Synthesis and Reversible Hydration of a Pseudoprotein, a Fully Organic Polymeric Desiccant by Multiple Single-Crystal-to-Single-Crystal Transformations

Article abstract of DOI:10.1002/anie.201806451

A diphenylalanine derivative, N3-Phe-Phe-NHCH2CCH, was designed for topochemical azide–alkyne cycloaddition (TAAC) polymerization. This dipeptide adopted β-sheet arrangement as designed, in its crystals, but the azide and alkyne were not fitly aligned for

Full text of DOI:10.1002/anie.201806451

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Accepted Article
Title: Synthesis and Reversible Hydration of a Pseudoprotein, a Fully
Organic Polymeric Desiccant via Multiple Single-Crystal-to-
Single-Crystal Transformations
Authors: Kana M Sureshan and Raja Mohanrao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806451
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Synthesis and Reversible Hydration of a Pseudoprotein, a Fully
Organic Polymeric Desiccant via Multiple Single-Crystal-to-
Single-Crystal Transformations
Raja Mohanrao and Kana M. Sureshan*
We have designed the diphenylalanine derivative, N3-Phe-Phe-
NHCH2CCH for its topochemical azide-alkyne cycloaddition (TAAC)
polymerization. This dipeptide adopted -sheet arrangement as
designed, in its crystals, but the azide and alkyne were not fitly aligned
for their topochemical reaction. However, the voids present around
these groups allowed them to attain a reactive geometry upon heating
and their consequent TAAC polymerization to a pseudoprotein in a
single-crystal-to-single-crystal (SCSC) fashion. This motion led to the
creation of channels in the product-crystal and it absorbed water from
the surroundings to fill these channels as H-bonded water wire. The
pseudoprotein undergo reversible hydration/dehydration in SCSC
fashion many times under mild conditions; hydration at low relative
humidity and dehydration at low temperature. Vapor sorption analyses
suggest that this fully-organic polymer might be useful as energy-
efficient desiccant material for controlling indoor humidity.
polymerization involved large motion of reacting partners and the
consequent creation of channels in the lattice enabled the
absorption of water from the surroundings to form
stoichiometrically hydrated polymer crystal. This polymer crystal
also undergoes SCSC reversible dehydration-rehydration
multiple times at ambient conditions suggesting that it might find
application as a fully-organic desiccant material.
The usefulness of natural protein materials such as silk, wool,
keratin, collagen, soy protein etc. for various applications
generated great interest in developing protein-mimics having
robust non-natural linkages as amide surrogates, in view of their
increased stability to chemical or enzymatic degradation.[1] 1,2,3-
Triazole motif has been established as a stable bioisostere of
amide linkage.[2] The conventional solution-phase synthesis of
triazole involves the use of metal-based catalysts.[3] It is difficult to
remove such metal catalysts from peptides due to strong
coordination.[4] In addition, premature precipitation of partially
oligomerized products is a major hurdle in the solution-phase
Figure 1. Plausible -sheet alignment of azide and alkyne-modified
diphenylalanine and its TAAC polymerization.
Diphenylalanine (FF) peptide derivatives are known to form
parallel -sheets in their crystals and other self-assembled
states.[ The propensity of FF unit to form -sheet assembly is
known in natural systems also. For instance, FF unit is considered
to be the key structural motif responsible for the aggregation of
polypeptides.[5]
synthesis
of
large
triazole-containing
8]
Topochemical Azide-Alkyne Cycloaddition (TAAC) reaction has
proven to be an efficient method for the synthesis of various
triazole-linked biopolymer mimics.[6] Like other topochemical
[9]
reactions,[7]
A-peptides through -sheet assembly to form amyloid fibrils. It
is therefore anticipated that diphenylalanine decorated with the
complementary reactive motifs (CRMs) viz. azide and alkyne
groups at its termini would crystallize in parallel -sheet
arrangement and thereby organize the CRMs of adjacent -
sheets at proximity in a direction perpendicular to the -sheet
direction (Figure 1). Such an arrangement would facilitate the
TAAC reaction along this direction to form pseudoprotein
containing FF repeating units. The attractive properties of
diphenylalanine-based materials and their possible applications
in various fields[10] encouraged us further to pursue this
hypothesis.
this proximity driven 1,3-dipolar cycloaddition
reaction of azide and alkyne in the crystal to form triazole, is
attractive as it (i) avoids solvent, catalyst, activators etc., (ii)
produce specific product in high yield, (iii) avoid chromatographic
purification and (iv) often give products that are not accessible by
conventional solution-phase reactions. Small peptides of
predictable supramolecular behavior can be exploited for TAAC
reaction to make large peptidomimetics.[6e] Herein we report the
Single-Crystal-to-Single-Crystal (SCSC) polymerization of
a
dipeptide to a pseudoprotein through TAAC reaction. This SCSC
The dipeptide (DP) was synthesized by adopting the standard
protocol (Scheme S1, Supporting Information) and was
crystallized (Figure 2) from a mixture of methanol and toluene (1:1,
v/v). Single crystal XRD analysis revealed that the DP crystallized
[*]
R. Mohanrao & Prof. Dr. K. M. Sureshan
School of Chemistry, Indian Institute of Science Education and
Research Thiruvananthapuram, Kerala-695551 (India).
E-mail: kms@iisertvm.ac.in
Homepage: http://kms514.wixsite.com/kmsgroup
1 1 1
in P2 2 2 space group. As expected, the molecules adopted -
sheet alignment in ‘a’-direction (Figure 2B) through strong NH…O
hydrogen bonds. This -sheet assembly is further stabilized by
two supplementary C-H…O hydrogen bonds and a weak …
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201xxxxxx.
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Figure 2. A) Chemical structure of dipeptide DP. B) -sheet alignment of DP in ‘a’ direction. C) ‘bc’ plane of crystals of DP showing two azides and two alkynes in
reaction cavity (shaded oval) and two possible propagations of polymer chain along ‘b’ or ‘c’ direction (shown as waves). D) Chemical structure of pseudoprotein-
hydrate (PPH). E) -sheet alignment and H-bonded water-wire along a-direction. F) ‘bc’ plane of crystals of PPH showing the polymer connectivityand its propagation
along b-direction.
stacking interaction (Table S2, SI). Interestingly, adjacent -
sheets are arranged in such a way that molecules of DP are
organized in head-to-tail fashion in both b- and c-directions
(
Figure 2C) in the crystal lattice. However, the static distances
between the reactive groups viz. azide and alkyne motifs and their
relative orientations were not suitable for the TAAC reaction, in
any of these directions. Nevertheless, the azide and alkyne
groups are present in a cavity-like environment with the phenyl
rings circumscribing the cavity (Figure 2C). Voids present in the
vicinity of alkyne and azide groups (Figure S2, SI) would allow
rotational flexibility of these groups. Rotations of azide and alkyne
groups allow two parallel reactive orientations of them such that
the chain can grow either along b-direction or c-direction (Figure
S3, SI).
1
o
H NMR of crystals of DP heated at 60 C for six days showed
the complete disappearance of the starting material and formation
of oligomers containing 1,4-disubstituted triazolyl linkages
(
Spectral data, SI). Presence of only one type of triazolyl protons
1
in the H NMR spectrum of heated crystals of DP suggested that
the solid-state cycloaddition reaction is regiospecific and the
reaction progressed under topochemical control. However, major
portion of the product obtained was insoluble in common solvents
(
Section 7, SI). Adding solubility promoters such as LiCl, CaCl
2
and TFA did not improve the solubility to a greater extent. (Figure
S4, SI). MALDI spectrum of the mixture of products contains
oligomers of molecular weights up to 13.1 KDa, corresponding to
Figure 3. A) TGA of DP and PPH. B) PXRD spectra of pseudoprotein-hydrate
(PPH) at rt and 50 oC (converted to PP).
1
05 aminoacids (Figure S5, SI). SCXRD analysis of the heated
crystals revealed that crystals of DP have undergone SCSC
polymerization. The pseudoprotein also retained the P2
space group. Interestingly, each asymmetric unit contains two
molecules of water, in addition to the repeating unit of the
1
2
1
2
1
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pseudoprotein (Figure 2D). While the unit cell parameters ‘a’ and
opposite directions for their TAAC reaction. Consequently, the
phenyl groups of two molecules also moved apart from each other
resulting in the expansion of cell parameter along ‘c’-direction and
the consequent creation of channels in the lattice. These channels
are occupied by water molecules, which form a hydrogen-bonded
water-wire per each channel along ‘a’-direction (Figure 2E).
Hence, it is an interesting case wherein the topochemical reaction
leads to the generation of channels in the crystal lattice, which
absorb water molecules from the surroundings.
‘
c’ increased by 1.8% and 8.3% respectively, the ‘b’ has
decreased by 7.3%, amounting to an overall 2.7% increase in unit
cell volume (Table S3, SI). While small crystals (0.2 mm x 0.12
mm x 0.05 mm) are morphologically intact, larger crystals (e.g., 4
mm x 0.9 mm x 0.2 mm) were found to undergo cracking after the
reaction (Figure S6, SI). However, the transparency and overall
shape of the crystals were preserved and the pieces did not get
detached from the cluster. We have isolated one of the fragments
from the cluster and analyzed it using SCXRD. The crystal
structure of the polymer PPH obtained by this method was
indistinguishable from the structure obtained by solving the whole
Thermogravimetric analysis (TGA) of the PPH crystals
revealed that it releases the water (weight loss of 8.1%) in the
temperature range 20-50 oC (Figure 3A) and this was further
supported by DSC experiments (Figure S8, SI). The low
temperature dehydration suggests that the water molecules are
loosely bound in the channels. This is not surprising as the water
is located in a hydrophobic channel, where it makes only a single
H-bond with the pseudoprotein. To investigate whether the
crystallinity is maintained after the dehydration, we have
(
small) single crystal.
A comparison of the crystal structures of DP and the
pseudoprotein-hydrate (PPH) revealed that the polymerization
happened along the crystallographic ‘b’ direction (Figure 2F),
which accounts for the contraction of cell in this direction. The -
sheet arrangement is retained in the ‘a’ direction (Figure 2E)
without much change in the intermolecular interactions, which
accounts for minimal change in the unit cell along ‘a’ direction. It
is clear from the overlay of crystal structures of DP and PPH that
the propargyl group and the azide (Figure S7, SI) group rotated in
o
compared its PXRD profiles at room temperature and at 50 C.
Figure 4. A) SCSC dehydration of the PPH to PP showing the sliding of polymer chains. B) 30 cycles of TGA of the PPH showing complete and reversible hydration-
dehydration. C) Water adsorption and desorption isotherms of the pseudoprotein at 20 oC showing sigmoidal graph.
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The PXRD profile at 50 oC was different from that at room
temperature and the sharp peaks indicate that dehydration is a
crystal-to-crystal process (Figure 3B). The PXRD profile of a
sample cooled from 50 oC to room temperature, in an open
atmosphere, was indistinguishable from that of the fresh as
formed PPH crystals, suggesting that the dehydrated sample
undergo hydration again. Thus these PXRD experiments reveal
that the hydration-dehydration is reversible and happens without
losing the crystallinity.
indoor atmosphere.[16] The pseudoprotein material described here
could be an efficient material for such purpose.
To verify the structural integrity and stability of PPH in various
solvents, its crystals were soaked in different solvents for 48 h.
The water sorption capacity of the PPH crystals was checked after
removing from the solvent and exposure to air. The pseudoprotein
showed unaltered water sorption efficiency as evidenced from the
TGA experiments (Figure S12, SI). Interestingly, the aged (3
months) crystals showed similar water sorption efficiency as the
freshly prepared crystals. All these crystals (fresh, aged, solvent
We could obtain the crystal structure of the dehydrated
pseudoprotein (PP) by carrying out the SCXRD at 50 oC,
suggesting that the dehydration is indeed a SCSC transformation
o
exposed) were found to be thermally stable up to 300 C.
In conclusion, we have designed and synthesized
a
(
Figure 4A). Upon cooling, the crystal again absorbed water,
phenylalanine based dipeptide decorated with complimentary
reactive groups viz. azide and alkyne at its termini having
propensity to undergo -sheet assembly for its topochemical
forming PPH as evidenced from SCXRD analysis. A comparison
of the crystal structures of PPH and PP reveal that upon
dehydration the adjacent polymer chains slightly slide in opposite
directions along b-axis to minimize the void previously occupied
by water molecules (Figure 4A). Hence, this mutual sliding of
polymeric chains acts as a gate for water absorption and
desorption.
In order to check the efficiency of hydration-dehydration over
several cycles, we have done 30 cycles of repetitive TGA of the
pseudoprotein by heating till 70 oC and then cooling to room
temperature in an open atmosphere (Figure 4B). It is clear that
polymerization to
a pseudoprotein. Though this monomer
crystallized with -sheet arrangement, the azide and alkyne motifs
were not in a ready-to-react orientation. Despite having a non-
reactive conformation, the crystal underwent
a
SCSC
polymerization through a regiospecific TAAC reaction forming
1,4-triazolyl-linked pseudoproteins. The spacious cavity and the
flexible linkages allowed the reacting motifs to rotate and
approach each other to form a reactive conformation and react
regiospecifically. Due to large motions of reactive motifs,
channels are generated in the polymer crystal and these channels
attract water from the surroundings to form a stoichiometric
hydrate. These crystals undergo dehydration under mild
conditions in a SCSC fashion and the dehydrated crystals
undergo further hydration, again in SCSC manner. The reversible
hydration/dehydration can be repeated multiple times without
losing the efficiency. This fully-organic polymeric desiccant might
be useful for various applications.
o
by 70 C the PPH released all of its water to form PP and while
cooling the PP absorbed water from the atmosphere in as short
as 10 min. to reconvert it back to PPH. The hydration-dehydration
efficiency was found to be uncompromised even after 30 cycles
with a hydration of 8.1-8.5 % by weight, which was in agreement
with the theoretical weight of 8.7 %. Interestingly, dehydration
o
started from 20 C itself under the conditions of TGA (Nitrogen
purging at a flow rate of 100 mL/min). This prompted us to
investigate the dehydration of PPH at lower temperature. An
isothermal TGA at 20 oC (Figure S11, SI) revealed that the
dehydration is complete within 20 min.
Materials that absorb/adsorb water reversibly are of great
significance for desiccant applications.[11] Most of the desiccant
materials known are either inorganic substances such as silica[12]
or metal containing substances such as MOFs.[13] There is a lot of
interest in fully-organic polymeric desiccants in view of their
processability, non-toxicity etc.[14] An important and desirable
Acknowledgements
KMS thanks Department of Science and Technology, Govt. of
India for a SwarnaJayanti Fellowship.
Keywords: click reaction • conformational motion • cycloaddition
•
organic desiccant • phenylalanine • pseudoprotein •
property of a desiccant is the energy-efficient low-temperature
dehydration.[15] We envisioned that our pseudoprotein material
topochemical reaction
could be used as fully-organic desiccant material in view of its
reversible water sorption at mild conditions.
The sorption capacity of PP was analyzed using dynamic vapor
sorption analyzer at every 10% intervals in relative humidity (RH)
starting from 0 to 100% (Figure 4C). At lower humidity levels (till
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Entry for the Table of Contents
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Raja Mohanrao and Kana M. Sureshan*
Page No. – Page No.
Synthesis and Reversible Hydration
of a Pseudoprotein, a Fully Organic
Polymeric Desiccant via Multiple
Single-Crystal-to-Single-Crystal
Transformations
We report a single-crystal-to-single-crystal TAAC polymerization of a Phenylalanine
based dipeptide to a pseudoprotein. The dipeptide experienced large conformational
motion to achieve the reactive orientation required for TAAC reaction. This resulted
in the creation of channels in the pseudoprotein leading to stoichiometric absorption
of water into these channels. The pseudoprotein undergoes reversible dehydration
and rehydration under mild conditions and hence find application as a desiccant
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