Relative contractile motion of the rings in a switchable palindromic [3]rotaxane in aqueous solution driven by radical-pairing interactions

Article abstract of DOI:10.1039/c4ob01228c

Artificial muscles are an essential component for the development of next-generation prosthetic devices, minimally invasive surgical tools, and robotics. This communication describes the design, synthesis, and characterisation of a mechanically interlocke

Full text of DOI:10.1039/c4ob01228c

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Relative contractile motion of the rings in a
switchable palindromic [3]rotaxane in aqueous
solution driven by radical-pairing interactions†
Leah S. Witus,a Karel J. Hartlieb,a Yuping Wang,a Aleksandrs Prokofjevs,a
Marco Frasconi,a Jonathan C. Barnes,a,b Edward J. Dale,a
Albert C. Fahrenbacha,c,d and J. Fraser Stoddart*a
Cite this: Org. Biomol. Chem., 2014,
12, 6089
Received 13th June 2014,
Accepted 24th June 2014
DOI: 10.1039/c4ob01228c
www.rsc.org/obc
Artificial muscles are an essential component for the development separated without breaking a covalent bond, the bond(s)
of next-generation prosthetic devices, minimally invasive surgical holding the molecule together is (are) called a mechanical
tools, and robotics. This communication describes the design, syn- bond. In the case of a rotaxane, a dumbbell-shaped component
thesis, and characterisation of a mechanically interlocked mole- is encircled by a ring, and the inclusion of functional groups on
cule (MIM), capable of switchable and reversible linear molecular the dumbbell, for which the ring has an affinity, provides
motion in aqueous solution that mimics muscular contraction and recognition sites. When the affinity for one recognition site is
extension. Compatibility with aqueous solution was achieved in increased or decreased relative to another one by a stimulus,
the doubly bistable palindromic [3]rotaxane design by using this bistability serves as the driving force for the ring moving
radical-based molecular recognition as the driving force to induce relative to the dumbbell. A unique interpretation of this design
switching.
has been reported6 previously wherein the dumbbell contains
two sets of identical recognition sites, in a constitutionally
symmetric, or palindromic, design. Thus, in this prototypical
design of a palindromic [3]rotaxane, the two rings achieve a
linear contractile motion as they are switched between the
inner and outer recognition sites.7 This motion (Fig. 1) mimics
the molecular motion present in actin and myosin proteins
within muscle tissue.3 In order to facilitate the next transition
of the doubly bistable [3]rotaxane switches – from isolated
molecules to integrated systems – and to enable biocompatible
applications, these switches must operate in aqueous solution,
the major medium of life itself.
The concept of controlling molecular motion at will is an
inspiring call to chemists.1 The design and synthesis of
organic molecules that are capable of achieving movement is
the first step towards translating that motion into a macro-
scopic effect through integration into larger systems.2 Some of
the most impressive evidence that molecular motion can
be translated into macroscopic motion comes from biology,
wherein proteins and assemblies of proteins – themselves
large molecules and supermolecules
– routinely achieve
incredible feats, from kinesin pulling organelles along micro-
tubules, to the rotary motion of ATP synthase, to the pro-
pulsion that results from bacterial flagellum.3 When we turn
to the synthetic world, there are still rather few comparable
systems.4 One class of organic compounds is particularly well
suited, however, to containing and exercising moving parts –
namely, mechanically interlocked molecules5 (MIMs). Since
MIMs consist of two or more components that cannot be
aDepartment of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, USA. E-mail: stoddart@northwestern.edu
bDepartment of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139, USA
cHoward Hughes Medical Institute, Department of Molecular Biology, and Center for
Computational and Integrative Biology, Massachusetts General Hospital, Boston,
Massachusetts 02114, USA
Fig. 1 (a) Graphical representation of the relative motion of the ring
components, which undergo a redox-stimulated contraction and expan-
sion in a doubly bistable palindromic [3]rotaxane. (b) The contraction of
the sarcomere in biological muscle is achieved through the ATP-driven
molecular motion of actin and myosin proteins sliding relative to each
other.
dEarth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguro-ku, Tokyo 152-8551, Japan
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob01228c
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Scheme 1 Synthetic route to the palindromic [3]rotaxane 3R·12PF6 using a threading-followed-by-stoppering approach.
Recently, we have discovered the potential of radical–radical charges on both the BIPY2+ units and the tetracationic ring
pairing interactions8 in the context of MIMs. It has been found upon re-oxidation.
that 4,4′-bipyridinium (BIPY2+) units, upon reduction to their
The synthesis of the bistable palindromic [3]rotaxane was
radical cationic state (BIPY+•), form strong inclusion complexes performed following the protocol outlined in Scheme 1. In
with the reduced diradical, dicationic (CBPQT2(+•)) form of order to achieve higher yields of the desired [3]rotaxane than
cyclobis(paraquat-p-phenylene) (CBPQT4+). The radical-based had previously been possible with clipping-based rotaxanation
pairing interaction in the reduced state is strong and re- approaches,6b we sought to use a threading-followed-by-stop-
presents the driving force for switching to the radical state co- pering protocol for the rotaxane formation. Therefore, the first
conformation8d (RSCC) in a MIM, a process that is eliminated step was synthesising an axle component, A·4PF6, with azide
in the oxidised state by electrostatic repulsions between the functionalities at each end to aid and abet the subsequent
positive charges on both components. Thus, redox stimuli stoppering reaction. The synthesis of the axle begins with a
initiate rapid and readily reversible switching, but what’s DNP-tri(ethylene glycol) unit9 1 which was subjected to a
more, this radical-based switching mechanism has been monotosylation to induce desymmetrisation.10 The mono-
shown to occur in aqueous solution. Here, we incorporate this tosylate 2 was reacted with sodium azide in order to install an
new switching mechanism into a palindromic [3]rotaxane.
azide group11 in 3. The remaining hydroxyl group on 3 was
The design of a doubly bistable palindromic [3]rotaxane then tosylated in order to form 4 prior to its reaction with a
requires two sets of identical recognition sites on the dumb- bisviologen-based core12 5·2PF6 to form the axle A·4PF6.
bell. Our design utilises 1,5-dioxynaphthalene (DNP) units as
Rotaxanation was achieved by a threading-followed-by-stop-
the outer recognition sites, which enter into donor–acceptor pering approach wherein an excess of CBPQT·4PF6 was in-
interactions with the CBPQT4+ rings in the oxidised ground cubated with the axle in MeCN at room temperature for a
state co-conformation (GSCC). After reduction, the diradical, week. An electron-deficient alkyne13 6 was added to form the
dicationic CBPQT2(+•) ring has a strong preference for the stoppers as a result of copper-free Huigsen cycloadditions.14
inner BIPY+• recognition sites and a decreased affinity for the The desired [3]rotaxane 3R·12PF6 was obtained in 46% yield,
DNP units, resulting in a shuttling motion whose reversal is along with a small amount (8%) of a [2]rotaxane byproduct
facilitated by the electrostatic repulsion between the positive (2R·8PF6). See synthetic procedures in the ESI.†
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1H NMR spectroscopy of 3R·12PF6 confirmed the hypo-
thesis that, in the non-reduced GSCC, the CBPQT4+ rings
reside on the DNP recognition units. Fig. 2 shows partial 1H
NMR spectra comparing the dumbbell D·4PF6, the [3]rotaxane
3R·12PF6 and the [2]rotaxane 2R·8PF6. An upfield shift was
observed for the peaks corresponding to the DNP protons
(labelled H4/8, H2/6, and H3/7) in the [2]- and [3]rotaxane, indi-
cating that the DNP units are encircled by CBPQT4+ rings. This
co-conformation was confirmed by through-space interactions
observed in the 1H–1H ROESY NMR spectrum, shown in
Fig. S9 in the ESI.† Variable temperature 1H NMR spectra,
which were recorded on the [2]rotaxane 2R·8PF6 demonstrate
that, in the GSCC, the inner BIPY2+ recognition sites serve as
electrostatic barriers preventing the shuttling of the ring from
one DNP recognition site to the other. See Fig. S11 in the ESI.†
Following the characterisation of the [3]rotaxane 3R·12PF6,
we were interested in investigating its switching properties,
particularly in aqueous solution. The solubility of the [3]rotax-
ane can be modulated by counterion exchange, given the fact
Fig. 3 (a) Differential pulse voltammetry (DPV) of the [3]rotaxane 3R12+
(purple curve), dumbbell D4+ (red curve) and CBPQT4+ (blue curve) per-
formed at 0.2 mM sample at 298 K in H2O–DMF (9 : 1, v/v) with 0.1 M
KNO3 as the supporting electrolyte.
−
that the PF6 salt is highly soluble in organic solvents such as
MeCN and Me2CO, and the Cl− salt is soluble in aqueous solu-
tion. Counterion exchanges were achieved using NH4PF6 and
nBu4NCl. The shuttling of the CBPQT4+ rings between the rec-
ognition sites, following reduction to the hexaradical hexa-
cationic RSCC, was monitored electrochemically and also by
UV-Vis-NIR spectroscopy since NMR characterisation of the
paramagnetic species was not possible.
The reduction potential for 3R·12Cl in aqueous solution
was determined (Fig. 3) by differential pulse voltammetry
using Ag/AgCl as a reference. Compared to the free BIPY2+
units in the dumbbell structure (−615 mV) and in the ring
(−410 mV), the reduction potential of the [3]rotaxane was sig-
nificantly shifted (−368 mV). The shift of the BIPY+• signal in
the [3]rotaxane toward more positive potential indicates the
formation of the trisradical complex between the BIPY+• and
CBPQT2(+•). Spectroelectrochemistry (SEC) performed (Fig. 4)
at an applied potential of −750 mV showed evidence for the
shuttling of the reduced diradical, dicationic CBPQT2(+•) rings
from the DNP recognition sites to the inner BIPY+• sites. This
Fig. 4 UV-Vis Spectra (60 μM, 298 K) of 3R6(+•) (purple curve), D2(+•)
(red curve) and CBPQT2(+•) (blue curve) conducted in H2O–DMF (9 : 1,
v/v) with 0.1 M KNO3 as the supporting electrolyte at an applied
potential of −750 mV vs. Ag/AgCl show different absorbances between
species in their reduced states.
was revealed by a change in the charge-transfer band corres-
ponding to the interaction between the CBPQT4+ rings and
the DNP units in the GSCC. A comparison of the spectra of
the reduced [3]rotaxane species with those of the dumbbell
D2(+·) and ring CBPQT2(+•) in their reduced states showed an
absorption band centred at 1125 nm characteristic of the tris-
radical interaction in the hexaradical, hexacationic [3]rotaxane
3R6(+·). SEC also showed significantly different absorption pro-
files for the reduced versus oxidised states of the [3]rotaxane.
See Fig. S2 in the ESI.† In addition to electrochemical stimuli,
switching was also achieved by chemical means – Na2S2O4 and
Fig. 2 Partial 1H NMR spectra (500 MHz, CD3CN, 298 K) comparing the
dumbbell D·4PF6, the [2]rotaxane 2R·8PF6, and the [3]rotaxane 3R·12PF6
reveal upfield shifts of the resonances for the DNP protons in the rotax-
anes (see Scheme 1 for proton labelling), indicating that the CBPQT4+
rings encircle the DNP units in the oxidised GSCC.
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Zn dust for reduction in H2O and MeCN respectively, and
O2 (air) for oxidation. See Fig. S3 and S4 in the ESI.†
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In summary, by combining new advances in radical-based
MIM motifs with a design that results in relative contractile
motion of the rings, we have produced a ‘next generation’
palindromic [3]rotaxane capable of redox switching in aqueous
solution. The radical-pairing recognition motif is incredibly
versatile, since it is amenable to switching by electrochemical
or chemical stimuli in different media, including aqueous
solution. This system brings the dream of artificial muscles
that function employing the same mechanism natural muscle
uses – namely molecular motion – one step closer.15 Operating
in aqueous solution enables the integration of this molecular
muscle mimic with biological interfaces. Future work will
focus on the preparation of derivatives that include conju-
gation handles for incorporating this molecular switch into
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Acknowledgements
The research reported in this publication was supported by
the National Institute of General Medical Sciences of the
National Institutes of Health (NIH) under Award Number
F32GM105403. The content is solely the responsibility of the
authors and does not necessarily represent the official views of
the NIH. This research is part (Project #34-949) of the Joint
Center of Excellence in Integrated Nano-Systems (JCIN) at King
Abdul-Aziz City for Science and Technology (KACST) and
Northwestern University (NU). The authors would like to thank
both KACST and NU for their continued support of this
research. A.C.F. was and E.J.D. is supported by the National
Science Foundation (NSF) Graduate Research Fellowship
Program. L.S.W. acknowledges support from the International
Institute for Nanotechnology (IIN) at NU. J.C.B. was supported
by a National Defense Science and Engineering Graduate
Fellowship from the Department of Defense and gratefully
acknowledges receipt of a Ryan Fellowship from the IIN while
at NU.
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