Guest-adaptable and water-stable peptide-based porous materials by imidazolate side chain control

Article abstract of DOI:10.1002/anie.201307074

The peptide-based porous 3D framework, ZnCar, has been synthesized from Zn²⁺ and the natural dipeptide carnosine (β-alanyl-L-histidine). Unlike previous extended peptide networks, the imidazole side chain of the histidine residue is deprotonated to afford Zn-imidazolate chains, with bonding similar to the zeolitic imidazolate framework (ZIF) family of porous materials. ZnCar exhibits permanent microporosity with a surface area of 448 m² g^-1, and its pores are 1D channels with 5 A openings and a characteristic chiral shape. This compound is chemically stable in organic solvents and water. Single-crystal X-ray diffraction (XRD) showed that the ZnCar framework adapts to MeOH and H₂O guests because of the torsional flexibility of the main His-β-Ala chain, while retaining the rigidity conferred by the Zn-imidazolate chains. The conformation adopted by carnosine is driven by the H bonds formed both to other dipeptides and to the guests, permitting the observed structural transformations. A peptide-based porous framework (ZnCar×DMF), which is formed by the assembly of Zn with carnosine, exhibits permanent microporosity and has pores of chiral shape. Zn-imidazolate bonding to the histidine side chain affords the chemical stability of the framework in water, and the His-β-Ala main chain provides flexibility while conferring structural adaptability on the framework in the presence of different guest molecules.

Full text of DOI:10.1002/anie.201307074

Angewandte
Chemie
DOI: 10.1002/anie.201307074
Metal–Organic Frameworks
Guest-Adaptable and Water-Stable Peptide-Based Porous Materials by
Imidazolate Side Chain Control**
Alexandros P. Katsoulidis, Kyo Sung Park, Dmytro Antypov, Carlos Martꢀ-Gastaldo,
Gary J. Miller, John E. Warren, Craig M. Robertson, Frꢁdꢁric Blanc, George R. Darling,
Neil G. Berry, John A. Purton, Dave J. Adams, and Matthew J. Rosseinsky*
Abstract: The peptide-based porous 3D framework, ZnCar,
has been synthesized from Zn2+ and the natural dipeptide
carnosine (b-alanyl-l-histidine). Unlike previous extended
peptide networks, the imidazole side chain of the histidine
residue is deprotonated to afford Zn–imidazolate chains, with
bonding similar to the zeolitic imidazolate framework (ZIF)
family of porous materials. ZnCar exhibits permanent micro-
porosity with a surface area of 448 m2 gÀ1, and its pores are 1D
channels with 5 ꢂ openings and a characteristic chiral shape.
This compound is chemically stable in organic solvents and
water. Single-crystal X-ray diffraction (XRD) showed that the
ZnCar framework adapts to MeOH and H2O guests because of
the torsional flexibility of the main His-b-Ala chain, while
retaining the rigidity conferred by the Zn–imidazolate chains.
The conformation adopted by carnosine is driven by the
H bonds formed both to other dipeptides and to the guests,
permitting the observed structural transformations.
patibility of the final products, enhance the structural and
chemical diversity of the internal surfaces of MOFs, and
afford chiral frameworks that may have unique separation
and catalytic properties.[6]
Peptides are particularly interesting as linkers because
dipeptides with hydrophobic residues that are held together
by H bonds form metal-free purely peptide-based porous
materials. These structures are divided into two groups, the
Val-Ala compounds with hydrophobic pores and the Phe-Phe
compounds with hydrophilic pores.[7] The Val-Ala structures
exhibit typical CO2 and CH4 adsorption for microporous
materials.[8]
In MOFs, peptides have the ability to act as connecting
ligands as they have at least one amino and one carboxylic
acid terminus that can coordinate metal ions. The dipeptides
Gly-Ala and Gly-Thr thus connect Zn2+ ions to form two
topologically distinct 2D-layered framework compounds,
Zn(Gly-Ala)2 and Zn(Gly-Thr)2, respectively.[9] The former
is a flexible porous material that displays an adaptable pore
conformation, which evolves continuously from an open to
a partially disordered closed structure in response to the level
of guest loading. The latter is structurally rigid to guest loss in
a manner characteristic of rigid MOFs and exhibits perma-
nent porosity with a surface area of 200 m2 gÀ1 after solvent
removal, as the framework is stabilized by the additional
H bonding between the OH functional group from the
threonine side chain and the NH2 terminal group. These two
examples clearly show how diverse the structures can be and
the strong control of adsorption behavior arising from small
changes in the peptide unit.
Here we present a new peptide-based MOF, ZnCar·DMF,
which is assembled from Zn2+ and carnosine (Car), a natural
dipeptide with the molecular structure b-alanyl-l-histidine.
The peptidic chain of carnosine contains an extra CH2 group
compared to classic dipeptides, because of the b-amino acid
structure of b-alanine. The histidine residue incorporates the
imidazole moiety that serves as an additional metal binding
site, and thus carnosine has two more potential linking points
compared to Gly-Ala and Gly-Thr. In reported histidine-
containing framework compounds the imidazole ring is
neutral and binds only one metal atom at N3.[10] ZnCar·DMF
is a 3D framework compound in which each carnosine
molecule links four tetrahedral Zn cations, two of which are
bridged by the deprotonated imidazolate ring. The structure is
flexible and displays 1D permanent porosity upon removal of
DMF. It is a microporous material with a specific surface area
of 448 m2 gÀ1 and exhibits strong binding affinity for small
molecules, such as CO2 and CH4. This MOF is not dissolved or
M
etal–organic frameworks (MOFs) are crystalline porous
materials composed of inorganic nodes, either single ions or
clusters of ions, bridged by organic linkers through metal–
ligand coordination bonds.[1] Recently, several biomolecules,
such as amino acids,[2] nucleobases,[3] saccharides,[4] and
peptides,[5] were used as organic linkers in MOF synthesis,
mainly because of the diversity of their metal binding sites.
The incorporation of biomolecules in MOFs also attracts
particular attention because they can improve the biocom-
[*] Dr. A. P. Katsoulidis, Dr. K. S. Park, Dr. D. Antypov,
Dr. C. Martꢀ-Gastaldo, Dr. G. J. Miller, Dr. J. E. Warren,
Dr. C. M. Robertson, Dr. F. Blanc, Dr. G. R. Darling, Dr. N. G. Berry,
Dr. D. J. Adams, Prof. M. J. Rosseinsky
Department of Chemistry, University of Liverpool
Crown Street, Liverpool, L69 7ZD (UK)
E-mail: m.j.rosseinsky@liverpool.ac.uk
Dr. J. A. Purton
Daresbury Laboratory, Science & Technologies Facilities Council
Kerkwick Lane, Warrington, WA4 4AD (UK)
[**] Financial support from the EPSRC under EP/J008834 and EP/
H000925 is gratefully acknowledged. We would like to acknowledge
the “Materials Chemistry Consortium” (the EPSRC grant EP/
L000202) for providing computational resources for this project.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307074.
ꢁ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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decomposed in either H2O or MeOH. Moreover, after
resolvation in MeOH and H2O, it adopts structures different
from that of the as-made or desolvated material. Single-
crystal X-ray diffraction studies showed that this flexibility is
driven by H bonds formed between the guests and the
dipeptide, leading to different torsional conformations of
carnosine.
Crystals of ZnCar·DMF in the shape of rectangular prisms
and 50 to 100 mm in length (Supporting Information, Fig-
ure S1) were obtained from the reaction of zinc nitrate with
carnosine in DMF/H2O at 1008C for 12 hours (see the
Experimental Section in the Supporting Information).
Under these conditions, both the imidazole N1 and the
carboxy group of the carnosine are deprotonated to form the
Car2À anion, allowing the formation of a 4:4 motif with Zn2+
(Scheme 1). The Zn cations are tetrahedrally coordinated to
framework is further stabilized by two H bonds, an intra-
molecular bond between the carboxy oxygen atom and amino
hydrogen atom of b-alanine, and an intermolecular bond
between the carboxylate oxygen atom of histidine and the
neighboring amide hydrogen atom (Figure 1a and c). The
arrangement of carnosine forms 1D square-shaped pores,
filled with DMF and running parallel to the crystallographic
b axis (Figure 1d). The chiral shape of the pore walls is
depicted using the Connolly surface representation (Fig-
ure 1e). The pores can be viewed as relatively large cavities,
with diameters of d1 = 5.18 ꢀ, connected in a zig-zag fashion
by narrow channels with diameters of d2 = 3.78 ꢀ.
The structure and the composition of ZnCar·DMF,
obtained from single-crystal X-ray diffraction data are in
good agreement with the characterization results of the bulk
sample. The powder XRD pattern perfectly matches the
simulated pattern from the single-crystal structure and is
indexed to the same unit cell (Figure S3 and Table S1). CHN
analysis results are very close to theoretical values (Table S2)
and the DMF content, calculated as 20 wt% from the single-
crystal data, was verified by thermogravimetric analysis
(TGA; Figure S4). The solid-state 13C CPMAS NMR spec-
trum of ZnCar·DMF displays all the expected resonances for
carnosine and three resonances for DMF (Figure S5) at room
temperature.
The ZnCar·DMF framework is structurally flexible and
displays permanent porosity after the removal of DMF, as
demonstrated by the variable-temperature single-crystal X-
ray diffraction data. While the crystallinity of the material is
retained over the whole temperature range that was studied,
the unit-cell volume decreases sharply from 762.82 to
726.78 ꢀ3 (by ꢀ 5%) between 388 and 394 K as DMF is
removed from the pores (Figure S6). The relaxation of the
ZnCar framework from the DMF-solvated state at 388 K
(Figure 2a) to the desolvated state at 394 K (Figure 2b)
results in an alternate linker conformation. The torsion angles
of the peptidic chain change by 20–308 (Figure S7) with the
exception of the rigid w angle C7-N8-C9-C14, which corre-
sponds to the peptide bond itself. The structure of the pore
was also altered, having cavities with diameters of d1 = 4.58 ꢀ
connected by channels with diameters of d2 = 4.18 ꢀ
(Table S3). In contrast, the three torsional angles responsible
for the orientation of the imidazolate ring and the carboxylate
group on histidine each changed by less than 28, indicating the
rigidity of the bc planes shown in Figure 1b.
Scheme 1. Formation of ZnCar·DMF from the reaction between
Zn(NO3)2 and carnosine.
four carnosine ligands (Scheme 1), and each ligand is
coordinated to four Zn2+ ions by the C-terminal His
carboxylate group, the N-terminal Ala amine group, and the
two nitrogen atoms of the imidazole ring (Scheme 1). The
chemistry and the bridging connectivity between the side
chain imidazole of histidine and the Zn cations are reminis-
cent of those seen in ZIF compounds.[11] The molecular cyclic
tetramer [Au(gly-l-his3À)]4·10H2O is the only other known
compound of histidine where metal–imidazolate bonding is
employed.[12]
ZnCar·DMF crystallizes in the chiral monoclinic space
group P21 and its 3D structure is directed by the carnosine
conformation (Figure 1a), as the Zn cations connected to the
histidine moiety define a plane and the residue of the N-
terminal b-alanine is extended to the third dimension. The
imidazolate rings (im) and Zn cations form a zigzag chain
where the rings adopt a trans conformation (Figure 1b) and
the Zn-im-Zn angle is 1388(Figure S2), which is smaller than
the 1458 angle found in ZIFs.[11a] This chain is similar to the
linear structures of univalent metal–imidazolate com-
pounds.[13] The chains are linked through the histidine
carboxylate group (Figure 1b) to form undulating layers
(parallel to the 100 plane) that are interconnected by the
antiparallel-oriented b-alanine residues (Figure 1c). The
The bulk desolvation of ZnCar·DMF was achieved by
evacuation on a high-vacuum line (10À5 mbar) at 1308C for
5 hours. The structure of the evacuated sample, ZnCar, was
confirmed by powder XRD and the complete removal of
DMF was verified by TGA (Figure S8). The porous proper-
ties of desolvated ZnCar were investigated with the CO2
adsorption–desorption isotherm (Figure S9) obtained at
195 K up to 1 bar. ZnCar is purely microporous, as the
isotherm shape corresponds to that of type I according to the
IUPAC classification. The BET surface area is calculated as
448 m2 gÀ1 using the relative pressure range, 0.01 < P/Po < 0.1.
The total pore volume, estimated at 0.19 cm3 gÀ1, is slightly
smaller than the solvent-accessible volume of the desolvated
ZnCar structure of 0.21 cm3 gÀ1. The isotherm shows no gate
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Chemie
Figure 1. a) 3D conformation of carnosine connected to four Zn cations. The dashed line corresponds to the intramolecular H bond between
a carboxy oxygen atom and an amino hydrogen atom. Three Zn cations are bound to histidine (above the line) and the fourth to b-alanine (below
the line). b) Zn cations and imidazolate rings form the zig-zag chain (highlighted) that is the rigid part of the structure. The chains are linked
through the histidine carboxylate group. c) Undulating layers of imidazolate rings interconnected by antiparallel-oriented b-alanine residues.
Intermolecular H bonds are formed between the carboxylate oxygen atom and the amide hydrogen atom. d) Space-filling representation of ZnCar
viewed along the b axis, showing the 1D pores. e) Connolly surface representation of the pore walls, calculated with a probe radius of 1.4 ꢂ. The
pores consist of cavities with diameters of d1 =5.18 ꢂ and narrow channels with diameters of d2 =3.78 ꢂ.
opening, but a low pressure hysteresis loop associated with
the strong adsorption of CO2.
compared to the desolvated structure (Figure 2c). The torsion
angles of carnosine in ZnCar·MeOH are in a range between
those in the structure containing DMF and the desolvated
structure (Figure S7). Using MD simulations, we found that
the carboxy oxygen atom of alanine in the desolvated
structure is sterically screened from forming an H bond with
a methanol molecule (Figure S10). Hence, the main structural
change caused by the addition of MeOH is the rotation of the
carboxy oxygen atom toward the pore where it can now form
an H bond with a guest molecule (each MeOH molecule also
works as an acceptor and forms an additional H bond with the
amine group) (Figure S11b). In the as-made material, the
same carboxy oxygen atom forms an intramolecular H bond
with one of the two hydrogen atoms on the amine group,
while the DMF molecule is bound to the other hydrogen atom
(Figure S11a). The DFT-calculated steepest descent energy
minimization of solvent-free ZnCar·DMF and ZnCar·MeOH
unit cells showed that both systems relax to assume the
structure of the empty framework. This means that both DMF
and MeOH guests distort the framework locally by H bonding
without inducing a phase transition in the framework itself
(Figure S12). MeOH vapor adsorption on ZnCar, collected at
The chemical stability of ZnCar·DMF was examined by
soaking fresh material in water under stirring for 3 days
(Table S4). ICP analysis of the supernatant aqueous solution
every 24 hours showed very low Zn leaching, corresponding
to 1.7 wt% of the Zn added to water in the form of
ZnCar·DMF. The stability of ZnCar·DMF in water, unlike
Zn(Gly-Ala)2 and Zn(Gly-Thr)2 frameworks, which are
stable only in organic solvents, is attributed to the Zn–
imidazolate bonding that is the basis of the chemical stability
of ZIFs.
To study the stability and the interaction of this frame-
work with different guest molecules, desolvated crystals of
ZnCar were immersed in H2O and MeOH. The resolvated
crystals resulted in two new structures, ZnCar·MeOH and
ZnCar·H2O. The ZnCar framework proved to be adaptable to
the presence of guest molecules, as the peptide conformation
changed in both cases without breaking the 4:4 connectivity
motif between Zn and carnosine. ZnCar·MeOH adopts the
monoclinic P21 space group with slightly altered unit-cell
parameters, but without significant changes in the pore shape
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tures, appears only in car-
nosine 2 in ZnCar·H2O. The
H2O molecules are inter-
connected by H bonds
forming a network inside
the pores. In particular, two
out of four distinct H2O
molecules in the wide pore
form the maximum possible
four H bonds, two of which
are with the framework
(Figure S16a).
In
the
narrow pore, there are two
distinct H2O molecules that
form three and two
H bonds, respectively (Fig-
ure S16b).The desolvated
ZnCar structure is obtained
after H2O removal (Fig-
ure S17) and the experi-
mentally measured pore
volume, 0.21 cm3 gÀ1 (Fig-
ure S18), equals to the esti-
mated value from the
ZnCar single-crystal struc-
ture. H2O vapor adsorption
isotherms on ZnCar exhibit
broad hysteresis loops and
Figure 2. The structures of a) ZnCar·DMF, b) desolvated ZnCar, c) resolvated ZnCar·MeOH, and d) resolvated
ZnCar·H2O. ZnCar·DMF, ZnCar, and ZnCar·MeOH show one type of pore with square or parallelogram shape.
ZnCar·H2O has two types of pores.
a
maximum uptake of
three different temperatures, reaches a maximum of 16 wt%
(Figure S13). Desolvation of ZnCar·MeOH again produces
the ZnCar structure (Figure S14), which is as porous as the
original DMF-desolvated ZnCar material (Figure S15), thus
demonstrating the repeatability of loading and unloading
MeOH from ZnCar.
15 wt% (Figure S19), and were cycled three times without
change. The exhibited reversible resolvation–desolvation of
the ZnCar framework with H2O proves that the properties of
ZnCar remain unaffected after its exposure to water. Unlike
ZnCar·MeOH and ZnCar·DMF, the DFT-calculated energy
minimization of the empty ZnCar·H2O structure did not
converge to that of the desolvated material, but relaxed to
a different stable state that was 53 kJmolÀ1 higher in energy
(Table S6). This means that the desolvated ZnCar and
ZnCar·H2O structures are separated by an energy barrier,
and the calculated energy increase for the framework is
counterbalanced experimentally by the formation of the
water network that is rich in H bonds. The framework
transformations, driven by the different guests, are facilitated
mainly by the torsional flexibility of the peptidic chain. The
2D layers constructed from the Zn–imidazolate chains con-
stitute the rigid part of the framework, as indicated by small
changes in the Zn-im-Zn angle, the Zn–Zn distance, and the
distance between the chains (Table S7).
High pressure CO2 and CH4 adsorption isotherms of
ZnCar were collected at 283, 293, and 303 K (Figure S20). The
gravimetric uptakes were found to be around 19 wt% for CO2
and 5 wt% for CH4 at 15 bars. The isotherms are of type I,
and in each case 80% of total uptake was adsorbed at 4 bars,
showing the entirely microporous character of this material.
The ZnCar MOF exhibits strong binding affinity for both
gases, as shown by the isosteric heat of adsorption Qst
(Figure S21). For CO2, the Qst value at zero coverage is
49 kJmolÀ1, which is amongst the highest ever reported for
MOFs. Cu-BTTri-mmen exhibits a Qst value of 96 kJmolÀ1,[14]
The structure of the water-resolvated ZnCar·H2O is
different, as carnosine adopts two distinct conformations,
carnosine 1 and carnosine 2 (Figure S7), which form different
sets of H bonds. Thus, two types of pores are formed, a wide
and a narrow one (Figure 2d), and the volume of the unit cell
is doubled compared to those of the other three structures.
The positions of the water oxygen atoms in both the narrow
and the wide pores were determined from the single-crystal
diffraction data with the hydrogen atoms added at calculated
positions. The orientations of the water molecules were then
refined using DFT calculations (Table S6). The carnosine 2
molecule defines a wide pore space, filled by eight H2O
molecules per unit cell, and exhibits a conformation similar to
those of the MeOH-containing structure, but with the amine
group pointing in the opposite direction. Carnosine 2 forms
six H bonds (Figure 3a), three within the framework and
three with H2O. The narrow pore is filled with four H2O
molecules and is bound by carnosine 1, in which the torsion
angles of the peptidic chain (Figure S7) were significantly
changed compared to those of all the other structures
(Figure 2a–c). Carnosine 1 forms three H bonds (Figure 3b),
one within the framework and two with H2O. The intra-
framework H bond between the carboxylate oxygen atom and
amide hydrogen atom, present in the three previous struc-
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Chemie
permanent porosity in the ZnCar·DMF compound.
ZnCar·DMF is an analogue of ZIFs where the involvement
of the imidazole ring of the histidine side chain in Zn–
imidazolate bonding both affords the chemical stability of the
framework in water and produces a 3D structure, in contrast
to previously studied peptide-based MOFs. The main His-b-
Ala chain connects the rigid imidazolate-based layers, retain-
ing its flexibility and conferring structural adaptability on the
framework in the presence of different guest molecules. The
ZnCar framework exhibits permanent microporosity and
strong adsorption affinity for CO2 and CH4.
Received: August 12, 2013
Revised: October 23, 2013
Published online: December 2, 2013
Keywords: imidazolates · metal–organic frameworks ·
.
microporous materials · peptides · structural adaptability
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and for the most microporous MOF, the Qst at zero coverage
ranges from 25–50 kJmolÀ1.[15] Our DFT calculations indicate
that most of this energy can be attributed to the dispersion
interactions (Table S8), in particular between the imidazole
ring and the CO2 carbon atom (Figure S22). The molecular
dynamics (MD) simulations of CO2 diffusion along the pores
have also shown this configuration to be the most favorable
for adsorption, and showed that the CO2 molecules diffuse by
hopping between the cavities in the pore structure through
smaller channels that connect the pores (Figure 1e and
Figure S22). The Qst for CH4 adsorption at zero coverage is
27 kJmolÀ1, which is also a high value for MOFs,[16] very close
to the highest reported Qst value 30 kJmolÀ1 for PCN-14.[17]
The selectivity of adsorption between CO2 and CH4 was
estimated using the IAST model at 303 K for the equimolar
gas mixture, and shows a value of 10 for the whole pressure
range (Figure S23). This is a moderate value for MOFs, lower
than that of Mg2(dobdc), which is 60,[18] in accordance with the
high adsorption affinity of ZnCar for both gases.
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