Stomata-like metal peptide coordination polymer

Article abstract of DOI:10.1039/c7ta08002f

The water-triggered reversible transformation plays a significant role in bio-systems. To develop artificial materials with reversible structural transformability under mild conditions would be helpful to understand how bio-systems work. Here we report on a flexible coordination polymer, Cu-(Gly-Thr)·2H₂O, whose structure can be controlled by humidity or temperature. This porous coordination polymer shows reversible single-crystal to single-crystal transformation with surprisingly high thermal stability. It was found that the ordered transformation occurs via coordination bonds breaking and reforming without changing the coordination numbers and valence of the copper ion. This reversible transformation makes it especially efficient at taking up CO₂ under wet conditions, which is similar to the function of plant stomata.

Full text of DOI:10.1039/c7ta08002f

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Zhang, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA08002F.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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Stomata-like metal peptide coordination polymer
a†
b
†
c
a
d
e
a
a
Ning Ma, Cong Lin, Nian Wu, Qi Liu, Jia-Le Ma, Wei Meng, Xiao-Shuang Wang, Lu Zhang,
a
b
c
d
b
a
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaohui Xu, Yifang Zhao, Lin Zhuang, Jun Fan, Junliang Sun, * Ren-Xi Zhuo, and Xian-Zheng
a
Zhang *
DOI: 10.1039/x0xx00000x
www.rsc.org/
The water-triggered reversible transformation plays a significant can be approximately divided into two categories: those
role in bio-systems. To develop artificial materials with reversible without any chain-scission degradation during adapting
structural transformability under mild conditions would be helpful process and those that undergo guest molecules response with
4
-6
to understand how bio-systems work. Here we report on a flexible bond breaking and reforming reactions. Most soft PCPs
7
coordination polymer, Cu-(Gly-Thr)•2H
2
O, whose structure can be adopt the former type of transformation, such as the
controlled by humidity or temperature. This porous coordination ‘breathing’ guanidinium 1,2,4,5-tetra(4-sulfonatophenyl)-
8
polymer shows reversible single-crystal to single-crystal benzene (G
transformation with surprisingly high thermal stability. It was {[Ni(bpe) (N(CN)
found that the ordered transformation occurs via coordination cooperative folding Zn(GS)
bonds breaking and reforming without changing coordination However, the structural transformation with breaking and
4
TSPB), two fold interpenetrating network
9
2
2
)](N(CN)
2
)(5H
2
O)}
n
and side-chain controlled
1
0
11-13
2
(GS = glycyl-serine), etc.
6
,14
numbers and valence of the copper ion. This reversible reforming of bonds in network linkage, is rarely reported,
1
5
transformation makes it especially efficient at taking up CO
2
under especially at room temperature.
Here we report on a flexible coordination polymer Cu-(Gly-
Thr)•2H O ( ), designated as WHU-3 (WHU stands for Wuhan
wet condition, which is similar to the function of plant stomata.
2
1
2
+
The stomata of leaves are specialized epidermal porous
University), which is built with Cu and an dipeptide linker
(Gly-Thr). The dipeptide is used to construct the coordination
complex due to its good stability, flexibility, polydentate and
structures that balance the regulation of water vapour and CO
molecules exchange between environment and the plants.
2
1
2
,16
The stomatal apparatus contains two guard cells that open and
close depending on whether the plants need to hold on or lose
biological activity. The structural transformation (Fig. 1) was
investigated by analyzing the change of chemical bonds and
water and CO . The guard cell signalling is functioned by the
2
charge distribution. In addition, the CO capture capability of
2
2
Stomagen (also called EPF-LIKE9) peptide.
Porous
the material was further studied in the absence or presence of
water.
coordination polymers (PCPs) were reported to show similar
structural transformation in response to specific stimuli, which
Water-triggered reversible transformation of the flexible
provides
a possibility for the design of stomata-like
,4
coordination polymer. Crystals of
1 in the shape of monoclinic
3
materials. Soft PCPs are defined as porous crystals that
possess both highly ordered networks and structural
4
transformability. Meanwhile, structural variations in soft PCPs
flakes (Fig. 2a) were obtained from the reaction of cupric
Fig. 1 Schematic diagram of structure transformation of Cu-(Gly-
Thr)•2H O (1).
2
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nitrate with Gly-Thr (GT) (Fig. S1) in water and methanol
To investigate the water triggered structural change, in situ
medium on a shaking table at 300 rpm for 20 minutes (see the powder X-ray diffraction (PXRD) was performed by controlling
Experimental Procedures in Supporting Information). The the humidity. Single crystal X-ray diffraction of and 1a
obtained complex (Fig. S2), is formed of a 1:1 motif of Cu and revealed that this structural change occurs via a single crystal-
1
2
+
2
-
2+
GT anion (Fig. S3). The Cu is tetra-pyramidically coordinated single crystal transformation (SCSCT), i.e., from the afore
to three GT ligands, and each ligand is coordinated to three Cu mentioned P2 structure of (a = 5.222 Å, b = 8.635 Å, c =
ions (Fig. 2b) by the C-terminal Gly carboxylate group, the N- 11.558 Å, β = 100.115°) to a new crystalline phase 1a with a
terminal Thr amine group, and the nitrogen atom and oxygen P2 symmetry (a = 5.287 Å, b = 8.718 Å, c = 17.773 Å).
atom of the peptide linkage (Fig. 2c). The intra-layer hydrogen Compared with the structure of , the dehydrated crystal 1a
1
1
1 1 1
2 2
1
bond between the -OH group from the Thr side chain and the shows a structural change, i.e., although the two dimensional
terminal carboxylate group (O4H4A···O3, 2.037 Å) restricts the layer structure is retained, every second layer seems to be
torsion of this flexible moiety, imposing the orientation of the rotated 180 degrees along the c-axis during dehydration (Fig.
water molecules and methyl groups (Fig. S4). Adjacent layers, 2e and 2f). In other words, the layers in
1 and 1a stack in the
with the same regular arranged layer structure, are bridged by AAAA and ABAB sequence, respectively (Fig. S6). However, the
hydrogen bonds through water molecules. The nitrogen atom rotation of the whole layer requires to overcome a high
1
7
of the terminal amino group forms hydrogen bonds with water reactive energy barrier. It is speculated and proved later that
guests (N1H1D···Ow1). And the close-connected H O (Hw3) is the structural transformation occured with breaking and
2
hydrogen-bonded with the oxygen (C2O1) from peptide in the reforming of bonds. Moreover, 1a switches the interlamellar
next layer (Fig. 2e). Meanwhile, it is surprisingly that the inter- spacing from 11.56 Å in
1 to 8.89 Å (the adjacent planes fit to
layer water molecules also form a stable layered hexagonal ice the layers and the spacing taken as the distance between
polymorph at room tempreture. As we will discuss later, the these planes). When the dehydrated single crystals of 1a were
2
+
tetra-pyramidically coordinated Cu ions and the hydrogen exposed to the air with the relative humidity greater than 60%
bonds contribute significantly to the structure transformation. for more than 1 day, 1a was able to return to the original
monoclinic crystalline structure of
1
.
Fig. 2 Representation of the water-triggered metal peptide coordination polymer. a) Optical microscope and SEM image of the
aggregated crystals of . b) Colour map of the atom name. c) Coordination pattern of the tetra-pyramidically coordinated Cu ions of
1a and 1b. d) PXRD pattern in different humidity. The top and bottom ones are simulated PXRD patterns of and 1a, respectively.
e) Crystal structure of . f) Crystal structure of 1a. g) Structural model of 1b. h) In situ PXRD at mentioned temperature ( C) in air.
Cu, green; C, grey; N, blue; O, red; H, white)
1
1,
1
o
1
(
2
| J. Name., 2012, 00, 1-3
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Thus, the SCSCT is reversible, pointing an achievable midday in the summertime) as shown in Fig. 1. Both
pathway for egress and ingress of the water molecule without transformations were triggered by the releasing and
pressure-assistance, accompanied by a reversible ‘Stomata-like intercalation of water molecules and involved in the breaking
breathing’ of the crystal, in which the open and close of the and reforming of coordination bonds while the peptide
‘
guard’ layers are controlled by the humidity. It is worth noting molecules remained intact.
that the PXRD patterns of the sample in different humidity at Experimental characterization and computational
room temperature confirmed the homogeneity of the modelling of the structural transformation mechanism. To
structure transformation (Fig. 2d). elucidate the possible mechanism of structural transformation
In situ variable temperature PXRD measurements (Fig. 2h) from the wet state ( ) to the dry state (1a or 1b), the in situ
and thermogravimetric analysis were implemented to examine variable temperature Fourier transform infrared (FT-IR)
the thermal stability of this porous framework. A small change spectra of were recorded as shown in Fig. 3a. No obvious
1
1
o
−1
−1
was observed at 42 C with a series of new peaks detected and shift was observed at 3445 cm and 3306 cm , which
o
subsequently, the peaks of
1
gradually disappeared from 42 C corresponds to a stretch asymmetric vibration νas(NH
to 50 C, indicating that the framework gradually changed. The symmetric vibration ν (NH ) of the terminal group on the
stepwise thermogravimetric curve (Fig. S7) of indicates that dipeptide ligands, indicating that the NH moieties remain
the release of the guest water molecules (measured loss 13.5 coordinated during the dehydration process. Notably, the
2
) and
o
s
2
1
2
o
−1
wt%, calculated 13.2 wt%), occurred at
∼50 C to give a new spectrum of 1 exhibited ν(O-H) bands at 3275 cm , and when
∼235 C. Therefore, the new the sample was heated to 72 C, this peak shifted to 3365 cm .
crystal form (1b) (Fig. 2g) can be accomplished by slow heating In the meantime, peaks of C-H bonds from 3000 cm to 2900
o
o
−1
crystal form, which is stable up to
−1
o
o
−1
up to the temperature range between 50 C and 235 C. On cm shifted to lower frequency with the temperature
further heating, the peptide molecule is decomposed at the increasing. These results indicated decrease in the
a
o
temperature about 237 C, forming a brown amorphous phase. conjugation of the system of hydrogen bonded six-water
Mass spectrometric study (Fig. S8 and S9) indicated that member ring, which might be owing to a charge-transfer
methyl groups, ammonia, water, nitric oxide and carbon oxides interaction between the lamellar structure and the water
7
were released at this point. No chemical decomposition was molecules. This is consistent with our initial assumption that
detected between the crystal transformation and ligand water molecules can induce changes in the coordination
decomposition temperatures.
environments around the copper atoms, and the evacuation of
water triggered the structural transition.
1
7,18
Based on the simulated PXRD (Fig. S11-13) by Diffax
and the in situ infrared spectra discussed later, it was
suggested that during the transformation from
intra-layer transformation occurred without very much inter- characteristic peak, ν (COO), is the asymmetric vibration of
The carboxyl group of the dipeptide coordinated with Cu
1
to 1b, the ions plays a key role for the transformation. The most
as
−1
layer rearrangement. This result indicates that sluggish water carboxyl groups around 1380 cm , and its peak position
-
evaporation upon heating can also change the intra-layer indicates that carboxylate group COO is bridging two Cu ions.
−1
−1
structure but retain stacking faults comparing with the This peak shifts from 1385 cm to 1378 cm . More
structure of 1a. Similar to the reversible phase transformation importantly, it became stronger from room temperature to 72
o
between
1
and 1a, the reversible phenomenon was also
C, and then turned weaker after further heating (Fig. 3a). The
detected in the case of 1b when it was exposed to moisture, increasing of its intensity during the transformation indicates
o
even after being heated up to 200 C (Fig. S14), indicating an that the coordination mode of Cu-OCO-Cu has changed during
extraordinary stability of GT in this crystalline form. It is the dehydration. The in situ FT-IR provided a possible
surprisingly for the peptide to be stable at such a high mechanism, which is related to a bond breaking and reforming
o
temperature since the bare peptide decomposes at 100 C (Fig. reaction occurred in a crystal by thermal induced desorption of
S15-17).
Thus, the flexible porous compound
reversible structural transformation between
water molecules.
1
1
shows both
The first-principles calculations were carried out to better
and 1a under understand the reaction pathway. According to charge density
2
+
different humidity at room temperature (reminiscent of the difference (CDD) schematic around Cu ions (Fig. 3b), each Cu
guard cells open the stomata under wet condition to abtain strongly binds with two N atoms and two O atoms. While,
more CO for photosynthesis) and between and 1b around there are obvious increment of electron density in the middle
1
2
o
the body temperature 38 C (reminiscent of the guard cells region of Cu and the ligand atoms. Other than this, the
close the stomata to hold on the water in the plants body at depletion of electron density is near the ligand atoms.
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Fig. 3 Experimental characterization and computational modelling of the structural transformation. a) In situ FT-IR of
1
at varying
temperatures under air, from blue to red represents temperatures from 25 C to 220 C. The initial and final curves were coloured
black for clarity. b) The isosurface maps of charge density difference for Cu-(Gly-Thr)/H O system. c) The CDD isosurface maps for
O-Cu/(Gly-Thr) system. The CDD isovalues of Cu-(Gly-Thr)/H O system and H O-Cu/(Gly-Thr) system are 0.006 and 0.003 e/Å . The
o
o
2
3
H
2
2
2
purple and cyan coloured isosurfaces correspond to negative and positive values, respectively. d) Relative energy of reaction
pathway for transformation from wet to dry states.
According to the infrared spectra, the coordination mode is no such bonding force or restriction for O1 and O3. Hence,
of Cu-OCO-Cu has changed, while the Cu-N coordination it’s reasonable to identify the breaking of Cu1-O3 during the
remained during the phase transformation. Because O2, N1 transformation. Due to the far distance of Cu1 and O1
2
+
and N2 are coordinated with the same Cu , the chelation and (Cu1···O1, 2.311 Å), there just exists depletion in the region
sterically hindered effect restrict the movement of the peptide, near O1, and no accumulation in the middle region between
which in turn influences the flexibility of Cu1-O2. While, there Cu1-O1, demonstrating the relatively fragile electrostatic
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interaction between them. It is also easy to identify the simplified in this calculation, for example, the remaining parts
+
2
breaking of Cu1-O1 coordination bond. In fact, Cu is penta- of peptides were not coordinated with copper, while they
coordinated and due to the Jahn-Teller effect, Cu1-O1 is the actually connected other coppers in the periodic system.
1
9,20
weakest bond among them.
Therefore, we can make a Moreover, the hydrogen bonding interactions have not been
reasonable assumption that the breaking of Cu1-O1 and Cu1- considered between water layers and function groups of the
+
2
O3 bonds provides a possibility for one group attacking Cu
and another group leaving away.
ligands. These interactions responsibly contribute to the
transformation mechanism and could possibly decrease the
To provide further insight into the effect of water transformation energy significantly.
molecules on the transformation, we calculated CDD for an Water regulated CO adsorption. To investigate the
isolated water (Fig. 3c), and it shows that hydrogen bond similarity of CO adsorption behavior regulated by water
2
2
formation leads to intermolecular stabilization of water between plant stomata and our material, we carried out
molecules. The hydrogen atom (Hw3) of water and the oxygen dynamic adsorption experiments in the absence and presence
atom of peptide bond (C2O1) hold together by hydrogen of water. We firstly investigated the static adsorption behavior
o
bonds, while O1 is coordinated with the Cu cation. The of the material at -78 C (Fig. 4a). The amounts of adsorbed
3
and 1a at the ordinary pressure are 45.90 cm /g and
increment of electron density around O1 is polarized by water, CO for
2
1
3
and the oxygen atom (Ow1) from water interacts with the 7.65 cm /g, respectively, and the potential selective
nitrogen atom of terminal amino group (N1H1). These results adsorption is very distinct. In addition, the tremendous
suggest that intra-atomic electron configurations play a hysteresis of water adsorption (Fig. S22) suggests a much
significant role in tipping the balance between water stronger interaction between water and framework, which
molecules and the frameworks, despite the larger changes in could be easily understood since the unusual flexibility of our
2
1
intermolecular energies. In fact, these interactions may PCPs is very closely related to water. The dynamic adsorption
provide the internal driving force for the structural experiments were subsequently implemented in the absence
o
(dry CO ) and presence (wet CO ) of water at 25 C (Fig. 4b).
transformation.
2
2
According to the periodic electronic structure, a possible For the dry CO , the breakthrough is rapid within only 19 s and
2
reaction pathway was animated to show the evolution of the calculated adsorbed amount can be compared with the
conformational state transition. Like domino effects, the result from static adsorption, but the breakthrough in the
constitutional units in the bulk phase are highly relevant presence of water is much slower with a 4 times longer
during the transformation. The center copper cation and its breakthrough time. It is also apparent that the ascent rate of
related fragments before and after transformation were the wet state CO after breakthrough is more sluggish. Such
2
abstracted from primitive structures in wet and dry states as differences in breakthrough behavior are induced by the
initial and final states, respectively (Fig. S20). Moreover, 9 introducing of water.
frames on the trajectory from the initial state to the final state
To further explore the origin of the distinctive adsorption
were generated via reaction preview in Material Studio (Movie behavior, the materials after breakthrough experiments were
S1 and S2). Then every single point energy was calculated (Fig. identified by PXRD (Fig. S23). The material in the dry CO is the
2
3d). From the energy change as reaction coordinates, the dehydrated 1a with a closed interlayer, making it hard for CO2
difference of total energy between the peak point and initial to penetrate from the interlayer. While the relative humidity
state is 2.14 eV. This is a relatively low energy for the total of the wet CO is only 34% and could not induce the entire
2
2
system compared with those in the literature, which reformation of
2
1. However, the fewer opened tunnels is
confirmed the reasonability of the reversible transformation at propitious for CO to travel through and interact with the pore
2
ambient conditions. Note that many factors have been walls, inducing a much larger adsorbed amount and longer
o
Fig. 4 Water regulated CO adsorption behavior. a) CO adsorption curves at -78 C. b) Breakthrough cycle under dry and wet
2
2
o
conditions at 25 C. Breakthrough time is highlighted in pink and light blue.
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breakthrough time. In the meantime, the small amount of water
3
4
5
6
S. Kitagawa, R. Kitaura, S. Noro. Angew. Chem. Int. Ed. 2004,
43, 2334-2375.
2
3
2
molecules also hinders the penetration of CO , which contributes
H. Satoshi, S. Shimomura, S. Kitagawa. Nat. Chem. 2009,
95-704.
1,
to the gentle ascent slope. We also believe that the water molecule
is dynamically moving along with the gas flow, thus constantly
6
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adsorption behavior of our PCPs material could be wisely regulated
2
by water. It is very rare for PCPs exhibiting water enhanced CO
2
24-26
adsorption,
and to the best of knowledge, our PCPs material is
7
8
9
1
the first one possessing such property through structure
27,28
transformation.
2
The CO adsorption behavior of our material is
also very similar to that of botanical stomata, since guard cells
would close stomata under very low ambient humidity thus
T. K. Maji, R. Matsuda, S. Kitagawa. Nat. Mater. 2007,
48.
6, 142-
1
preventing CO
to take up CO
2
taking up, and open stomata under wet ambience
for photosynthesis.
0 C. Martí-Gastaldo, D. Antypov, J. E. Warren, M. E. Briggs, P.
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D. Louër, G. Férey. J. Am. Chem. Soc. 2002, 124, 13519-
3526.
2
Cu-(Gly-Thr)•2H O, was prepared with structural transformative
1
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pore wall of the PCPs and the water molecules. It was found that a
reversible SCSCT with bond breaking and reforming reactions
occured without changing coordination numbers and valence of the
copper ion. Adsorption experiments indicate our material possesses
1
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We acknowledge funding from the National Natural Science
Foundation of China (51690152, 51233003 and 21471009). We
thank Prof. Shaw Ling Hsu and Dr. Fang Chang Tsai for
discussions; we thank the Department of Polymer Science and
Engineering at University of Massachusetts (Amherst) for
assistance with in situ FT-IR experiments and facilities. We
thank Prof. Hexiang Deng for assistance with PXRD
experiments and facilities.
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Conflict of interest
There are no conflicts to declare.
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