A Phenylalanine Derivative Containing a 4-Pyridine Group Can Construct Both Single Crystals and a Selective Cu-Ag Bimetallohydrogel

Article abstract of DOI:10.1002/ejic.201900078

Metallohydrogels are attractive biomaterials, whereas formation of a selective bimetallogel in aqueous solution has rarely been explored. In this study, we show that a phenylalanine derivative containing a 4-pyridine group can not only assemble to form si

Full text of DOI:10.1002/ejic.201900078

A Journal of
Accepted Article
Title: A phenylalanine derivative containing a 4-pyridine group is
able to construct both single crystals and a selective Cu-Ag
bimetallohydrogel
Authors: Chuan-Wan Wei, Xiao-Juan Wang, Shu-Qin Gao, Ge-Bo
Wen, and Ying-Wu Lin
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900078
Link to VoR: http://dx.doi.org/10.1002/ejic.201900078

10.1002/ejic.201900078
European Journal of Inorganic Chemistry
COMMUNICATION
A phenylalanine derivative containing a 4-pyridine group is able to
construct both single crystals and a selective Cu-Ag bimetallohydrogel
Chuan-Wan Wei 1, Xiao-Juan Wang 1*, Shu-Qin Gao 2, Ge-Bo Wen 2, and Ying-Wu Lin 1,2*
Abstract: Metallohydrogels are attractive biomaterials, whereas
formation of a selective bimetallogel in aqueous solution has rarely
been explored. In this study, we show that a phenylalanine derivative
containing a 4-pyridine group can not only assemble to form single
crystals with copper acetate, but also selectively generate a Cu-Ag
bimetallohydrogel upon further addition of silver nitrate, which
exhibits excellent multi-responsiveness. This study represents a novel
example of a selective bimetallohydrogel in aqueous solution, which
provides insights into the formation of a bimetallohydrogel and clues
for future design of more functional biomaterials.
advantages of non-covalent interactions (such as hydrophobic and π–π
stacking interactions), hydrogen(H)-bonds, as well as the coordination
interactions. Although we have applied Phe derivatives to construct
[2e,
4d]
several metallohydrogels,
our attempts to create
bimetallohydrogel were not successful until recently. As shown in this
study, we found that a Phe derivative containing a 4-pyridine group
(denoted as 4-PF), can not only assemble to form single crystals with
copper acetate (Cu(Ac)2), but also selectively generate a novel Cu-Ag
bimetallohydrogel, namely 4-PF-Cu-Ag, in an aqueous solution upon
further addition of silver nitrate (AgNO3) (Fig. 1).
Introduction
Synthetic materials such as nanoenzymes and supramolecular
hydrogels have attracted much attention recently due to their potential
[1]
applications.
In combination of metal ions to confer new functions,
metallohydrogels are particularly attractive.[2] To date, although
various metallohydrogels have been reported,[1f-n,
2-3]
most of them
contain only one type of metal ion, which may result in limited
properties and functionalities. For example, several groups, including
ours, have reported the formation of Cu2+-metallohydrogels using
different gelators, and metallohydrogels triggered by other metal ions,
4]
such as Ag+, Co2+, Ni2+, Pb2+ and Cd2+.[2e,
Recently, we have
constructed the first La3+-selective metallohydrogel using a gelator of a
phenylalanine (Phe) derivative containing an imidazole group.[5]
Despite these achievements, the studies on formation of bimetallogels
are still limited. For example, Lin and co-workers prepared several
metallogels containing two metal ions (such as Ca2+/Fe3+ and Cu2+/Al3+)
Figure 1. Chemical structure of 4-PF (a) and digital photos of the mixture of 4-PF and
Cu(Ac)2 (b), which formed 4-PF-Cu(Ac)2 crystals after several weeks (c), or a
metallohydrogel of 4-PF-Cu-Ag by addition of AgNO3 with sonication (d).
Results and Discussion
in organic solvents.[6] Yin and co-workers prepared
a Cu-Zn
The compound 4-PF was a white solid with good solubility in
alkaline aqueous solutions. 4-PF could not self-assemble to form gels
by addition of Cu(Ac)2 (Fig. 1b). Nevertheless, 4-PF-Cu(Ac)2 single
crystals were grown in this system after several weeks (Fig. 1c). Both
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) images of the crystals were shown in Fig. S1. We
then determined the X-ray crystal structure of 4-PF-Cu(Ac)2 complex
(Fig. 2a, CCDC number 1853310). Both 4-PF and acetate acted as
ligands for the Cu ion, where the O atom of carboxyl group in 4-PF
and acetate, the N atom in the main chain (secondary amine), and the N
atom of pyridine group from the another 4-PF coordinate to the Cu ion,
with an additional H2O ligand (Fig. S2). Another H2O molecule was
found to form H-bonds with both 4-PF and acetate. The crystals belong
to a monoclinic system with a space group of P1 21 1 (Table S1), in
which the 4-PF-Cu(Ac)2 complex was stacking layer by layer (Fig. 2b),
as stabilized by the π–π stacking interactions between the benzene ring
and the pyridine ring. Moreover, many H2O molecules were found to
form a super H-bond network with both O and N atoms among the
layers (Fig. S3).
bimetallogel using specific anions in dimethyl sulfoxide.[7] In contrast
to those prepared in organic solvents, the formation of bimetallogel in
aqueous solution has rarely been explored, which prompted our
attempts in this study. Here we report
a selective Cu-Ag
bimetallohydrogel, which is a novel example of formation of a
selective bimetallogel in aqueous solution.
In order to construct a biocompatible metallohydrogel with metal
selectivity, it is important to choose a suitable gelator. Nature uses only
22 amino acids to construct metalloproteins and less than a half can act
as a ligand for the metal ions.[8] To overcome this limitation, unnatural
amino acids have been used for design of functional metalloenzymes.
[8-9]
As inspired, our group focused on the design of selective
metallohydrogels by using synthetic derivatives of amino acids, such as
Phe derivatives containing an imidazole group or a pyridine group with
4d, e, 5]
the N atom at different positions.[2e,
These designs took the
[1] Dr. C.-W. Wei, X.-J. Wang, Prof. Dr. Y.-W. Lin
School of Chemistry and Chemical Engineering, University of South China,
Hengyang 421001, China
E-mail: 1052961032@qq.com (Dr. X.-J. Wang);
linlinying@hotmail.com; ywlin@usc.edu.cn (Prof. Y.-W. Lin)
Homepage URL: http://hgxy.usc.edu.cn/info/1151/3863.htm
[2] S.-Q. Gao, Prof. Dr. G.-B. Wen, Prof. Dr. Y.-W. Lin
Laboratory of Protein Structure and Function, University of South China,
Hengyang 421001, China
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/anie.2014xxxxx.
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10.1002/ejic.201900078
European Journal of Inorganic Chemistry
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The prepared 4-PF-Cu-Ag gel was found to be stable at pH 8-
10, with a minimum gelation concentration (MGC) of 0.08 M at
room temperature (RT) when the ratio of 4-PF, Cu(Ac)2 and
AgNO3 was 2:1:1. Rheological tests showed that the average
value of the storage modulus (G’) was ~6-fold higher than the
loss modulus (G’’) (Fig. S7). Both G’ and G’’ exhibited slight
dependence on frequency over the entire measurement range (0.1-
100 Hz), and were stabilized within 600 s (Fig. S8), which
indicates the formation of a stable gel phase.
Moreover, the 4-PF-Cu-Ag bimetallohydrogel was found to
possess multi-responsiveness to several physical and chemical
stimuli. As shown in Fig. 4, the gel turned suspension when
titrated with NaOH, and appeared as a blue solution by addition
of HCl, which was irreversible by further addition of HCl or
NaOH, respectively. After addition of solid crystalline EDTA, the
gel turned into a clear solution within 20 min, due to the strong
affinity between EDTA and metal ions. Trifluoroacetic acid (TFA)
could induce the collapse of the gel, and the change of sol to gel
occurred quickly by addition of ammonia. The thixotropic
property of the gel was further studied by the shaking-resting test
and by a time-dependent step-strain rheological experiment (Figs.
S9 and S10), and the results demonstrated an excellent elastic
response[10] for the 4-PF-Cu-Ag bimetallohydrogel. When the
Figure 2. (a) X-ray structure of 4-PF-Cu(Ac)2 and (b) the molecule assemble. Red
spheres represent O atoms, blue N atoms and gray C atoms.
To test whether 4-PF could act as
a gelator to form a
metallohydrogel, we carefully study the self-assembly behavior of 4-
PF with 16 common metal ions, including alkaline earth metal ions
(Mg2+ and Ca2+), first-row and second-row transition metal ions (Fe2+/3+
Co2+, Ni2+, Cu2+ and Ag+, etc), and lanthanide metal ions (La3+ and
Ce3+, etc) (Fig. S4). Meanwhile, the results showed that the complexes
of 4-PF with these metal ions either formed precipitations or appeared
as clear solutions. We then investigated the combination of two
different metal ions. It was interesting to observe that upon addition of
AgNO3 to the mixture of 4-PF and Cu(Ac)2, or mixing 4-PF, AgNO3
and Cu(Ac)2, the system transformed into a blue metallohydrogel (4-
PF-Cu-Ag), after a while with sonication (Fig. 1d). For the presence of
a single metal ion, both 4-PF-Cu(Ac)2/Cu(NO)3 and 4-PF-AgNO3
complexes generated precipitations (Fig. S5), which suggests that the
combination of Cu2+ and Ag+ is crucial for the formation of
bimetallohydrogel. Moreover, we systematically tested the
combination of Cu2+ or Ag+ with another metal ion, and none of them
could form a metallohydrogel, yielding precipitations or clear solutions
(Fig. S6 and Table S2). These results showed that the gelator of 4-PF
has excellent selectivity for Cu2+ and Ag+ bimetal ions.
,
o
temperature was increased to 66 C, the gel began to collapse and
the sol-to-gel transition occurred after resting at room temperature,
suggesting a thermo-reversible property. Moreover, the gel-to-sol
transformation temperature (Tgel) was found to increase with an
increase of the gelling concentrations (Fig. S11).
To analyze the relationship between structural details and the
gelation behavior, we synthesized a series of compounds of 2-PF, 3-PF
and BF (Fig. 3a). 2-PF and 3-PF are position isomers of 4-PF, and BF
lacks a pyridine N atom compared with the PF molecule. Interestingly,
we found that neither the complex of 2-PF-Cu-Ag, 3-PF-Cu-Ag nor
BF-Cu-Ag could form a metallohydrogel with Cu2+ and Ag+ under the
same conditions (Fig. 3b). These observations indicated that both the
presence of a pyridine N atom and its position are important for the
gelation of the 4-PF-Cu-Ag bimetallohydrogel.
Figure 4. Gel-sol transitions of the 4-PF-Cu-Ag bimetallohydrogel triggered by
various stimuli (mechanical, thermal, pH, and chemical reactions).
To gain insight into the self-assembly mechanism of the 4-PF-
Cu-Ag metallohydrogel, we performed SEM, UV-Vis,
fluorescence and FT-IR spectroscopy, mass spectrometry (MS)
and X-ray diffraction (XRD) studies. SEM was carried out to
provide the morphological details of the 4-PF-Cu-Ag
metallohydrogel. As shown in Figs. 5a and S12a, the scaffold of
the gel presented as intertwined nanofibers with a diameter of 80-
100 nm and several micrometers in length. The SEM energy
dispersive X-ray spectrum (EDS) showed that both Cu and Ag
elements uniformly disperse in the region of nanofibers (Fig.
S12b). Nevertheless, the complex of 4-PF-Cu assembled to form
nanosheets (Fig. 5b), and the gelator 4-PF self-assembled into
short columnar nanofibers (Fig. S13). These significantly
different morphologies suggest that the presence of Ag+ ion is
very important for the gelation.
Figure 3. Chemical structures of 2-PF, 3-PF and BF (a), and digital photos of their
complexes with both Cu2+ and Ag+ ions (b).
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XRD studies further showed that the pattern of 4-PF-Cu-Ag
xerogel was different from that of 4-PF-Cu(Ac)2 complex
(powder before formation of single crystals) (Fig. 6b). The XRD
pattern of 4-PF-Cu(Ac)2 complex exhibited characteristics of a
crystal structure, which is in accordance with the SEM images of
the 4-PF-Cu complex (Fig. 5b). Nevertheless, in the XRD pattern
of 4-PF-Cu-Ag xerogel, a common broad diffraction peak at 20°
showed an amorphous nature.[11] The reflection at 3.60 Å
suggested strong π–π stacking interactions[12] among these
complexes. In addition, a typical peak at a d-spacing of 2.3 Å
could be assigned to the strong H-bonds among the networks.
These results further indicate that the presence of Ag+ ions make
differences in the arrangement of atoms, the non-covalent
interactions between the atoms and the various synergy effects for
the 4-PF-Cu-Ag system.
Figure 5. SEM images of (a) 4-PF-Cu-Ag xerogel and (b) 4-PF-Cu complex.
UV-Vis and fluorescence spectra of 4-PF-Cu-Ag were found
to be different from those of 4-PF (Figs. S14 and S15,
respectively), which suggests a direct coordination of the metal
ions by the ligand. FT-IR was used to further identify the
coordination interactions. As shown in Fig. 6a, for both 4-PF-Cu
complex and 4-PF-Cu-Ag xerogel, the peak at 3436 cm−1 (ν–NH
shifted to 3426 cm−1, and the peak at 3030 cm−1 (ν–COOH
)
)
disappeared, which suggest that both amino N atom and carboxyl
O atom participate in the coordination between the gelator and
metal ions. The peak of 1627 cm−1 in 4-PF due to the pyridine
ring skeleton stretching vibration remained the same at that in the
4-PF-Cu complex, which suggests that the pyridine N atom may
not directly coordinate to the Cu2+ ion, different from that in the
X-ray crystal structure. Meanwhile, the peak blue-shifted by 5
cm−1 to 1622 cm−1 in 4-PF-Cu-Ag xerogel, which may be
attributed to the direct coordination of Ag+ by the pyridine N
atom serving as a bridge for gelation.
ESI-MS were also carried out to indentify the coordination
status of both 4-PF-Cu(Ac)2 and 4-PF-Cu-Ag complexes. The
mass spectrum of 4-PF-Cu complex showed several ionized forms,
such as [Cu(PF)2]− (573 Da), [Cu(PF)2+Ac]− (632 Da), [Cu(PF)3]−
(828 Da) (Figs. S16 and S17), which suggests that PF may act as
a bidentate ligand to provide the amino N and carboxyl O as the
coordination atoms, in agreement with the results of FT-IR.
Moreover, Fig. S16 revealed three major ionized forms of the 4-
PF-Cu-Ag complex, [Cu(PF)2 + Ag]+ (682 Da), [Cu(PF)2 + Ag +
Cu(PF+Ac)]+ (1059 Da), and [Cu(PF)2 + Ag + Cu(PF)2]+ (1255
Da) (More details in Figs. S16 and S18), which suggests that Ag+
ions may play a bridging role in the gelation process.
Figure 6. FT-IR spectra of 4-PF, 4-PF-Cu complex, and the 4-PF-Cu-Ag
xerogel (a), and XRD patterns of the 4-PF–Cu complex and 4-PF-Cu-Ag
xerogel (b).
The mixture of 4-PF and Cu(Ac)2 could not self-assemble to
form gels, with or without sonication, whereas the system can
transform into a blue metallohydrogel upon addition of AgNO3.
These observations suggested that Ag+ plays a key role in
formation of the bimetallohydrogel by preferring a linear
coordination. Based on all above results, we proposed a plausible
mechanism for the gelation of 4-PF-Cu-Ag bimetallohydrogel.
Schematic representation of the probable local structure of
bimetallohydrogel was shown in Scheme 1. First, 4-PF acts as a
bidentate ligand, which binds to Cu2+ and forms a coordination
compound of 4-PF-Cu, likely due to the preference of the ligand
for the nature of Cu2+ ion, such as the ionic radius and
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Conclusion
In summary, we have synthesized a facile gelator of a Phe
derivative containing a 4-pyridine group, which can not only
assemble to form single crystals with Cu(Ac)2, but also
selectively generate a Cu-Ag bimetallohydrogel upon further
addition of Ag(NO3) in aqueous solution. Both Cu2+ and Ag+ ions,
as well as the position of the pyridine N atom, were found to be
crucial for formation of the novel bimetallohydrogel. Moreover,
the 4-PF-Cu-Ag gel exhibits excellent multi-stimuli responsive
properties to various chemical and physical stimuli. More
importantly, this study represents a novel example of a selective
bimetallohydrogel in aqueous solution, which provides insights
into the formation of a bimetallohydrogel by the combination of
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This work was supported by the National Science Foundation of China
(21807058), the Doctoral scientific research foundation of the
University of South China (2014XQD32, 2017XQD10), and Scientific
Research Fund of Hunan Provincial Education Department (14C1004).
Received: xxxx xx, 20xx
Published online: xxxx xx, 20xx.
Keywords: Metallohydrogels • Bimetallogel • Multi-responsiveness •
Phe derivative • Cu-Ag
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Bimetallohydrogel
COMMUNICATION
Copper-Silver Hydrogel: A
Chuan-Wan Wei 1, Xiao-Juan Wang 1*
,
phenylalanine derivative containing a
4-pyridine group can construct not only
single crystals, but also a novel
selective Cu-Ag bimetallohydrogel in
aqueous solution.
Shu-Qin Gao 2, Ge-Bo Wen 2, and Ying-
Wu Lin 1,2*
Page No. – Page No.
A phenylalanine derivative
containing a 4-pyridine group is able
to construct both single crystals and
a selective Cu-Ag bimetallohydrogel
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