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A molecular information ratchet


Motor proteins and other biological machines are highly efficient at converting energy into directed motion and driving chemical systems away from thermodynamic equilibrium1. But even though these biological structures have inspired the design of many molecules that mimic aspects of their behaviour2,3,4,5,6,7,8,9,10,11,12,13,14,15, artificial nanomachine systems operate almost exclusively by moving towards thermodynamic equilibrium, not away from it. Here we show that information about the location of a macrocycle in a rotaxane—a molecular ring threaded onto a molecular axle—can be used, on the input of light energy, to alter the kinetics of the shuttling of the macrocycle between two compartments on the axle. For an ensemble of such molecular machines, the macrocycle distribution is directionally driven away from its equilibrium value without ever changing the relative binding affinities of the ring for the different parts of the axle. The selective transport of particles between two compartments by brownian motion in this way bears similarities to the hypothetical task performed without an energy input by a ‘demon’ in Maxwell’s famous thought experiment16,17,18,19. Our observations demonstrate that synthetic molecular machines can operate by an information ratchet mechanism20,21,22, in which knowledge of a particle’s position is used to control its transport away from equilibrium.

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Figure 1: A photo-operated molecular information ratchet.
Figure 2: 1 H NMR spectra (600 MHz, CD 3 OD, 298 K) of a working nanomachine.
Figure 3: Operation of a molecular information ratchet.
Figure 4: Operation of rotaxane 2, featuring only intermolecular photosensitization of the α-methyl stilbene gate.

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We thank P. J. Camp and his research group for the free energy calculation, and the EPSRC, the EU project Hy3M and the Carnegie Trust for financial support. D.A.L. is an EPSRC Senior Research Fellow and holds a Royal Society-Wolfson research merit award.

Author Contributions V.S., C.-F.L. and E.R.K. contributed equally to this work.

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Correspondence to David A. Leigh.

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Supplementary information

Supplementary Information

This file contains Supplementary Notes and Supplementary Figures S1-S7 with legends and additional references. The file contains details of synthetic procedures for the preparation of thread and rotaxane molecules, the results from a repeat run of the photochemical experiments carried out on rotaxane 1, and a Supplementary Figure containing the 1H NMR spectra relevant to the experiment on control rotaxane 2 (reported in Fig. 4 of the main text). Also included are the results of photochemical experiments carried on model compounds and a discussion of these in relation to the kinetic mechanism of operation of the molecular machine. Explanatory notes are also provided in relation to the effect of stilbene isomerization on the macrocycle binding affinities for the ammonium sites and the calculation of the extent to which the molecular machine is moved away from equilibrium. (PDF 1674 kb)

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Serreli, V., Lee, CF., Kay, E. et al. A molecular information ratchet. Nature 445, 523–527 (2007).

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