Metallo-Helicoid with Double Rims: Polymerization Followed by Folding by Intramolecular Coordination

Article abstract of DOI:10.1002/anie.202010696

In this study, we established a feasible strategy to construct a new type of metallo-polymer with helicoidal structure through the combination of covalent polymerization and intramolecular coordination-driven self-assembly. In the design, a tetratopic monomer (M) was prepared with two terminal alkynes in the outer rim for polymerization, and two terpyridines (TPYs) in the inner rim for subsequent folding by selective intramolecular coordination. Then, the linear covalent polymer (P) was synthesized by polymerization of M via Glaser-Hay homocoupling reaction. Finally, intramolecular coordination interactions between TPYs and Zn(II) folded the backbone of P into a right- or left-handed metallo-helicoid (H) with double rims. Owing to multiple positive charges on the inner rim of helicoid, double-stranded DNA molecules (dsDNA) could interact with H through electrostatic interactions. Remarkably, dsDNA allowed exclusive formation of H with right handedness by means of chiral induction.

Full text of DOI:10.1002/anie.202010696

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
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Accepted Article
Title: Metallo-Helicoid with Double Rims: Polymerization Followed by
Folding via Intramolecular Coordination
Authors: Guangqiang Yin, Sneha Kandapal, Chung-Hao Liu, Heng
Wang, Jianxiang Huang, Shu-Ting Jiang, Tan Ji, Yu Yan,
Sandra Khalife, Ruhong Zhou, Libin Ye, Bingqian Xu, Hai-Bo
Yang, Mu-Ping Nieh, and Xiaopeng Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202010696
Link to VoR: https://doi.org/10.1002/anie.202010696

10.1002/anie.202010696
Angewandte Chemie International Edition
RESEARCH ARTICLE
Metallo-Helicoid with Double Rims: Polymerization Followed by
Folding via Intramolecular Coordination
Guang-Qiang Yin, Sneha Kandapal, Chung-Hao Liu, Heng Wang, Jianxiang Huang, Shu-Ting Jiang,
Tan Ji, Yu Yan, Sandra Khalife, Ruhong Zhou, Libin Ye, Bingqian Xu, Hai-Bo Yang, Mu-Ping Nieh,
Xiaopeng Li*
Abstract: In this study, we established a feasible strategy to
construct a new type of metallo-polymer with helicoid-like structure
through the combination of covalent polymerization and
intramolecular coordination-driven self-assembly. In the design, a
tetratopic monomer (M) was prepared with two terminal alkynes in
the outer rim for polymerization, and two terminal terpyridines
(TPYs) in the inner rim for subsequent folding by selective
intramolecular coordination. Then, the linear covalent polymer (P)
guest systems and emissive materials.
Introduction
In geometry, helicoid is the third minimal surface to be
known, after the plane and catenoid.[1] As the name derived from
its similarity to the helix, helicoidal structure consists of
numerous parallel-arranged helices as its boundary (Fig.1). As
one type of topological structure with inherent chirality, helical
structures are ubiquitous in nature.[2] Helical or spiral
biopolymers play prominent roles in sophisticated biological
processes such as genetic encoding, chiral catalysis as well as
self-assembly of cellular components.[3] To mimic the complexity
and functionality of helical biopolymers in nature, a variety of
artificial helical oligomers and polymers have been successfully
constructed with the aid of reversible non-covalent forces, i.e.,
was synthesized by polymerization of
M through Hay-Glaser
homocoupling reaction. Finally, intramolecular coordination
interactions between TPYs and Zn(II) folded the backbone of P into
a right- or left-handed metallo-helicoid (H) with double rims. Due to
multiple positive charges on the inner rim of helicoid, double-
stranded DNA molecules (dsDNA) could interact with
H via
electrostatic interactions. Remarkably, dsDNA allowed exclusive
formation of H with right handedness by means of chiral induction.
Furthermore, the interaction with DNA molecules led to the
enhancement of luminescent efficiency of H through restricted
intramolecular motions. The processes of encapsulation of DNA
molecules were able to be probed by time-dependent fluorescent
spectroscopy. Our study not only broadened the scope of metallo-
polymer, but also paved a new avenue for the development of host-
metal-coordination
interactions[4],
hydrogen
bonding[5],
electrostatic attraction[6], and π-π interactions[7]. Compared to
the flourish of helical polymers, the spiral polymers with
helicoidal structures still remain less explored, perhaps because
of the challenge of design and synthesis of multi-rimmed or
multi-layered monomer to afford parallel-arranged helix as the
boundary helicoidal polymers in a well-controlled manner.
Among the artificial helical polymers, acetylene–based
helical polymers have been widely studied on account of their
facile synthesis, handleability, the host ability of tubular cavity,
and structural diversity.[8] However, the spiral conformation of
most of the reported acetylene–based helical polymers with
single chain (or rim) of backbone was sensitive to the solvents
and temperature due to the weak interactions. In the past few
years, 2,2′:6′,2″-terpyridine (TPY) has been broadly utilized to
construct various metallo-polymers owing to its versatile
binding affinity towards transition metal ions.[9] In TPY-based
metallo-polymers, TPYs could be introduced into conventional
polymers as pendant groups for further coordination, or act as
[☆]
Dr. G.-Q. Yin, Dr. H. Wang, Dr. X., Li*
College of Chemistry and Environmental Engineering, Shenzhen
University,
Shenzhen, Guangdong 518055, China
E-mail: xiaopengli@szu.edu.cn
S. Kandapal, Dr. B. Xu
Single Molecule Study Laboratory, College of Engineering and
Nanoscale Science and Engineering Center, University of Georgia
Athens, Georgia 30602, United States
C.-H. Liu, Dr. M.-P. Nieh
Department of Chemical and Biomolecular Engineering, University
of Connecticut
Storrs，Connecticut 06269, United States
S.-T. Jiang, T. Ji, Dr. H.-B. Yang
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, Department of Chemistry, East China Normal University
Shanghai 200062, China
the terminal chelation moieties for supramolecular
9d, 10]
polymerization.[9a,
The systems with multiple TPYs as
pendant groups generally experienced both intermolecular and
intramolecular coordination without precise control; While
monomers with two terminal TPYs were polymerized by
intermolecular coordination to form metallo-supramolecular
polymers with labile main chains.
Y. Yan, S. Khalife
Department of Chemistry, University of South Florida
Tampa, Florida 33620, United States
Dr. L. Ye
Department of Cell Biology, Microbiology and Molecular Biology,
University of South Florida
Tampa, Florida 33620, United States
Dr. M.-P. Nieh
Polymer Program, Institute of Material Science, University of
Connecticut
Storrs, Connecticut 06269, United States
Dr. J. Huang, Dr. R. Zhou
Institute of Quantitative Biology, Zhejiang University
Hangzhou, Zhejiang, 310027, China
This article is protected by copyright. All rights reserved.

10.1002/anie.202010696
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 1: Cartoon representation of helix (left) and helicoid (right).
To tackle the challenge of helicoidal polymers mentioned
above, we combined acetylene–based polymer chemistry (i.e.,
covalent bond) and TPY-based supramolecular chemistry (non-
covalent interaction) into a hybrid system to construct metallo-
helicoid (H) with double rimmed structure. As shown in scheme
1, a tetratopic monomer (M) was designed with two terminal
alkynes in the outer rim for polymerization, and two terminal
terpyridines (TPYs) in the inner rim for subsequent
intramolecular coordination driven self-assembly. Because of the
different length of two rims, intramolecular coordination played a
more predominant role in folding the metal-free polymer (P) to
form energy-favorable secondary structure with helicoidal
conformation compared to intermolecular coordination. We
envisioned that H might be able to interact with DNA molecule
through electrostatic interactions between positive charged
tubular cavity and phosphate anions of DNA molecule. As such,
the helicity of H may be induced by DNA to achieve controlled
helical orientation. Moreover, tetraphenylethylene (TPE) as an
iconic aggregation-induced emission (AIE) fluorophore was
incorporated into the backbone of H.[11] The coordination of
<TPY-Zn(II)-TPY> as well as the encapsulation of DNA resulted
in restricted intramolecular motions (RIMs) of TPE, and thus
leading to the enhancement of fluorescent intensity.[12]
Figure 2: MALDI-TOF mass spectrum of P with corresponding number of
repeating units. Inset: higher molecular weight region.
Figure 3: 1H-NMR spectra of (a) monomer M, (b) covalent polymer P and (c)
metallo-helicoid H (600 MHz, 300 K, d6-DMSO).
Results and Discussion
Synthesis and Characterization of Metallo-helicoid (H)
Tetratopic monomer M was synthesized with two TPY units
and terminal alkyne groups in the inner and outer rim of TPE
core, respectively through several steps of Suzuki and
Sonogashira cross-coupling reactions in decent yields as
outlined in Schemes S1-2. To increase the solubility and
balance the hydrophobicity/hydrophilicity of M, two triethylene
glycol (TEG) chains were modified onto the periphery. The
intermediate precursors and M were fully characterized by
nuclear magnetic resonance (NMR), including 1H, 13C, Two-
Dimensional Correlation Spectroscopy (2D-COSY), Nuclear
Scheme 1. The preparation of metallo-helicoid (H) by polymerization and
folding via intramolecular coordination-driven self-assembly successively. The
linear covalent polymer (P) was folded by Zn(II) to give rise to H.
2
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Overhauser Effect Spectroscopy (2D-NOESY), high resolution
Electrospray Ionization-Time of Flight (ESI-TOF) and Matrix-
Assisted Laser Desorption/Ionization-Time of Flight (MALDI-
TOF). The covalent polymer (P) was prepared by polymerization
of M through efficient Hay-Glaser homocoupling in an O2
atmosphere (Scheme S3).[13] Gel permeation chromatography
(GPC) analyses gave a number average molecular weight (Mn)
of 36 k Da and a polydispersity index (PDI) of 1.95 for P (Fig.
S37). In the MALDI-TOF mass spectrum of P (Fig. 2), a series of
peaks with a specific interval approximate 1525 Da could be
observed, corresponding to the molecular weight of a repeating
unit. For example, a high molecular weight signal up to ca.
53375 Da was observed with the degree of polymerization of 35,
indicating the successful synthesis of corresponding metal-free
polymers. It is worth noting that MALDI-TOF generally displays
an asymmetrical distribution of peaks for polymers with higher
abundant signals at low m/z range due to the high ionization
efficiency of oligomers with low molecular weight.[14]
Figure 5: TEM images of (a-c) metal-free polymer (P) and (d-f) metallo-
helicoid (H).
Compared with the sharp 1H NMR of M (Fig. 3a), the
spectrum of P (Fig. 3b) showed broad signals with many
overlapping in both aromatic and aliphatic region. This is
attributed to the rigid π-conjugated TPE-containing polymers
with high polymerization degree and multiple conformations.[15]
Metallo-helicoid H was prepared by adding Zn(NO3)2•6H2O (0.5
equiv of TPY units) into a DMSO solution of P at 80 ℃ under
vigorous stirring for 12 h. Upon adding Zn(II), various the peaks
of H (Fig. 3c) in aromatic region were shifted downfield
compared with P due to TPY-Zn(II) coordination.[16] For instance,
signals of 3', 3'' and 4', 4'' of H were shifted toward the downfield
by ca. 0.5 ppm compared with corresponding signals of M and P.
b
On the contrary, the signals of the outer rim such as H, cH, kH
and lH were slightly shifted to downfiled. Similarly, the peaks of
oH and pH in the inner rim of H showed minor shift as a result of
both shielding effect from the helical conformation and losing
1
electron density by coordination. In contrast, the H NMR of H
displayed sharper signals than P, indicating the presence of
ordered conformation with relatively immobilized skeleton.
Saturation Transfer Difference (Decay)-NMR Spectroscopy
(STD-NMR) experiment was also performed on the H ⊃ dsDNA
complex to examine if the dsDNA was encapsulated into the
H.[17] As displayed in the Fig. S33, an internal hydrogen signal
(peak 6,6'') at 7.91 ppm derived from the H was saturated while
an off-resonance reference was pulsed at 11.00 ppm. STD
difference was clearly observed for dsDNA signals, indicating
the interactions between dsDNA and internal cavity of H. The
Figure 4: Energy-minimized structures of H with 36 repeating units, (a) side-
view, (b) top-view, and (c) basic unit. TEG chains were omitted for clarity.
Diffusion-Ordered
NMR
Spectroscopy
(DOSY-NMR)
experiments on free dsDNA and the complex of H ⊃ dsDNA
further confirmed this interaction. The decay rate of dsDNA was
decreased after binding with H as revealed in the Fig. S34,
implying that the larger hydrodynamic radius of H ⊃ dsDNA
complex compared with free dsDNA. Besides, as shown in
Ultraviolet-visible (UV-vis) absorption spectra (Fig. S47), an
obvious hypochromic effect[8e, 18] was also observed with respect
to the formation of H. Such reduced UV absorbance further
confirmed the successful preparation of corresponding H.
From the modeling structures (Fig. 4), we reasoned that the
large dimension of this polymer system could facilitate further
structural characterization by transmission electron microscopy
(TEM), atomic force microscopy (AFM), scanning tunneling
microscopy (STM) and small angle X-ray scattering (SAXS)
experiments. Indeed, in TEM imaging of P, plenty of nano-fibers
3
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with a uniform diameter (ca. 5 nm) were observed on Cu grid,
which is probably attributed to the self-assembly of P with both
rigid hydrophobic moieties (i.e., TPE and TPY) and hydrophilic
TEG (Figs. 5 and S38).[19] AFM imaging of P was consistent with
TEM results, showing fiber-like nanostructures on freshly
insufficient resolution and unavoidable tip broadening effect of
AFM.[20] As such, the observed helical pitch of H by AFM was
around 10.5 nm, which could be a sum of multiple real pitches
(Fig.S43). Both right- and left-handed
structures) existed equally on HOPG owing to the same
during intramolecular
H (racemic helical
cleaved highly oriented pyrolytic graphite (HOPG) surfaces (Figs. probability of helical wrapping of
P
6a-c and S40). The formation of H was first evidenced by TEM.
Compared with P, the TEM images of H exhibited jagged
surface morphologies with larger size. The images revealed
uniform rod-like structures with a diameter ca. 7 nm (Figs. 5d-f
and S39), which is in good agreement with modeling structure
without TEG (Fig. 4B) on account of the low contrast of TEG in
TEM imaging. This result demonstrated that H was successfully
formed as the energy-favorable structure by means of
intramolecular coordination-driven self-assembly as we
designed. However, the handedness of H is indistinguishable
due to its narrow pitch and low resolution in TEM.
To further investigate the structure of H, we performed AFM
experiments on HOPG substrates. As shown in Figs. 6d-f and
S41, the images exhibited helical rod-like structures with
comparable measured diameter ca. 10 nm (Figs. 5d-f). The
value of the diameter was slightly bigger than that obtained from
TEM results and molecular simulation (Fig.4) due to the
coordination-driven self-assembly.
Considering the positively charged inner rims of H, we
reasoned that H might be able to interact with negatively
charged DNA molecules through electrostatic interactions.[21]
Interestingly, in the presence of double strand DNA (dsDNA)
molecules (Figs. 6g-i and S42), the length of H was further
elongated. We speculated that DNA molecules could bridge
several metallo-helicoids together. In the case of H binding with
dsDNA, the helical nanorod was further extended up to tens of
micrometres. Remarkably, only right-handed complexes (H ⊃
dsDNA) could be observed. This result implies that the right-
handed dsDNA could induce the chiral conversion of host H to
give rise to preferred right-handed conformation. It was also
found that the angle between the crest and main axis of H was
dreased from ca. 40° to 10° after binding with dsDNA due to
multiple electrostatic interactions.
Figure 6: AFM images of metal-free polymer (P), metallo-helicoid (H) and its complex with dsDNA. AFM images of (a-c) P, (d-f) H, and (g-i) H ⊃ dsDNA. The
cross-sectional profiles (rightmost) are along the white lines, including P (top), H (middle), and H ⊃ dsDNA (bottom).
With enhanced resolution, STM could provide more accurate
structural information of supramolecules.[7d, 22] In order to gain
more insights into the structural characterization, STM
experiments were conducted for H and its complexes with DNA.
As revealed in Figs. 6d-f and S41, both right- and left-handed H
with relatively uniform diameter were observed on the HOPG
4
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10.1002/anie.202010696
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RESEARCH ARTICLE
surface, agreeing well with TEM imaging. Similarly, as shown in
the Figs. 7a-c and S44, both right- and left-handed of H with a
diameter ca.7 nm in average can be observed. On the contrary,
in terms of H ⊃ dsDNA, only right-handed H can be detected as
exhibited in the Figs. 7d-f and S45. This result further proved
that the helicity of H could be successfully induced by dsDNA.
From the height of section analysis, dsDNA could be
encapsulated in the cavity of H (Figs. 7d-e).[23]
the existence of enantiomeric helical conformations of H. On the
contrary, the complex of H and dsDNA (H ⊃ dsDNA) showed
characteristic induced CD signals in the vicinity of 375 nm,
indicating that the chirality was transferred from right-handed
dsDNA to the host H. Meanwhile, the CD spectrum of dsDNA
exhibited a positive band at around 280 nm and a negative band
at ca. 250 nm, which are characteristic signals of a right-handed
B conformation. This result implied the integrity of the chiral
structure of dsDNA during the complexation with H. Furthermore,
UV-vis absorbance of the complex (H ⊃ dsDNA) was reduced
compared with H originating from the hypochromic effect (Fig.
8b). Such results suggested that dsDNA could stabilize the
helical conformation of H with the aid of multiple electrostatic
interactions.
Absorbance and Emission Properties of
Complexes with DNA
H
and Its
Circular dichroism (CD) spectroscopy was further utilized to
validate the chirality of helical polymers.[24] In terms of H, the
Cotton effect was absent in DMSO solution (Fig. 8a), indicating
Figure 7: STM images of H and its complex with dsDNA. STM images of (a-c) H, and (d-f) H ⊃ dsDNA. STM line profiling measurements (bottom) are performed
along the white lines for H and complexes, and red lines for dsDNA.
5
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RESEARCH ARTICLE
Figure 8: (a) Chiral induction: CD spectra of dsDNA, H, and H ⊃ dsDNA, (b) UV absorbance of H and H ⊃ dsDNA. The concentrations of dsDNA and H are 1.2
μM, 12 μM in DMSO, respectively. (c) Time-dependent fluorescence spectra (λex = 325 nm), (d) UV absorbance of H ⊃ dsDNA, the concentration of dsDNA and H
are 0.01 μM, and 12 μM in DMSO, respectively.
Emission and RIM effects
mixed with metal-free polymer (P). As displayed in the Figs. S48,
the emission intensity of the mixtures almost remained constant
in the duration of the incubation.
Time-dependent fluorescence spectroscopy was carried out
to monitor the process of DNA encapsulation. It is anticipated
that the compact helical H and its complex with DNA would emit
intensively in solution state on account of RIM by coordination
and complexation (Figs. S46). The maximum emission peak of
H is located at ca. 610 nm in DMSO solution. In addition, the
binding of DNA led to the enhancement of emission intensity
because of further immobilization of TPE fluorophore. Indeed, as
shown in Fig. 8c, upon the addition of DNA, the solution
exhibited consecutive increments of emission intensity until
reaching equilibrium (ca. 20 minutes). It was attributed to the
introduction of additional RIM via multiple electrostatic
interactions. As revealed in Fig. 8c, the emission peak of the
complex of H and dsDNA centered in ca. 610 nm. Notably, the
emission intensity of H showed a constant increase from 1.40 x
106 to 1.75 x 106 with the addition of dsDNA. In addition, the
time-dependent UV-vis absorption spectra further evidenced the
encapsulation of dsDNA molecule (Fig.8d). The absorbance was
decreased until reaching equilibrium along with time due to the
further stabilization of H by electrostatic interactions. Also, we
performed control experiments where DNA molecules were
The SAXS data of H and H ⊃ dsDNA provides the important
insight into the transformation of achiral H to chiral H ⊃ dsDNA.
Due to the nature of SAXS, the interpreted structure should be
representative of the whole sample. Fig. 9 shows the SAXS data
of H and H ⊃ dsDNA in DMSO. The fact that the SAXS data of H
can be well described by a core shell cylindrical (CSC) model
with a Gaussian peak agrees with the morphology from TEM
images. The best fitting shell thickness and inner radius are (0.7
± 0.2) and (4.7 ± 0.1) nm, respectively, with a length of the
cylinder > 100 nm (beyond the SAXS probing range), yielding an
outer diameter of 10.8 nm. The inclusion of the Gaussian peak
at q = 0.2 Å-1 is required in order to fit the SAXS data in the
higher q regime better and the peak position corresponds to a
well-defined distance, D = 2ꢀ/ꢁ ~ 3.1, consistent with the
pitch distance from the molecular modeling (Fig. 4a) as well. The
SAXS data of H ⊃ dsDNA, as compared with that of pristine H,
reveal distinct differences. First, steeper intensity decay was
found in the low q region, causing deviation from the CSC model,
while the scattering feature at medium- and high- q regime is
6
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RESEARCH ARTICLE
similar to that of H. Here, we applied a combinational scattering
model of power law decay, CSC and a broad Gaussian peak.
The best fitting power law decay yields q-3.5, corresponding to
the surface scattering from large aggregates. Most intriguingly,
the best fit using CSC model yields a smaller inner radius of (4.0
± 0.04) nm but similar shell thickness of (0.7 ± 0.1) nm and
slightly larger pitch distance of 3.5 nm compared to H tube. The
original best-fitting CSC for helicoiled H does not agree well with
the SAXS data of H ⊃ dsDNA (Fig. S50), confirming the
reduction of coil radius.
density of positive charge was able to accommodate dsDNA as
the guest molecule. Remarkably, the helicity of the helicoid
could be induced by the right-handedness of dsDNA. The
success of this study inspired us to explore a strategy to
regulate the formation of helicoidal metallo-polymers with
specific handedness but without using chiral inducing agents for
ongoing study.
Acknowledgements
The authors acknowlede the support from Shenzhen
University and University of South Florida.
Keywords: metallo-polymer, helicoid, terpyridine, coordination-
driven self-assembly, tetraphenylethylene
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Figure 9: The 1-D SAXS patterns of H and H ⊃ dsDNA. The solid lines are the
best fits for the two data sets. Rescaling factors are applied to the H with
dsDNA for better visual clarity.
The fact that the increased pitch distance (~ 13%)
corresponds to the reduced tuluous diameter (~ 13%) well
implies possible stretching of the H helicoil after interaction with
the dsDNA. We hypothesize that electron-negative dsDNA is
likely driven into the coil by the electrostatic attraction in the
presence of electron-positive chelated Zn(II) sites which are
presumably located at the vicinity of the inner wall of the polymer
coil. As a result, the entrapped dsDNA slightly modulates the
pitch spacing of H from 3.1 nm to 3.5 nm and slightly reduces
the inner diameter of the H coil. It should be noted that the
diameter of dsDNA is much smaller than the inner diameter of
the coil and therefore such entrapment would not significantly
reduce the total entropy of the system. The short-range
interaction between dsDNA and H is expected to introduce
chirality of the dsDNA to the system as shown in Fig. 8a. The
fact that the shell thickness remains the same (even in the
presence of dsDNA) suggests that the induction of molecular
chirality does not require large quantities of dsDNA to be at
presence.
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Guang-Qiang Yin, Sneha Kandapal,
Chung-Hao Liu, Heng Wang, Jianxiang
Huang, Shu-Ting Jiang, Tan Ji, Yu Yan,
Sandra Khalife, Ruhong Zhou, Libin Ye,
Bingqian Xu, Hai-Bo Yang,Mu-Ping
Nieh, Xiaopeng Li*
[Page No. – Page No.
Metallo-Helicoid with Double Rims:
Polymerization Followed by Folding
via Intramolecular Coordination
A new type of metallo-polymer with helicoid-like structure was constructed through the combination of
covalent polymerization and intramolecular coordination-driven self-assembly. Due to the multiple
positive charges on the inner rim of helicoid, double-stranded DNA molecules (dsDNA) could interact
with metallo-helicoid (H) via electrostatic interactions. Remarkably, dsDNA allowed exclusive
formation of H with right handedness by means of chiral induction. Furthermore, the interaction with
DNA molecules led to the enhancement of luminescent efficiency of H through restricted
intramolecular motions.
9
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