gelation efficiency of
aromatic amino acid containing known gelators15-20,27 by
modifying the aromatic ring. This actually increases the modes
of different non-covalent interactions namely π-π interactions,
hydrogen bonding, hydrophobic and electrostatic interactions
between the gelator molecules and results in the enhanced
gelation properties. Several chemical modifications of the
aromatic ring were attempted to improve the mode of
interactions. In this regard, pioneer works were done by Nilsson
and co-workers17,19. Ring substituted phenylalanine derivatives
of 9-fluorenylmethyloxycarbonyl [Fmoc-Phe(X)-OH] exhibited
efficient hydrogelation compared to the simple Fmoc-Phe-
OH21,22. Their works suggested that gelation efficiency depends
upon the nature and number of halogen group/s and their
position in the aromatic ring19,20. Metrangolo and co-workers
highlighted the role of halogen bonding5,18,23,24 interactions in
the para-substituted halogen derivatives of Fmoc-Phe-OH18 and
short amyloid peptide23. Lloyd recently showed how viscoelastic
property and solid-state packing change on the introduction of
a.Department of Chemistry, Indian Institute of Engineering Science and Technology,
Shibpur, P.O.- Botanic Garden, Howrah-711103, West Bengal, India. Email-
jayanta2017@chem.iiests.ac.in & drjayantananda16@gmail.com
b.Department of Chemistry, Jadavpur University, Jadavpur,Kolkata-700032
c. Polymer Science Unit, Indian association for the Cultivation of Science,
Jadavpur,Kolkata-700032
d.Solid State Physics Division, Bhabha Atomic Research Centre Trombay, Mumbai,
400085 India
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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halogen group in the ring of phenylalanine25. So, the dissolved into solution. Serendipitously, we noticed that 4NP
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DOI: 10.1039/D0SM00584C
introduction of different substituents in the aromatic ring of forms beautiful reddish coloured transparent organogel within
amino acid enhanced the gelation efficiency significantly5,18,20,25- the NMR tube upon cooling at ambient temperature. The gel
27 .
.
formation of 4NP in regular DMSO solvent was further checked
in a glass vial by ‘inverted test-tube’ technique. One interesting
observation is that 4NP crystal is almost colourless, but when it
was dissolved in DMSO, the solution turns into a reddish colour.
The gelation efficiency of 4NP in DMSO was measured and its
gelation efficiency is better than native Phe residue14. The
measured minimum gelation concentration (MGC) is 19.2 mM.
It is calculated that 4NP forms almost 2.6 times efficient gel in
DMSO than its native form14 in terms of MGC. This observation
suggested that slight modification of the structure leads to
better gelator in DMSO. Organogel exhibits thermo-reversible
nature. The gel-sol transition temperature i.e. gel melting
temperature (Tgel) was measured at different concentrations
(mM) and plotted as Fig. 1b. Tgel values of DMSO-gel increase
with increasing concentration of 4NP46.
Scheme 1. Synthetic scheme of 4-nitrophenylalanine (4NP). Reagents and condition:
mixed acid (Conc. HNO3 and Conc. H2SO4) at 0 °C.
Inspired by these works, we have introduced one simple
functionality (nitro group) in the aromatic ring of phenylalanine
to evaluate the effect of ring substitution on self-assembly.
Regular nitration method was used for the incorporation of the
nitro group at the cold condition. Nitro group is incorporated in
the para position of the phenyl ring of Phe. Here, presence of
nitro group makes the aromatic ring electron-deficient.
According to our strategy, we are interested in the self-
assembly of electron-deficient 4-nitrophenylalanine (4NP) in
different organic and aqueous solution. In this work, we have
serendipitously found that 4NP can form efficient organogel in
dimethyl sulphoxide (DMSO). It is already known that 4NP forms
crystal in water28. DMSO and water are miscible solvents. So, it
will be interesting to study the self-assembly property of 4NP in
water, DMSO and their mixtures. In pure DMSO solvent, it forms
reddish coloured organogel. Nanoscale morphology using FE-
SEM and SANS studies suggested the formation of nanofibers in
Fig. 1 a) Image of 4NP-organogel in an inverted glass vial and b) concentration-
dependent thermal stability of 4NP organogel in DMSO.
DMSO. Crystallization and gelation are the nucleation process Variable temperature NMR (VT-NMR) experiment was done to
of the molecule which lead to the emerging properties of know the effect of temperature on the gelation. The fine
materials29-45. In the gel, molecules undergo self-assembly to changes due to different supramolecular interactions are
form unidirectional fiber, whereas in crystal highly directional, usually reflected in an NMR experiment. Here, a temperature-
three-dimensional arrangements occur. A recent report29 dependent H NMR study was carried out to understand the
1
suggests that the nucleation process in gelation is kinetically role of different non-covalent interactions present in the
favoured however crystallization is thermodynamically organogel state. It is obvious that with the increase of
favoured process. In this work, we have studied the self- temperature, the interactions of the gelator-gelator and
assembly of 4NP in two pure solvents and in their mixture. It is gelator-solvent molecules will be influenced. During VT-NMR
observed that the ratio of the solvent dictates the possible final study, the temperature was increased from 40 to 90 °C (ESI, Fig.
self-assembly state. Competition between charge-transfer and S3). It is observed that with the increase of temperature
hydrogen bonding mediated interactions were studied in the aromatic protons are down-fielded47,48 i.e. the aromatic-
mixed solvent using several techniques including UV-Vis, FE- aromatic interactions of 4NP were diminished which leads to
SEM, rheology and XRD analysis.
the gel to sol transition. So, aromatic-aromatic interactions play
a pivotal role in gel formation.
Results and Discussion
Gelator compound was synthesized by the nitration28 of
phenylalanine at 0 C (Scheme 1). Compounds were purified by
the crystallization from water. Faint yellow coloured crystalline
4NP was used further for all the experiments.
SANS study
4NP gel material was studied using small-angle neutron
scattering (SANS) experiment. Fig. 2 shows the scattering profile
of 4NP at 30 oC. In the low-Q region of the data, the scattering
intensity decreases in almost a straight line as 1/Q2 and it
indicates the cross-section of bundle-like structures in these
systems. The absence of lower cut-offs in the data indicates that
the length of these bundles could be much larger than what can
be determined from the present Qmin and therefore the length
was kept fixed higher than to a value 2π/Qmin, i.e., ~ 400 Å.
Self-assembly in DMSO
For characterization of newly synthesized material, NMR
technique was used. 4NP crystals are not readily soluble in
DMSO-d6 solvent and under heating condition, 4NP was
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These bundles of the fiber-like structure have been 4NP (1.0 mg) was added in different vials having 0.25 ml solvent.
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DOI: 10.1039/D0SM00584C
characterized by the cross-sectional radius or thickness as the The resulting solutions were heated until the clear solutions
measurement of the length is limited by the Qmin of the SANS were obtained. All these solutions were kept at room
instrument. The average thickness of the bundle of the fiber-like temperature for equilibration. Within a few minutes, a red-
structure has been found to be 28 Å.
coloured gel was formed in 100% DMSO (extreme left vial in Fig.
3b). With the increase of the water percentage in the solution,
the reddish colour was diminished and turned to form an almost
colourless (faint) solution in pure water (extreme right). Around
30 minutes, needle-shaped crystal formation took place from
the vial with 100 % water (extreme right vial in Fig. 3b). Within
one hour, it was observed that solutions (% of DMSO 90 and 80)
were transformed into gel i.e. 4NP can form self-supported gel
even in presence of 20 % water. Longer time duration indicates
that the presence of water slowed down the gel formation
kinetics. But, the rest of the vials remained as a solution. An
interesting observation was noticed after overnight
equilibration. The solutions present in the rest of the vials did
not form gel rather needle-shaped crystals of 4NP in the bottom
of the glass vials.
Fig. 2. SANS scattering profile for 4NP-DMSO gel measured at 30 C.
Self-assembly in Mixed solvents (DMSO and Water)
During the gel formations, apart from gelator-gelator
interactions, gelator-solvent interactions play a crucial role. To
understand the solvent-gelator interactions in the gel state, as
well as, competitions among different solvents for a gelator in
the gel state, another solvent system (water) was introduced in
DMSO solution of 4NP. DMSO is one of the highly polar solvents
and highly miscible in water. Our studied gelator 4NP is also
soluble in water and forms crystal in it28. We studied the self-
assembly of 4NP in the mixed solvent system (varied ratios of
DMSO and water) at a fixed concentration of 4NP. In the solvent
mixture, two solvents will interact with 4NP using different non-
covalent interactions and try to modulate the final self-
assembly state as like their self-assembly in pure solvent.
Fig. 4 PXRD patterns of xerogel of 4NP in DMSO (red) and simulated XRD pattern of 4NP
grown in DMSO-water mixture (orange) and water (yellow).
a)
b)
In the presence of water (>20%), non-covalent interactions of
water with 4NP molecules dominated in the solutions and water
facilitated the formation of thermodynamically stable crystal in
the rest of the solutions. We studied the powder XRD pattern of
the xerogel (from DMSO) and the simulated XRD pattern of
crystals grown from pure water and DMSO-water mixture
(ratio= 50:50) in Fig. 4. It is clear that packing pattern is same for
the crystals which are grown in water or water-DMSO medium.
The XRD pattern of 4NP in DMSO is quite different from the
crystal. During this study, we have varied the gelator
concentration and noticed that increased in concentration of
4NP can gelify the mixed solvent even above 20 % of water
content.
Fig. 3 Optical images of glass vials containing 4NP in different DMSO-water mixtures: a)
under the hot condition and b) after equilibration of 24 hrs. The extreme left vial
contained 100 % DMSO and the water content was increased 10 % in each next vial.
A set of mixed solvent (DMSO+water) was prepared by
increasing the percentage of the water. In Fig. 3a, only pure
DMSO was taken in extreme left vial (100 % DMSO), after that,
in each vial water content was increased 10 % till the extreme
right vial where only 100 % water was taken. Then, crystalline
Hansen Parameter
It is already noticed that gel and crystal formation of 4NP was
dictated by the solvent ratio. Recently, some researchers have
applied the ‘Hansen solubility parameter’ for understanding gel
formation/crystallization ability in a similar type of solvents by
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accounting three types of interactions: polar (δp), van der Waals competition between DMSO (through CT interaction) and water
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(δd) and hydrogen bonding (δh) interactions of a solvent. By (through hydrogen bonding interactioDnO) I:f1o0r.1043N9/PD0iSnM0m05ix8e4Cd
balancing these three interactions, a molecule can form a gel or solvent. In presence of water (> 20 %), hydrogen bonding
remains soluble or insoluble in a particular solvent44,49. Here, interactions of 4NP-water overcomes the charge transfer
the physical state of 4NP in DMSO, water, and mixture of DMSO- interactions of 4NP-DMSO and favours the formation of the
water was compared using the Hansen solubility parameter (δp, crystal in the mixed solvents.
δd, δh). For the DMSO-water mixture system, the Hansen
solubility parameter was calculated by assuming a linear
relationship49
휹풎풊풙풕풖풓풆 = 풙water  휹water + (ퟏ – 풙water)
where 풙water is the fraction of water in the mixture (v/v). For
DMSO and water, van der Waals interaction (δd) values are
respectively 18.4 and 15.5 and also polar interaction (δp) values
are 16.4 and 16.2 respectively. These two interaction values are
quite similar for DMSO and water. But the hydrogen bonding
parameter of DMSO and water are 10.2 and 42.4 respectively
which has a huge difference. In DMSO, 4NP molecules self-
assemble by balancing three solubility parameters. During the
addition of water in the DMSO medium, hydrogen bonding
propensity of resulting solvent was increased linearly.
Therefore, hydrogen bonding (δh) interactions with 4NP are
getting more preferences and guides the overall system
towards crystallization (ESI, Fig. S4). Up to 20% of the water in
the DMSO, 4NP can retain its gel behaviour and after that
crystallization of 4NP occurs.
Fig. 5. a) UV-Vis spectra of 4NP (50 µM) in different solvents. The composition of the
solvents is given in the figure legends. b) The variation of charge-transfer band (477 nm)
intensity with the percentages (%) of water in the mixed solvents.
Morphology of Gel
UV-Vis spectroscopy study
The interesting colouration of 4NP in solution and gel state
(Fig.3) makes us curious to know the reason behind it. For this
purpose, we studied the UV-Vis absorption spectra of 4NP in
water, DMSO and mixed solvents. We prepared solutions of
4NP (50 µM) in different mixed solvents and solutions were
equilibrated for 30 minute prior to the UV-vis spectroscopic
study. In water, a characteristic peak at 274 nm was noticed,
which is due to the π-π* transition of nitroaromatic
functionality present in 4NP. In DMSO, apart from a peak at 300
nm, a characteristic peak at the longer visible wavelength (477
nm) was observed. The colouration in DMSO is due to the
interactions of 4NP with DMSO. This is a charge-transfer (CT)
interactions50-52 between electron-rich DMSO with electron-
deficient 4-nitro phenylalanine. This intense visible region peak
was diminished, upon addition of water into the DMSO solution.
It is assumed that DMSO solvent and 4NP are participating to
form charge-transfer complex, which is diminished in presence
of water having high hydrogen bonding capability.53
Competition between two solvents for 4NP is monitored by UV-
Vis spectroscopy in a mixed solvent (Fig. 5a). Upon changing the
solvent system from 100% DMSO to 100% water, the charge-
transfer band is diminished. The decrease of the absorbance
value at 477 nm with the % of water in the mixture was plotted
in Fig. 5b. It is clear from the plot that the extent of CT
interaction was drastically decreased upon addition of water.
a)
b)
Fig. 6 FE-SEM micrographs of 4NP in a) 100% DMSO and b) 20 % water in DMSO-water
mixture. Scale bar is 200 nm.
For understanding the self-assembly behaviour and nanoscale
morphology of 4NP in mixed solvents, field emission scanning
electron microscopy (FE-SEM) was used. FE-SEM image of
DMSO-gel is shown in Fig. 6a. 4NP forms dense, cross-linked
nanofibers in DMSO. These fibers are several micrometer long
and the average fiber-width is around 60 nm (ESI, Fig. S5a). The
effect of water on the self-assembly process in the mixed
solvent (20 % water in DMSO) was also investigated by FE-SEM
study. It is observed from the image (Fig. 6b) that the density of
fiber decreased noticeably and the average fiber-width is
around 40 nm (ESI, Fig. S5b). SEM study demonstrates that 4NP
molecules are self-associated in DMSO to form kinetically stable
1D nanofibers which are cross-linked to each other and trap
Moreover, the π-π* transition band (300 nm) was shifted
towards shorter wavelength (Fig. 5a), i.e. blue shift occurs. This
blue shift is possibly due to the continuous breaking of extended
aromatic-aromatic interactions of the 4NP in DMSO. So, all
these observations clearly indicate that there is a strong
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DMSO solvent to form a gel. In addition to water, the self- dissimilarity with the crystal behaviour of Phe. InteVrieewstAirntigclleyO, ntlhinee
assembly process of 4NP in DMSO was affected by the b axes of 4NP unit cell (b = 5.293(4)) isD1O/I6: 1t0i.m10e39s/hDo0SrtMe0r0t5h8a4Cn
competing hydrogen bonding interactions of 4NP and water. that of Phe (b = 30.8020 (17)) [ESI, Table-S1). In solid-state,
This results in the less dense, thinner fiber formation.
three 4NP molecules are interlocked by a single H2O molecule
along a and b axes (Fig. 8b & ESI Fig. S6-S7). The intermolecular
H-bonding distances for N9-H9BO11, O11-H11AO7 and
Rheology of gel
The strength of gels, prepared in different solvents, were O11-H11BO8 are 1.885 Å, 1.883 Å and 2.020Å respectively.
examined with the help of a rheometer. Viscoelastic gel Such hydrogen bonding interaction is absent in Phe since it does
materials exhibit storage energy as well as loss energy. The not contain any water of crystallisation In the lattice, a network
higher value of storage modulus (G
constant frequency represents the gel behaviour30,54 At critical hydrophobic zone due to their tail-tail and head-head
stress value, storage modulus (G ) and loss modulus (G interactions. In the hydrophilic zone, the deprotonated
) than loss modulus (G) at arrangement of 4NP creates one hydrophilic and one

)
crossed each other and gel becomes sol. Here, to evaluate the carboxylic group and protonated amine group were connected
addition of water on the self-assembly process of 4NP in DMSO, via electrostatic attraction (with distance 2.909 Å) and stabilized
oscillatory stress sweep experiments of 4NP-gel were via water-mediated H-bonding interaction. This type of
performed in two conditions by preparing gels in 100% and 80 hydrophilic and hydrophobic zones are also observed in the
% DMSO solvent at a fixed concentration of 4NP (0.75 % w/v). crystal structure of Phe. Along c axis, hydrogen bonding
From the Fig. 7, it is observed that in the linear viscoelastic distance of ring-hydrogen (H4) of 4NP with oxygen (O9) of the
region (LVR), the storage modulus (G) values are higher than nitro group of another 4NP is 2.720 Å. This intermolecular
loss modulus (G) in both solvent system55,56. The storage strong hydrogen bonding interaction helps to form head to
modulus value in the LVR region of 4NP gel (in 100% DMSO) is head arrangement of the aromatic ring along c axis. Here,
comparable with the native L-phenylalanine gel in DMSO14 additional C-H interaction is identified along b axis. The
reported earlier. The measured critical stress value in DMSO is interaction between β-H (H12A) and phenyl ring of another 4NP
201 Pa, while in 20 % water in DMSO, critical stress value is 171 molecule along b axis with distances 2.823 Å, made two
Pa. These results suggest that with increasing of water fraction aromatic ring parallel.
in the solvent mixture, the gel loses its strength.
a)
c)
b)
d)
Fig. 7 Oscillatory stress sweep experiment of 4NP-gel in a) 100% DMSO and b) 80% DMSO
in DMSO-water mixture respectively. The concentration of gelator was kept 0.75 % (w/v).
The three-dimensional arrangement of 4NP in the crystal was
characterized by single-crystal X-ray technique. Needle shaped
crystal was grown in water by slow evaporation.
A
Fig. 8 a) ORTEP diagram of 4NP. Different non-covalent interactions present in the
crystals: b) water-mediated hydrogen-bonding interactions and aromatic-aromatic
interactions and c) H-bonding interaction and CH-π interactions. d) Formation of the
hydrophilic zone due to ionic interactions of COO- and NH3+ of two adjacent 4NP
molecules in the higher-order packing.
monohydrated 4-nitrophenylalanine crystalized in the
monoclinic space group P1211. The asymmetric unit of 4NP is
consisting of one 4NP molecule in zwitterion form and one
water molecule. We have also compared the crystal structure
4NP with the native Phe which crystalized in monoclinic space
group P21. 59 We found that 4NP have sufficient similarity and
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Fig. 9: a) Fitting of simulated and experimental powder X-ray crystallographic patterns of 4NP packed in DMSO; b) head-to-tail arrangement of 4NP in unit cell of nanofiber; c)
intermolecular interactions in 4NP molecules; d) arrangements of the 4NP molecules in three dimension with the help of supramolecular interactions; e) higher order packing
of 4NP; f) formation of nanofibers through self-assembly of 4NP.
Incorporation of the nitro group introduces new intermolecular in intermolecular hydrogen bonding is responsible for the
H-bonding interactions which help to arrange the molecules in higher solubility as well as the non-gelation tendency of 4NP in
a zig-zag manner along c axis. However, in the Phe crystal, this water. The molecule crystallizes with water form hydrogen
type of zig-zag formation is grown along the b axis.
bond with the acid group inside the unit cell and forms a
hydrophilic zone. In the presence of less polar and aprotic
solvent DMSO, the 4NP molecules preferred to form less-polar
Computational Simulations
Understanding the change of arrangements of 4NP molecules head-to-tail arrangement (Fig. 9b). The aprotic nature of solvent
inside the crystal which form the nanofibers in less polar solvent helps to form strong hydrogen bonding (1.43 Å) between
(water to DMSO) is important to unravel the structure- ammonium and nitro groups and gives a stable zig-zag chain
photophysical relationship of 4NP. Here, we have assumed that (Fig. 9c). Such multiple chains can stack parallel way with each
same number of molecules present in a unit cell creating the other through different supramolecular interactions (Fig. 9c).
nanofibers in DMSO, like its single-crystal formed in water. We Here,  staking of nitrobenzenes ring, C-H… and -cation
have changed the molecular orientation randomly with interactions are in the behind of the association of large number
different crystal symmetry and calculated the powder x-ray of molecules in an organized way (Fig. 9c). The three-
pattern of each orientation to fit with the pattern of nanofibers dimensional repetition of such arrangements in high-order
found experimentally. During this process, the cell parameters packing is able to produce a nanofiber like structures (Fig. 9d-f).
obtained from single-crystal X-ray crystallography have been These theoretical results corroborate the experimental
used. It was found that the powder X-ray pattern of 4NP packed observations satisfactorily.
with Pc space group fits very well with the experimental pattern
The studies have been extended to TD-DFT for finding the
(Fig. 9a) and the unit cell is shown in Fig. 9b. Computationally reason of the solvatochromic behaviour of 4NP during self-
generated system contains a glide plane perpendicular to [0, 1, assembly in DMSO. The calculations show that the molecular
0] plane with glide component [0, 0, ½], where the single crystal arrangement presented in Fig. 9b absorb at λmax 490 nm
system has 2-fold screw axis with direction [0, 1, 0] at 0, y, 0 (experimental value, λmax = 477 nm; Fig. 4a) with the 0.068
with screw component [0, ½, 0].
oscillation strength (Fig. S9). The transitions from HOMO-2 to
The molecular arrangements are very helpful to realize the LUMO (26%), HOMO-1 to LUMO (62%) and HOMO to LUMO
nanofiber formation mechanism with the help of intermolecular (5%) contribute mainly for the absorption (Fig. S9). The energy
weak interactions. In presence of water, the zwitterionic and difference for HOMO-1 to LUMO energy levels is 3.1 eV. The
highly polar 4NP molecules trend to interact more with water detail of the studies are given in ESI. The closely fitting of PXRD
through hydrogen bonding rather than its intermolecular and λmax value obtained from theory with the corresponding
hydrogen bonding to form nanofiber. This solvent interruption
6 | J. Name., 2012, 00, 1-3
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Soft Matter
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ARTICLE
experimental data validate the assumptions and support the also forms crystal. In the presence of a nitro group, several
View Article Online
DOI: 10.1039/D0SM00584C
mechanism of formation nanofiber in DMSO.
additional non-covalent interactions take part to form a three-
dimensional structure in solid-state which are absent in the
crystal structure of simple phenylalanine59. Out of them, water
Hirshfeld analysis
Hirshfeld analysis57 was used to understand the role of different molecule-mediated hydrogen bonding interactions with three
mode of interactions in the lattice packing of the 4NP molecule, 4NP molecules and CH- interaction are of special interest. We
which stabilize the three-dimensional arrangement. In the have also studied the self-assembly of 4NP in the mixed solvent
crystal packing, the Hirshfeld surfaces express the volume of DMSO and water, which is an interesting study indeed.
occupied by a monomeric molecule in the lattice by dividing Solvent molecules are interacted with the 4NP using different
crystal electron density into a molecular fragment58. The non-covalent interactions like hydrogen bonding interaction
Hirshfeld surfaces of the molecule is mapped by dnorm (water) and charge-transfer interaction (DMSO). We find that
(normalized for the atom size) function. The intense colour on final self-assembly state in mixed solvent is dictated by the
the surfaces represents the different type of intermolecular solvent composition.
interaction zone, which present in the molecule. Here, the
intense red colour zones signified the strong hydrogen bonding
Experimental Section
Synthesis of 4-Nitro-Phenylalanine (4NP)
interaction site which brings the molecule in the close vicinity.
Here, the red zones represent the hydrogen bonding sites (COO-
+
and NH3 group) which form hydrogen bonds with a water
4-Nitrophenylalanine was synthesized using the procedure
reported in the literature28, 61. 1.65 g (10 mmol) of L-
phenylalanine was taken in a round bottom flask along with 2
mL conc. H2SO4 and mixed properly by a stirrer at the ice-cold
condition. After 15 min, mixed acid (0.7 ml conc. H2SO4 and 0.7
ml conc. HNO3) was added drop-wise with continuous stirring
for 1 hr. Then, the solution was allowed to stir at room
temperature for 4 hrs. The reaction mixture was poured into
100 ml ice-cold water with continuous stirring. The resulting
solution was basified with liquor ammonia. The volume of the
resulting solution was reduced until the precipitation appeared.
The faint yellow coloured solid mass was obtained and
recrystallized in water. Shining needle-shaped crystal was
grown in the water medium.
molecule (Fig. 10 a). Hydrogen bonding interactions present in
the crystal is shown in Fig. 10b via two-dimensional fingerprint
plot. The contribution of donor-acceptor close contact is plotted
in the fingerprint plot by calculating the nearest interior (di) and
exterior (de) nuclei distances to the Hirshfeld surfaces. The
close contact contribution of O⋯H interactions are visually
observed in the 2D fingerprint plot (Fig. 10b). For 4NP, in the
fingerprint plot, the spikes represent the maximum close
contact contribution of strong O⋯H interactions. It can be
concluded from the Hirshfeld surfaces that the arrangement of
the 4NP molecule is specially stabilized by hydrogen bonding
interaction (46.3 %) via a water molecule.
Materials and Methods
a)
b)
L-phenylalanine was purchased from SRL chemical, India. Conc.
H2SO4, conc. HNO3 and liquor ammonia were purchased from
Spectrochem, India. Distilled water was used throughout the
experiment. All analytical grade solvents were purchased from
Finar, India and distilled prior to use.
1
NMR: H NMR study of 4NP was conducted using BRUKER 300
MHz NMR spectrometer. Compound 4NP was dissolving in
DMSO-d6 at 0.6 % (w/v) by heating. With the gel materials, the
temperature-dependent 1H NMR study was conducted at 30 to
95 °C range by raising the temperature. Before every scan at a
different temperature, the system allowed to equilibrate for 10
min.
Fig. 10 a) Hirshfeld surface of the 4NP mapped over dnorm., b) Two-dimensional
fingerprint plots for 4NP and relative contributions of the atom pairs (O…H) to the
Hirshfeld surface.
HRMS: For molecular weight determination, ESI-MS+ spectra
were recorded on a QTOF Micro YA263 mass spectrometer. The
compound 4NP was taken with acetonitrile solvent.
Gel melting temperature measurement: The gel-melting
temperature was measured using a digital oil-bath. Gel was
prepared in a screw cap glass vial (capacity 1mL, diameter 10
mm) and the melting temperature was noted using the
‘inverted test-tube method’. Temperature of water-bath was
Conclusions
In this article, we have successfully demonstrated that the
incorporation of new functionality in the ring of aromatic amino
acid residue modifies the supramolecular interactions. Here, we
are first time reporting that the presence of the nitro group in
the phenylalanine (4NP) makes it a better gelator than the
native form in DMSO solvent in term of gelation efficiency. In
DMSO, a reddish colour gel is obtained due to charge transfer
complex formation. The gel materials are extensively
characterised using different techniques including FE-SEM, XRD
and rheology. In water, the same modified aromatic amino acid
increased 1 C/min. For each concentration, three readings
were taken and average gel-melting temperature was plotted
against the corresponding concentration.
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J. Name., 2013, 00, 1-3 | 7
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SANS: Small-angle neutron scattering experiments were temperature (293 K). The collected X-ray data weVreiewinAtrteicglerOantleinde
performed at the SANS diffractometer at Guide Tube using SAINT-NT software package andDwOiIt:h10.t1h0e39/hDe0lSpM0o0f58th4Ce
Laboratory, Dhruva Reactor, Bhabha Atomic Research Centre, SADABS program, absorption correction was made. The
Mumbai, India60. In SANS, one measures the coherent structure was reduced by the help of the SHELX 2013
differential scattering cross-section (dΣ/dΩ) per unit volume as program56, and refined by full-matrix least-squares on F2. For
a function of wave vector transfer Q (= 4π sinθ/λ, where λ is the all non-hydrogen atoms, anisotropic thermal displacement
wavelength of the incident neutrons and 2θ is the scattering parameter was employed. The X-ray structure refinement
angle). The mean wavelength of the monochromatized beam parameters and details of the X-ray structure were summarised
from the neutron velocity selector is 5.2 Å with a spread of Δλ/λ in tabular form.
~ 15%. The angular distribution of neutrons scattered by the Computational Simulations: The structure of 4NP was
sample is recorded using a 1 m long one-dimensional He3 optimized in solid-state with the help of DMol3 using LDA-PWC
position-sensitive detector. The instrument covers a Q-range of functional and DND basis set with 3.5 basis file. These results
0.015–0.30 Å-1. The data have been analyzed by comparing the were visualized by Discovery studio. TD-DFT calculations were
scattering from different models to the experimental data.
performed by Gaussian 09W with D.0158 revisions at B3LYP
UV-Vis: UV-visible spectra were recorded on a Shimadzu 2401 level of theory through 6-31g basis set.
PC UV-vis spectrophotometer. The samples were prepared in Hirshfeld Analysis: The electrostatic surface of the compound
100% DMSO, 100% water and mixed solvents. The 4NP, designed by dnorm (Normalized for the atom size) function
concentration of 4NP was 50 µM in each solvents. During using CrystalExplorer software. Crystallographic Information
measurement of each different 4NP solution, spectrometer was File (cif) of compound 4NP was used in Hirshfeld surface
calibrated using that particular solvent system.
analysis.
Xerogel Preparation: First, gel was prepared by dissolving 4NP-
crystals in DMSO under heating condition. The resulting
solution was cooled at room temperature to get the organogel.
Then, solvent was removed from this gel by connecting with
high vacuum pump in a vacuum desiccator for two days to get
xerogel.
Conflicts of interest
There are no conflicts to declare.
PXRD: For the determination of powder X-ray diffraction (PXRD)
patterns of the xerogel, Bruker D8 Discover instrument using Cu-
Kα (λ = 1.5406 Å) radiation was conducted.
Acknowledgements
JN gratefully acknowledges DST-INSPIRE Faculty Grant (IFA16-
CH246) and Department of Chemistry, IIEST-Shibpur for
financial assistance. We are thankful to Prof. A. K. Nandi for
rheological study. JN is sincerely thankful to SAIF-IIEST for single
crystal X-rays diffraction study.
Field Emission Scanning Electron Microscope (FESEM): By using
a ZEISS SIGMA FE-SEM instrument, Field-emission scanning
electron microscopy (FE-SEM) experiment was performed. For
the sample preparation, 4NP was taken in different glass vial
with 100 % DMSO and 20% water in DMSO at 0.6 % (w/v)
concentration. Then, using heating cooling technique, gels were
prepared. The freshly prepared gel was placed on the clean
glass coverslip. This organogel was first dried in air. Before the
gold coating, samples were placed in high vacuum for 2 days.
Fiber width measurement: The width of the nanofiber, which
was obtained by the FESEM study was measured by the help of
the Image-J software.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the matter
under discussion, limited experimental and spectral data, and
crystallographic data.
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Table of Contents
Incorporation of nitro (-NO2) group in phenylalanine makes it an efficient gelator than the native amino acid. The self-assembly and
photophysical properties of 4-nitrophenylalanine (4NP) are changed with the alteration of solvent and final self-assembly state of 4NP in
competitive solvent mixture are dictated by solvent ratio.