Strong positive allosteric cooperativity in ternary complexes based on hydrogen-bonded aromatic amide macrocycles

Article abstract of DOI:10.1039/C9CC00925F

Three new hydrogen-bonded aromatic amide macrocycles with eight residues were synthesized. The first single crystal structure of this class of larger macrocycles was obtained, which reveals a saddle-like conformation. Interestingly, in sharp contrast to previous negative cooperativity in binding paraquat with cyclo[6]aramide, strong positive allosteric cooperativity in ternary complexes was observed. This may open an avenue for the construction of mechanically interlocked molecules with these larger H-bonded macrocycles.

Full text of DOI:10.1039/C9CC00925F

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DOI: 10.1039/C9CC00925F
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Strong positive allosteric cooperativity in ternary complexes
based on hydrogen-bonded aromatic amide macrocycles
Received 00th January 20xx,
Accepted 00th January 20xx
Zhiyong Peng,a Xuwen Guo,a Weitao Xu,b Jian Li,a Pengchi Deng,a Xin Xiao,b Wen Feng,a and Lihua
Yuana
DOI: 10.1039/x0xx00000x
www.rsc.org/
Three new hydrogen-bonded aromatic amide macrocycles with
Hydrogen-bonded (H-bonded) aromatic amide macrocycles,9
eight residues were synthesized. The first single crystal structure emerged in recent years as a class of shape-persistent10 cyclic
of this class of larger macrocycles was obtained, which reveals a compounds featuring full amide linkages with pretty rigidified
saddle-like conformation. Interestingly, sharply contrasted to backbones due to the presence of intramolecular hydrogen
previous negative cooperativity in binding paraquat with bonds. They have exhibited a variety of intriguing properties as
cyclo[6]aramide, strong positive allosteric cooperativity in ternary indicated by their uses in extraction,11 separation technology,12
complexes was observed. This may open an avenue to transmembrane channels,13 liquid crystal,14 as well as
construction of mechanically interlocked molecules with these catalysis.15 Our previous studies demonstrated that hydrogen-
larger H-bonded macrocycles.
bonded aromatic amide macrocycles with six residues,16
namely, cyclo[6]aramides, are able to form a complex with
paraquat in 2 : 1 stoichiometry .17 Nevertheless, only negative
cooperativity is observed. Herein we report a rare example of
H-bonded aromatic amide macrocycles with eight residues 1
(cyclo[8]aramides for brevity) that show strong positive
allosteric cooperativity towards binding bipyridinium G1-G3
(Fig. 1 and Fig. S1-S6, ESI†). Also important to this work is the
first successful growth
Cooperativity1 is ubiquitous in biological systems arising from,
e.g., the interplay of entities capable of assembling as a result
of collective intermolecular non-covalent interactions.
Understanding such cooperativity effect holds a significant
position in molecular recognition and supramolecular self-
assembly. Positive or negative cooperativity results when one
non-covalent interaction favors or disfavors another1a. Positive
cooperativity is advantageous in constructing multicomponent
architectures in supramolecular chemistry by overcoming the
entropy penalties. In effect, stabilization of biological
assemblies is closely associated with such positive
cooperativity.2 Among the four types of cooperativities
categorized so far,1d which include cooperative aggregation,
allosteric cooperativity, chelate cooperativity, and interannular
cooperativities, allosteric cooperativity is probably the most
recognized and has found wide applications in artificial
receptors-guest-based systems. Particularly, some of these
systems involve the use of macrocycles such as calixarenes, 3
cyclodextrins,4 crown ethers,5 cucurbiturils6 and other cyclic
compounds.7 However, few shape-persistent macrocycles
exhibit a positive allosteric cooperativity.8
a.College of Chemistry, Key Laboratory for Radiation Physics and Technology of
Ministry of Education, Analytical & Testing Center, Sichuan University, Chengdu
610064, Sichuan, China. Email: lhyuan@scu.edu.cn; Tel: +86-28-85412890
b.Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province,
Guizhou University, Guiyang 550025, China.
†Electronic Supplementary Information (ESI) available: Further details of synthesis,
characterization, 1H and 2D NMR experiments and computational modelling. CCDC
1850273. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
Fig. 1 (a) Chemical structures of cyclo[8]aramides (1a-1c) and bipyridinium salts G1-G3.
(b) Intermolecular cooperative system and the definition of cooperative factor (α).
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of single crystals of cyclo[8]aramide for analysis of the efficient intermolecular stacking.19 Therefore, 1a containing
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molecular structure since the first report on their synthesis a linear n-dodecyl groups was selecteDdOI:fo10r.10t3h9e/C9fCoCll0o0w92in5gF
decade ago.18
cooperatively binding experiments to promote such
Compounds 1a-1c bearing different alkyl groups as side intermolecular interaction, and 1b having branched hexadecyl
chains were synthesized by coupling trimeric diamines with groups was used as a control.
diacid chlorides. Reprecipitation from methylene chloride and
Recently, we reported several host-guest systems based on
acetone (or methanol) afforded cyclo[8]aramides with the rigid cyclo[6]aramide,20 some of which exhibit 2 : 1 binding (H
isolate yields of ca. 30%. The macrocyclization proceeded to G) and present negative cooperativity.1717 We speculate
under non-high dilution condition because of persistency of that the steric hindrance from peripheral alkyl side chains may
folded backbone in assisting the cyclization reaction.9a It is hamper the second macrocycle to approach, thus leading to
highly challenging to grow single crystals of cyclo[8]aramide negative cooperativity. To test this assumption, 1b bearing
because of the difficulty in balancing the solubility and branched alkyl groups was examined first for the binding of
selection of peripheral groups. This was finally accomplished guest G1. The results from Job plot experiments indicated 1 : 1
by exploiting 4,4’-dibutylbipyridinium hexafluorophosphate to stoichiometry for the inclusion complexes in CHCl3/CH3CN (5 :
solubilize 1c bearing 2-ethylhexyl in a mixed solvent of 1) (Fig. S10, ESI†), which is corroborated by matrix-assisted
chloroform and methanol (9 : 1, v/v), followed by slow laser desorption ionization-time of flight (MALDI-TOF) mass
evaporation. The brownish yellow cubic plates of 1c suitable spectrometry (Fig. S13, ESI†). Therefore, indeed, the
for single-crystal X-ray structure determination was thus considerably strong repulsion between peripheral bulky
obtained (Fig. 2). The diameter of the cavity measures 11.6 Å groups prevents the second cycle from threading on G1,
(Fig. 2a, Fig. S7-S8 and Tab. S1-S3, ESI†), which is sufficiently demonstrating anticooperative effect. The macrocycle 1b also
large to engulf cationic guests such as G1-G3. Different from fail to display any cooperativity with G2 and G3, all giving a 1 :
cyclo[6]aramide in
a
near-planar conformation, this 1 stoichiometry (Fig. S11-S12 and S14-S15, ESI†). In sharp
macrocycle has a saddle-like conformation (Fig. 2b). Face-to- contrast, 1a offers a 2 : 1 stoichiometry with G1 according to
face - stacking between the macrocyclic aromatic the Job plot (Fig. S16, ESI†) and MALDI-TOF results (Fig. S19,
backbones is observed with the curved backbone only partially ESI†). Similar 2 : 1 binding modes were observed for guests G2
1
overlapped (Fig. S9, ESI†). We attribute it to the presence of and G3 (Fig. S17-S18 and S20-S21, ESI†). H NMR spectra of
sterically hindered groups that preclude the rings from
G1-G3 binding by 1a recorded in CDCl3/CD3CN (1 : 1) show
pronounced downfield shifts of guest protons in the
complexes with respect to the free guests (Fig. S22-S24, ESI†).
This is examplified by G2, which produces separated protons
for the bound macrocycle upon addition of 1a to the guest (Fig.
3). It should be noted that the low solubility of 1a in the mixed
solvent deters us from obtaining a NMR spectrum for
comparison. Since the signal from the free guest is not
observed even in the presence of excess G1 (Fig. 3b), it
indicates a fast chemical exchange on the NMR time scale. The
response to the cationic guests, as reflected in the change of
chemical shifts, suggests site-specific binding by the host.
Fig. 3 Partial 1H NMR spectra (600 MHz, CDCl3/CD3CN = 1 : 1, 298 K) of (a) 2 : 1
mixture of 1a (4.0 mM) and G2 (2.0 mM), (b) 1 : 1 mixture of 1a and G2 (each 2.0
mM) and (c) G2 (2.0 mM). The spectrum of 1a is not shown due to low solubility.
Fig. 2 X-ray crystal structure of cyclo[8]aramide 1c. (a) Top view, (b) Side view in the
CPK model, illustrating a saddle-like macrocyclic backbone. The dashed blue lines in (a)
indicate hydrogen bonds
.
2 | J. Name., 2012, 00, 1-3
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Threading of the guests through the cavity of a macrocycle is equiv. of the guest in the titration process, it suggests that the
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evidenced by the results from 2D nuclear overhauser effect intermediate is maintained at a very lowDcOoI:n1c0e.1n0t3r9a/tCio9CnC. 0A0t9t2h5eF
spectroscopy (NOESY). NOESY experiments performed with a same time, the 2 : 1 bound species 1a2⊃G3 starts to
solution (CDCl3/CD3CN, 1 : 1) of 1a and G2 (or G3) reveal cross predominate at 0.34 equiv. of G3. Taken all these results
peaks between bipyridinium protons of G2 (or G3) and the together, this demonstrates the presence of strong positive
internal aromatic protons Hb and Hc of 1a (Fig. S25-S26, ESI†). allosteric cooperativity in the complex 1a2⊃G3.
No through-space NOEs are observed with the mixture of 1a
To quantitatively evaluate allosteric cooperativity,
and G1 due to severely broadened signals caused by cooperative factor  was determined by UV-vis spectroscopy.
aggregation at high concentration (Fig. S83, ESI†). These 1H The extent to which cooperativity works in a H-G system relies
22
NMR experiments with macrocycles of contrasted side chains heavily upon types of guests and solvent polarity.
Thus,
(linear and branched) reveal that the steric barricade on the binding behaviours of three structurally isomerized
macrocyclic periphery plays crucial role in effecting bipyridinium cations and variation of solvent polarity were
intermolecular interactions between cyclo[8]aramide explored (Tab. S4-S5 and Fig. S36-82, ESI†). Partial data along
a
molecules, which is manifested in two significantly different with their ternary binding constants of 1a with G1-G4 are
binding modes (2 : 1 and 1 : 1). As such, the follow-up listed in Table 1. Isothermal titration calorimetry (ITC)
discussions only involve the use of 1a for cooperativity experiments were also performed; however, the large error as
experiments.
a result of a very small change in heat prevented us from
A qualitative evaluation of the allosteric cooperativity in retrieving accurate binding constants. The 1H NMR titration
these systems was implemented by titration of guests into experiments also failed due to low solubility of 1a.
solutions of 1a. The formation of complexes 1a⊃G1-G3 and
1a2⊃G1-G3 and the presence of free 1a were analyzed by
fitting experimental data21 from UV-vis spectroscopy (Fig. 4
Table 1. Association constants (Ka/M-1)a for complexation of various guests
(G1, G2, G3 and G4) by host 1a at 298 K.
Ka/M-2
b
Guest
Solvent
CHCl3/CH3CN
1:1
K1
K2
and S27–S32, ESI†).
(×104)
(×104)
(×1010
3.4
1.7
1.1
1.0
2.4
12
)
Positively cooperative systems are characteristic of the
presence of only fully bound species (the final products) and
fully unbound species (the starting host or guest) in the
binding event.1a This is usually signalled by the persistence of a
low concentration of intermediates throughout the titration
process (Fig. S33-S35, ESI†). In the case of G3, the
concentration profiles were obtained by fitting the entire
series of spectra to the model shown in Fig. 1b. The mole
fraction of the 1 : 1 complex (intermediate) only reaches 7%
when titrating 1.0 equiv. of G3 into the solution of 1a in
CHCl3/CH3CN (1 : 1) and slowly rises to a maximum of 11% with
1.7 equiv. of G3 (Fig. 4). Since less than 10% of G3 was
observed to exist in the form of the complex 1a⊃G1 with 1
G1
G2
G3
G3
G3
G3
G3
G4
59 ± 8.6
4.1 ± 1.0
1.6 ± 0.3
3.2 ± 0.5
8.4 ± 1.7
24 ± 1.4
40 ± 9.0
2.9 ± 0.1
5.7 ± 0.4
42 ± 0.3
66 ± 6.6
32 ± 1.9
28 ± 2.8
51 ± 5.5
70 ± 10
0.39
41
165
40.
13
8.5
7
1:1
1:1
1.3:1
1.5:1
1.7:1
2:1
28
1:1
-
-
-
a
The association constant Ka values were obtained by UV-vis titration spectroscopy†,
b
the concentration of the host was fixed at 50 μM;  represents the cooperativity
factors defined as  = 4K2/K1.
All cooperativity factors in Table 1 for the complexes
between 1a and G1-G3 are larger than ca. 7 except for G1,
indicative of a positive cooperativity. The binding affinity for
the guest by the first macrocycle is comparable to that of the
methylated pyridinium G4 alone as indicated by a K1 value of
2.91 × 104 M-1. The increase of solvent polarity for G3 brings
about enhanced positive cooperativity. This holds true for G2
(Tab. S4, ESI†), and 1a2⊃G1 still exhibit negative cooperativity.
Particularly noteworthy is the complex 1a2⊃G3, which gives a
 value of 165. For allosteric cooperative systems associated
with shape-persistent macrocycles, such high positive
cooperativity is considered pretty large through weak
interaction excluding ion-pair systems.
To gain a better understanding of the positive allosteric
cooperativity, computational simulation based on the density
function theory (DFT) method was performed with 1a2⊃G3 as
a representative example (Fig. S84-S86 and Tab. S6, ESI†). The
result indicates that multiple π-π stacking interactions are
presented, which is consistent with the π stacking observed in
the crystal structure of 1c. There are several C-H···O hydrogen
bonds between two cyclo[8]aramides. Cation-dipole
interaction between cyclo[8]aramide and bipyridinium salt
also contributes to positive allosteric cooperativity.
Fig. 4 Mole fraction evolution of existing species 1a2⊃G3, 1a⊃G3 and 1a in the system
when titrating G3 into a solution of 1a (50 μM, CHCl3/CH3CN = 1 : 1) as monitored by
UV-vis spectroscopy. Mole fraction is defined as the amount of a constituent
(1a2⊃G3, 1a⊃G3 or 1a) divided by the total amount of all constituents in the
mixture.
ChemThis journal is © The Royal Society of Chemistry 20xx
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1-7; (c) X.-L. Ni, H. Cong, A. Yoshizawa, S.ViRewahArmticalenO,nliHne.
Furthermore, the difference in the binding mode of
macrocycles 1a and 1b (2:1 vs. 1:1) suggests that the van der
Waals force constitute another factor to effect the cooperative
action in forming the complexes. However, the essence of
each specific interaction that is responsible for the collective
cooperativity is still hardly understood.
In conclusion, we demonstrate strong positive allosteric
cooperativity in solution of shape-persistent cyclo[8]aramide-
based ternary complexes. A cooperativity factor of 165 is
achieved with a bipyridinium salt as a result of combined weak
interactions including π-π stacking, hydrogen bonding, cation-
dipole interaction and van der Waals force. Increasing the
polarity of solvent system tends to raise positive allosteric
cooperativity. Also noteworthy in this work is the first single
crystal structure of this series of H-bonded aromatic amide
macrocycles of larger size. These findings offer opportunities
for constructing mechanically interlocked molecules with H-
bonded aromatic amide macrocycles of large lumen.
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We are grateful to the National Natural Science Foundation
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