Metal Actuated Ring Translocation Switches in Water

Article abstract of DOI:10.1021/acs.orglett.8b01019

Among a series of metal ions in water, silver is the only one to remotely and reversibly switch cucurbit[7]uril (CB[7]) movements (translocation or uptake) on a rigid and linear three-station viologen-phenylene-imidazole (V-P-I) derivative, avoiding undesired pH actuation. ¹H NMR, UV-vis spectroscopy, mass spectrometry, ITC, and modeling were combined to show that ring translocation or uptake along a molecular thread is possible in water by Ag⁺ as a metal stimulus.

Full text of DOI:10.1021/acs.orglett.8b01019

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Metal Actuated Ring Translocation Switches in Water
Hang Yin,† Roselyne Rosas,‡ Didier Gigmes,§ Olivier Ouari,§ Ruibing Wang,*,†
Anthony Kermagoret,*,§ and David Bardelang*,§
†State Key Laboratory of Quality Research in Chinese Medicine, and Institute of Chinese Medical Sciences, University of Macau,
Avenida da Universidade, Taipa, Macau SAR, China
‡Aix Marseille Univ, CNRS, Spectropole, FR 1739, Marseille, France
§Aix Marseille Univ, CNRS, ICR, Marseille, France
S
* Supporting Information
ABSTRACT: Among a series of metal ions in water, silver is the only
one to remotely and reversibly switch cucurbit[7]uril (CB[7])
movements (translocation or uptake) on a rigid and linear three-
station viologen−phenylene−imidazole (V−P−I) derivative, avoiding
undesired pH actuation. 1H NMR, UV−vis spectroscopy, mass
spectrometry, ITC, and modeling were combined to show that ring
translocation or uptake along a molecular thread is possible in water
by Ag+ as a metal stimulus.
CB[7] over a simple, rigid molecular axle containing a viologen
(V), a phenylene (P), and an imidazole (I) station (V−P−I,
Figure 1) or pull a second CB[7], without affecting the pH
deleteriously.
olecular switches are increasingly investigated because of
Mtheir high potential as elementary building blocks for the
next generation of molecular machines.1 Remarkable examples
have been reported by several groups, with designs comprising
flexible axle and ring molecules.2 Even if water is attractive due
to its lack of toxicity, possible enhanced affinity by hydrophobic
effects,3 and ability to make molecular switches more amenable
for biological applications,4 most molecular switches proceed in
organic solvents.5 Yet, water-soluble molecular switches are
much less investigated and, even if pH6 is an essential stimulus
to trigger ring translocation, nature also uses metal ions or ATP
hydrolysis, for instance to trigger reversible protein ring
contraction in ion channels or to drive ring-shaped motor
proteins over DNA strands to achieve essential functions in
cells.7 To make efficient molecular switches work in water in
various contexts, one would ideally be able to choose the best
stimulus to actuate ring translocation. Expanding the palette of
available stimuli working in water, beyond pH, is thus desirable.
For example, when buffers are undesired and H+ unusable due
to fragile motifs on hosts or guests, metal ions could be valuable
stimuli.8 However, there are, to the best of our knowledge, only
a handful of examples of metal actuated ring translocation
switches and none work in water.9 Reasons for that may be
because of difficulties in combining functions recognizable by a
metal ion, in both the host and guest, or because most metal
ions can behave as Brønsted acids in water and thus affect the
pH,10 rendering unbiased analyses more complicated.
Figure 1. Molecular structures of V−P−I and of CB[7] and
representation of the silver actuated molecular switches.
Imidazole V−P−I[Cl2] (2 Cl− counterions) was prepared in
a three-step sequence, and details are given in the Supporting
Information (SI). Imidazole V−P−I[Cl2] is water-soluble and
possesses two pKa values, as the imidazole station can be either
protonated (pKa1 = 4.0) or deprotonated (pKa2 = 10.4, Figure
S6). Therefore, working at near-neutral pH was essential
without buffers to (i) warrant having benzimidazole structures
(Figures 1 and 2; i.e., not the protonated or deprotonated
forms) and (ii) avoid complications due to cations that are
Cucurbit[7]uril (CB[7])11 is a ring shape receptor
presenting several advantages to test metal ions as triggers of
ring translocation because it is water-soluble and possesses (i) a
versatile hydrophobic cavity and (ii) two carbonyl-laced portals
amenable for metal ion binding. In line with our continuous
work on CB[n],12 we tested a series of commonly used water-
soluble metal salts and found that Ag+ selectively translocates
Received: March 30, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.8b01019
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1
Figure 2. Excerpts of the aromatic region of H NMR spectra for the CB[7] titration of V−P−I[Cl2] (1 mM, a) and V−P−I[Cl2] (1 mM) with
AgNO3 (8 equiv, b) in D2O with (c) proposed structures of the complexes (Ag+ ions: red spheres).
known to interact significantly with CB[7] carbonyl rims.13
First, CB[7] complexation on the V station was easily
monitored by H NMR in D2O (Figure 2a). Assignment of
0.25 ppm, carbonyl rims symmetry broken, Figures S9 and S11,
1 equiv, right). AgNO3 had a weak, but significant effect on the
spectrum of V−P−I alone (Figure S12), affecting H7, H8, and
H9 that are located nearby the imidazole function and thus
reflecting weak binding on the imidazole function (significant
effects were also observed by UV−vis spectroscopy, Figure
S14). CB[7] Titration of V−P−I[Cl2] with AgNO3 instead
showed CB[7] binding on station P (slow exchange on the 1H
NMR time scale, Figure 2b). Silver ions have previously been
shown to behave as soft Lewis acids,15 and imidazoles16 or the
carbonyl rims of cucurbiturils17 have been reported to be
possible ligands for Ag+. We postulate a silver-induced host
translocation by multiple binding on the guest-imidazole
function and by the oxygen carbonyl rims. Addition of 2
equiv of AgNO3 was necessary to neutralize the effect of the
two chloride ions present in V−P−I[Cl2]. Exchanging chlorides
to nitrates could have been relevant to probe anion effects, but
anion exchange adding AgNO3 (AgCl precipitation) to isolate
V−P−I[(NO3)2] could lead to a product containing a slight
excess or default of salts potentially biasing further analyses.
1
the 1H NMR signals was without ambiguity, except for protons
8 and 9 despite the record of several Heteronuclear correlation
spectroscopy experiments, probably because of the presence of
the nearby imidazole function and the associated H/D
exchange in D2O. Then, NaCl, CsF, AgNO3, CaCl2, MgCl2,
CoCl2, NiCl2, ZnCl2, CuSO4, MnCl2, FeCl2, FeSO4, FeCl3,
GdCl3, Cr(NO3)3, and AlCl3 were assessed for CB[7]
translocation. NaI, CsI, CaII, MgII, CoII, NiII, ZnII, MnII, and
1
GdIII had essentially no effect on H NMR spectra, even after
addition of several equivalents in water (up to 10 equiv Figure
S7, pH reduced to 6.10 in the worst case). Conversely, Cr, Al,
and Fe salts changed the spectrum of V−P−I·CB[7] to
assignable resonances, while Cu showed broad signals. The
obtained spectra for CrIII, AlIII, and FeIII are very similar to that
of V−P−I−H+·CB[7] (Figure S7) and are probably the result
of metal-induced acidification as reflected by the measured pH
values of the solutions (Figure S7). The addition of these
metallic salts thus resulted in undesired pH triggered CB[7]
translocation. Buffering the solutions to limit pH changes
triggered by Fe, Cu, Al, or Cr would result in precipitates of
hydroxides complexes.14 pH Actuation of the present system is
also interesting, but will be reported elsewhere since it exceeds
the scope of this study. However, AgI resulted in new NMR
spectra without altering too much of the pH. Without Ag+, as
expected, CB[7] quantitatively binds V−P−I on station V while
a further increase in the CB[7] concentration does not change
this 1:1 binding mode (Figure 2a). Split chemical resonances
for free and complexed V−P−I suggest slow exchange on the
NMR time scale. The signals of protons 3 and 4 are shifted
upfield by ∼1.5 ppm in line with the CB[7] location on station
V, with one carbonyl rim downfield-shifting the signals of
protons 6 and 7 by 0.37 and 0.13 ppm respectively (⟨Δδ⟩ ≈
1
However, H NMR spectra in the presence of 2 equiv of
AgNO3 lead to V−P−I with two nitrates in water (precipitation
of 2 equiv of AgCl) and no significant difference was observed
compared to the spectrum of V−P−I[Cl2] (see Figure S18),
showing no detectable anion effect in these conditions. This is
further supported by the reversibility experiments where large
excesses of NaCl and AgNO3 were alternatively added (see
below, and Figures S25 and S26). 1H NMR titrations of AgNO3
for a 1:1 V−P−I[Cl2]:CB[7] mixture showed large chemical
shift changes up to ∼10 equiv of Ag+ for species rapidly
exchanging on the 1H NMR time scale (Figure S18). This fast-
exchange regime was presumably due to the lability of the
silver−nitrogen bond upon Ag+ binding on the imidazole
function of V−P−I accompanied by the gradual translocation
of CB[7] from station V to station P. This allowed the indirect
B
DOI: 10.1021/acs.orglett.8b01019
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determination of the binding constant for Ag+ (Ka = 3470
550 M−1, fit of experimental points using a 1:1 binding model)
toward the V−P−I·CB[7]1 complex and suggested a 1:1:1
stoichiometry for the V−P−I·CB[7]1·Ag+ complex (Figures
S18 and S27). When V−P−I[Cl2] was titrated against CB[7]
with enough Ag+ (8 equiv of AgNO3 kept constant, Figure 2b),
signals of protons 5, 6, and 7 upfield shifted by 1.00, 0.99, and
0.60 ppm respectively up to 1 equiv of CB[7] (ring on station
P). While the threaded CB[7] seems to exchange fast on the
binding sites with respect to the NMR time scale, the host
prefers remaining on station P. However, excess CB[7]
completely changed the NMR spectra with large upfield shifts
for both sets of protons, 3,4 (1.51 ppm) from station V and 8,9
(⟨Δδ⟩ ≈ 0.80 ppm) from station I while the signals of protons
6 and 7 are shifted downfield by approximately double the value
as for the 1:1 binding on V−P−I (⟨Δδ⟩ ≈ 0.50 ppm, Figure 2b
bottom). This agrees with a CB[7] binding on the V station
and a second CB[7] located on the I station. The P station
protons experienced significant deshielding effects from two
carbonyl rims (one rim from each of the two CB[7]), while no
further changes were observed for more than 2.0 equiv of
CB[7]. Thus, Ag+ ions were observed to (i) translocate CB[7]
from the V to the P station at 1:1 guest/host ratio and (ii) pull
a second CB[7] onto the I station at a 1:2 guest/host
stoichiometry. Simultaneous binding of two CB[7] on two
vicinal stations (V and P or P and I) was not observed. Mass
spectrometry (MS) showed peaks assigned to V−P−I, and V−
P−I·CB[7]1 together with the corresponding H+ adduct. With
AgNO3, ion mobility separation prior to analyses allowed us to
observe additional peaks at m/z 545.1390 (theo. 545.1390) and
m/z 848.7045 (theo. 848.7027) respectively corresponding to
3+
2+
the formulas C66H62N32O14Ag1 and C66H62N33O17Ag1 in
line with complexes V−P−I·CB[7]1·Ag+ and V−P−I·CB[7]1·
Ag+·NO3 (Figure S19).
−
DFT minimized structures of V−P−I·CB[7]1 complexes18
with CB[7] positioned on the V, P, or I stations (frequency
checked and corrected for BSSE; see SI) showed a marked
preference for the complex with CB[7] on station V (Figure
Figure 3. Proposed structures (DFT minimized) in line with 1H NMR
for the complexes (a) V−P−I·CB[7], (b) V−P−I·CB[7]1·Ag+, and (c)
V−P−I·CB[7]2·Ag+.
1
3a), which is in good agreement with H NMR. With a silver
ion (1 Ag+ considered as suggested by NMR and mass
spectrometry), the complex with CB[7] on station P is the
most stable (Figure 3b, SI), also consistent with the H NMR
NMR signals of V−P−I near the imidazole function and
maximum absorbance in the UV−vis spectra of V−P−I was
also significantly shifted (Figures S12 and S14 respectively).
The observed silver selectivity for ring translocation could be
explained by (i) the charge of the metal ion (a monocationic
metal ion is more likely to be bound by a CB[7] carbonyl rim
for an overall 2+ charged complex), (ii) the soft lewis acid
nature of silver capable of coordinating both an imidazole
function and carbonyl groups of CB[n],15−17 and (iii) because
AgI cations do not generate insoluble metal oxides at neutral
pH.14
The present system was also investigated by ITC (Figure 4).
Without silver ions, the binding affinity between CB[7] and V−
P−I was determined to be 7.25 × 105 M−1 (Figure 4a). The
binding stoichiometry is 1:1, consistent with NMR, DFT, and
MS. In the presence of Ag+, results of ITC titration of V−P−I
with CB[7] are consistent with a sequential complexation, with
apparent binding constant values of Ka1 = 6.45 × 106 M−1 and
Ka2 = 3.75 × 105 M−1.
1
of V−P−I·CB[7]1·Ag+, V−P−I·Ag+ and the UV−vis spectra of
V−P−I·Ag+ which suggested an interaction between Ag+ and
N-imidazole. In this binding geometry, there are several
hydrogen bonds between CB[7] and the guest, and the silver
ion is 2.20 Å distant from the imidazole nitrogen atom (Ag+···
N); there are two additional Ag+···O coordinating bonds (2.32
and 2.45 Å) with one CB[7] carbonyl rim (Figure 3b). The
combination of these two kinds of interactions seems to be
responsible for driving CB[7] from the V to the P station. The
situation is similar for the 1:2 complex, except that one CB[7]
is on station V and the second CB[7] is instead on station I
(Figure 3c).
In this case, the silver ion interacts (i) with the imidazole
nitrogen atom (2.20 Å) and (ii) with two carbonyl oxygen
atoms of the CB[7] of the I station (2.33 and 2.46 Å). Thus,
the imidazole function behaves as a pivot for Ag+ which in turn
attracts CB[7] providing an oxygen-rich environment to further
stabilize Ag+. Of course, other positions for Ag+ near a carbonyl
rim and around the imidazole station cannot completely be
ruled out, but such other positions lead to less stable 1:1:1
Below 1 equiv of CB[7], corresponding heat changes can
tentatively be assigned to one Ag+-assisted CB[7] binding on
V−P−I (Ka1). The larger binding of CB[7] toward station P in
the presence of Ag+ is well consistent with NMR. The heat
1
complexes by DFT and Ag+ also significantly affected the H
C
DOI: 10.1021/acs.orglett.8b01019
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Letter
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01019.
Preparation and characterization of V−P−I[Cl2], 1D-
NMR and 2D-NMR spectra, details for mass spectrom-
etry measurements, additional NMR titrations, ITC data,
pKa measurements and details of DFT calculations
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: rwang@umac.mo.
*E-mail: anthony.kermagoret@univ-amu.fr.
*E-mail: david.bardelang@univ-amu.fr.
Figure 4. Microcalorimetric titration of V−P−I with CB[7] in
aqueous solution, in the absence (a) and in the presence (b) of Ag+.
Top: thermograms of CB[7] injected into V−P−I solutions. Bottom:
dependence of ΔH for each injection of CB[7] against the molar ratio
between CB[7] and V−P−I (the solid line represents the best fit using
“one set of binding sites” and “two separate one set of binding sites”
models, respectively.
ORCID
Hang Yin: 0000-0002-2898-844X
Ruibing Wang: 0000-0001-9489-4241
David Bardelang: 0000-0002-0318-5958
Notes
The authors declare no competing financial interest.
changes above 1 equiv of CB[7] are hard to interpret because
of the suspected entanglement of two effects: (i) CB[7]
translocation back to station V and (ii) binding of a second
CB[7]. However, ITC confirmed the proposed stoichiometry
and the process is both enthalpically and entropically driven
(SI). Interestingly, the switching processes were simply
reversed by addition of NaCl (Figure 2) precipitating Ag+ in
the form of AgCl. The reversibility of the molecular switches
(1:1 and 1:2 guest/host ratios) was examined over two cycles
by repetitive alternation of the position of CB[7] on stations V
and P (Figure S25), or repetitive catch-and-release of a second
CB[7] (Figure S26). Finally, the role of the anion should not
be left unconsidered with a potentially binding imidazole
function on the axle and cations ubiquitous in this system, but
the carbonyl rims of CB[7] are rather expected to behave as
anion repellants.
ACKNOWLEDGMENTS
CNRS and Aix-Marseille Universite
■
́
are acknowledged for
financial support. Valer
for MS analyses (Aix Marseille Univ, CNRS, Centrale Marseille,
FSCM, Spectropole, Marseille, France).
́
ie Monnier is gratefully acknowledged
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In summary, we report silver-triggered, reversible molecular
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E
DOI: 10.1021/acs.orglett.8b01019
Org. Lett. XXXX, XXX, XXX−XXX