Goldup-2015-Nature Nanotechnology

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Controlling molecular motion
to reproduce the behaviour of
macroscopic machines is a rapidly
developing eld of chemistry1. In the last
two decades, researchers have reported
examples of Brownian ratchets2, rotary
motors3 and linear motors4. However,
designing systems that can work repetitively
to move progressively further from
equilibrium, a key property of natural
molecular machines, remains challenging.
Writing in Nature Nanotechnology,
FraserStoddart and co-workers at
Northwestern University now report a
minimalistic molecular machine that can
successfully pump molecules from solution
into a collecting region, iteratively moving
the system further fromequilibrium5.
e authors base their molecular
machine on a rotaxane, a molecule
comprising a linear axle that is able to
restrict the motion of a ring-shaped
component threaded onto it. Although
rotaxanes are ubiquitous features of
many molecular machines, in this case
the chemical structure of the axle is such
that the motion of the rings can only
occur in one direction through a complex
mechanism that involves two one-way
valves. e key structural features (Fig.1)
of the machine are (i) a positively charged
pyridinium unit (red) that acts as the
rst one-way valve; (ii) a viologen unit
(orange) that acts as the pump; (iii) a bulky
isopropylphenyl ‘speed bump’ that acts as
the second one-way valve (purple); and
(iv) an alkyl chain (green) that acts as the
collection unit, terminated with a group
too large to allow the rings to dethread. e
driving force is provided by a reduction–
oxidation cycle6 that reversibly loads and
expels tetracationic cyclophane rings from
the viologen moiety and this, combined
with the one-way valves, allows the machine
to operate repetitively to move rst one,
then another ring from solution to the ring
collection moiety.
e cycle begins with chemical reduction
of the ring and the viologen group (step1)
that establishes an attractive interaction
between them. As a result, a ring from
solution slips past the pyridinium group to
encircle the viologen moiety. Subsequent
oxidation (step2) removes this attractive
interaction; indeed, on oxidation the
ring and viologen electrostatically repel
one another. At this point, although
thermodynamic considerations dictate that
the ring should escape back to solution, the
combined repulsion between the ring in its
oxidized state and the pyridinium end group
and viologen moieties is sucient to ensure
that the ring takes the path of least resistance
and moves to become trapped between the
viologen and the speed bump. is repulsive
interaction provides the rst one-way valve
in the cycle. From here the ring slowly slips
over the bulky speed bump (step3) to reach
the ring-collecting region of the axle. us,
over the rst cycle the ring is pumped from
solution, its thermodynamically preferred
position, to the ring collection region of
the axle where it experiences reduced
translational freedom (lower entropy) and
little or no attractive interactions with the
alkyl chain.
To start the second cycle the mixture is
reduced once again. Although on reduction
the thermodynamically preferred outcome is
for the trapped ring to return to the viologen
unit, the speed bump acts as the second one-
way valve by preventing its passage; without
the repulsive interaction between the ring
and viologen unit, the ring lacks the energy
to overcome the activation barrier to slip
over the speed bump. us, reduction leads
to the loading of a second ring from solution
onto the viologen unit without the rst ring
escaping. Subsequent oxidation then sends
the second ring over the speed bump to
thecollector.
With two rings captured, the system is
now further from the preferred equilibrium
ARTIFICIAL MOLECULAR MACHINES
Two steps uphill
An axle-shaped molecule pumps charged rings from solution into an alkyl collection unit, a mechanism that, in two
repetitive cycles, takes the system increasingly further from equilibrium.
Steve Goldup
O
N
NN
O
NN
N
N N
NN
8
12
2 + 3 1
3
First pumping cycle
Second pumping cycle
a
b
Figure 1 | Working principle of a molecular pump. a, Chemical structure of the molecular machine.
Pyridinium unit, red; viologen, orange; isopropylphenyl, purple; alkyl chain with end group, green; ring,
blue. b, Schematic of the molecular pump operation over two cycles. Step1,reduction; Step 2, oxidation;
Step 3, slippage of the ring past the speed bump. Equivalent chemical and mechanical components are
colour coded.
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position (both rings in solution). e rings
are now at a higher eective molarity and
experience signicantly larger inter-ring
electrostatic repulsion than in solution. e
pump therefore does work on the system.
On each cycle, the system captures some
of the energy consumed by the pump
as chemical potential energy and moves
further from equilibrium, although the
exact amount of potential energy stored is
hard to estimate. To achieve this, Stoddart
and co-workers demonstrate ne control
over the kinetics and thermodynamics of
the motion of the rings. is is an extremely
challenging task and it must be noted that
even relatively small modications to the
spacing between the viologen and the
pyridinium are sucient to obliterate the
working of the molecular machine.
e researchers liken the behaviour
of their machine to biological machines
that transport ions and molecules across
membranes against a concentration
gradient. In keeping with this analogy, the
next task would be to extend this device
beyond two cycles to do more work and
store more energy. However, in this initial
version, the ring-collecting region is too
small to accommodate more than two
rings. is limitation could be overcome
either by simply extending the axle or, more
excitingly, by embedding such machines in
membranes to pump molecules between
compartments, in a form of articial
activetransport.
In addition to the aesthetic value of
producing minimalist models of complex
biological machinery, a long-term challenge
in the eld of articial molecular machines
is to nd ways to do useful work either at
the macroscopic level on the surrounding
environment or at the molecular level by
controlling the chemistry of the system
and eventually materials properties. e
work by Stoddart and co-workers takes this
possibility two steps closer. ❐
Steve Goldup is in the Department of Chemistry,
University of Southampton, Southampton,
Hampshire SO17 1BJ, UK.
e-mail: s.goldup@soton.ac.uk
References
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2. Chatterjee, M.N., Kay, E.R. & Leigh, D. J.Am. Chem. Soc.
128,4058–4073 (2006).
3. Ruangsupapichat, N., Pollard, M.M., Harutyunyan, S.R. &
Feringa, B.L. Nature Chem. 3, 53–60 (2011).
4. von Delius, M., Geertsema, E.M. & Leigh, D.A. Nature Chem.
2,96–101 (2010).
5. Cheng, C. etal. Nature Nanot ech. http://dx.doi.org/10.1030/
nnano.2015.96 (2015).
6. Cheng, C. etal. J.Am. Chem. Soc. 136, 10–13 (2014).
Published online: 18 May 2015
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