Synthetic molecular motors

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Sep 9, 1999 - Established molecular actuators include systems capable of cistrans isomerism, where groups lie on the same (cis) or opposite (trans) sides of ...
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100 YEARS AGO Many observers in different countries, noticing the fact that malaria is most prevalent at the most active period of mosquito life, have attributed malaria to the agency of this insect. Dr. Patrick Manson, in 1894, first brought the subject forward in England … . Bignami and Bastianelli, who had been trying unsuccessfully to infect a man by allowing mosquitoes to bite him, attributing their want of success to the use of the wrong kind of mosquito, and, acting on the observations of Grassi, tried again with some mosquitoes imported from the malarious district. This time they succeeded in infecting the man with malaria of the same type that prevailed in the district from which the mosquitoes came. … Whether the Anopheles can be extirpated from a locality, and by what means, will be the problem for scientific workers resident abroad to settle; fortunately they seem to be confined to small areas, so the suggestion of Ross to draw off the water from stagnant pools may not be so hopeless a task as it would at first appear.

effect’, could allow males with the same father to recognize one another, whether they were reared together or apart, as often occurs in species where males mate with many females. I believe that this is the most likely explanation. However, confirmation requires identifying what heritable cues males use (for example, calls, plumage or odour), altering them experimentally, and observing whether manipulated males indeed prefer similar but unrelated lek-partners. Petrie and colleagues’ results2 are important for several reasons. First, they (and Höglund et al.1) suggest that lek-joining in some species is best analysed by considering inclusive fitness, not just male–male competition. Second, although phenotype matching has been observed in many invertebrates and vertebrates10 (most recently in chimpanzees13), this is the first report for a lekking species. Third, the likelihood of self-referencing in peacocks should galvanize studies of this intriguing, yet controversial10,11,14,

kin-recognition mechanism, particularly among species in which social learning is an inadequate or misleading guide to relatedness. ■ Paul W. Sherman is in the Department of Neurobiology and Behavior, Cornell University, Ithaca, New York 14853, USA. e-mail: [email protected] 1. Höglund, J., Alatalo, R. V., Lundberg, A., Rintamäki, P. T. & Lindell, J. Proc. R. Soc. Lond. B 266, 813–816 (1999). 2. Petrie, M., Krupa, A. & Burke, T. Nature 401, 155–157 (1999). 3. Höglund, J. & Alatalo, R. V. Leks (Princeton Univ. Press, 1995). 4. Deutsch, J. C. Behav. Ecol. Sociobiol. 34, 451–459 (1994). 5. Widemo, F. & Owens, I. P. F. Nature 373, 148–151 (1995). 6. Watts, C. R. & Stokes, A. W. Sci. Am. 224, 112–118 (1971). 7. Kokko, H. & Lindström, J. Proc. R. Soc. Lond. B 263, 919–923 (1996). 8. McDonald, D. B. & Potts, W. K. Science 266, 1030–1032 (1994). 9. Petrie, M. Nature 371, 598–599 (1994). 10. Sherman, P. W., Reeve, H. K. & Pfennig, D. W. in Behavioural Ecology, 4th edn (eds Krebs, J. R. & Davies, N. B.) 69–96 (Blackwell, Oxford, 1997). 11. Sherman, P. W. Ethol. Sociobiol. 12, 377–386 (1991). 12. Dawkins, R. The Extended Phenotype (Freeman, San Francisco, 1982). 13. Parr, L. A. & deWaal, F. B. M. Nature 399, 647–648 (1999). 14. Alexander, R. D. Ethol. Sociobiol. 11, 241–303 (1990).

Nanotechnology

Synthetic molecular motors Anthony P. Davis

From Nature 7 September 1899.

50 YEARS AGO For the woman, Dr. Turquet saw two possibilities fraught with anxiety and frustration and therefore likely to add to the sum of aggression in society. One is her entry into an occupational life in which her opportunities compare unfavourably with a man’s. If, on the other hand, she prefers the older domestic role with its exemption from the need to compete on the male pattern, she then faces the anxiety of choosing and securing a husband, on whose adequacy her own social status will largely depend, and to whom she will lose some of her individual identity. In either case the woman meets an atmosphere of masculine superiority which contrasts sharply with her childhood experience of the mother’s seeming superiority in the home. Aggression results from the inferiority which she feels in such an atmosphere. “Hence,” said Dr. Turquet, “ambivalent feelings towards motherhood, the declining birthrate, and an unconscious desire to reject the more fundamental aspects of the feminine role”. From Nature 10 September 1949. Many more extracts like these can be found in A Bedside Nature: Genius and Eccentricity in Science, 1869–1953, a 266-page book edited by Walter Gratzer. Contact Lisa O’Rourke. e-mail: [email protected]

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he construction of miniature, ‘nanoscale’ machines is a goal of modern science and technology, inspired by Richard Feynman’s remark that “There’s plenty of room at the bottom”1. Chemists, by the nature of their discipline, are already at the bottom, manipulating the smallest entities that have complex shapes (molecules), and which can therefore be used as engineering components. While engineers and physicists explore the top-down approach to nanoscale engineering through lithography and scanning probe microscopy, chemists are well placed to pursue the bottom-up strategy, whereby molecular -scale components are created using chemical synthesis and then self-assembled into devices by pre-programmed intermolecular forces2. Among the more interesting challenges in this area is the design and synthesis of ‘molecular actuators’, molecules that can undergo changes in shape in response to external stimuli and thereby, in principle, perform mechanical work. To date, most research has concentrated on two-state systems, ranging from classical cis–trans isomerism to more elaborate ‘rotaxanes’ and ‘catenanes’ (Fig. 1)3,4, and biomolecular constructs, such as a device based on the transition of right-handed to left-handed DNA5. These systems, in which movement is driven by chemical, electrochemical or photochemical forces, are best described as molecular

T

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switches or shuttles, and they have great potential in, for example, molecular-scale information processing. However they are not capable of the continuous, unidirectionhν Trans-cis interconversion under the influence of light

Station 1

Station 2

Stopper

Stopper

Rotaxane – a ring imprisoned on a string, shuttling between two stations

Catenane – a ring imprisoned on another ring, again moving between two stations

Figure 1 Two-state molecular systems. Established molecular actuators include systems capable of cis–trans isomerism, where groups lie on the same (cis) or opposite (trans) sides of a double bond, and more complex strucures such as rotaxanes and catenanes. The rings in the rotaxanes and catenanes may be driven between stations by chemical, electrochemical or photochemical input. NATURE | VOL 401 | 9 SEPTEMBER 1999 | www.nature.com

news and views al motion we would expect of a true molecular motor. Now, on pages 150 and 152 of this issue, Kelly et al.6 and Koumura et al.7 report two different routes towards this seductive goal. The design of Kelly et al.6 is based on a previous attempt to construct a ‘molecular ratchet’. In this earlier work, the rotation of a three-pronged ‘wheel’ was hindered by a curved ‘brake’ (Fig. 2a), whose asymmetrical shape was intended to favour rotation in one particular direction8. Such unidirectional motion, driven only by ambient thermal energy, would have violated the second law of thermodynamics9, and unsurprisingly failed to occur. Careful NMR measurements showed that the ratchet rotated in either direction with equal probability. Kelly et al. have now modified their system, using the asymmetry of their brake to produce chemically driven (and therefore thermodynamically allowed) motion in one direction. Functional groups are attached to the brake and one spoke of the wheel, allowing the two to be linked covalently. The linkage pulls the molecule into a strained conformation, so that it becomes energetically favourable for the spoke to slip under the brake. Cleavage of the linkage returns the system to its original chemical state, but with the wheel having undergone one-third of a rotation. The system is not a true molecular motor, as no provision is made for further movement, but it does demonstrate a method by which chemical energy may be transduced, on a molecular scale, into unidirectional rotary motion. Understanding this and similar processes may be relevant to natural molecular motors, such as ATP synthase10,11, which are also powered by chemical reactions. A critical aspect of the above system is its chirality (handedness), without which a direction of rotation could not be defined, let alone pursued. The design of Koumura et al.7 also obeys this principle. Their system is an alkene molecule (that is, it contains a carbon–carbon double bond), and uses the photochemical cis–trans interconversion illustrated in Fig. 1. But whereas simple alkenes are planar and achiral (that is, superimposable on their mirror images), the system of Koumura et al. has a complex structure which prevents planarity, introduces chirality and results in four distinguishable states, two trans and two cis (Fig. 2b). As expected, both trans–cis interconversions can be achieved by illumination with ultraviolet light of an appropriate wavelength. The singular feature of this system is that, for subtle structural reasons, one of each pair of trans- and cis-isomers is more stable than the other. Provided the temperature is high enough to overcome the activation barrier between the two trans or cis isomers, the more stable of the pair is formed irreversibly from its partner. So, light-driven creation of NATURE | VOL 401 | 9 SEPTEMBER 1999 | www.nature.com

a

b From molecular ratchet...

cis-1

trans-1

Spindle Spokes

hν Brake

... to molecular motor

Irreversible, thermallydriven rotations

1

2 cis-2

trans-2



3

Figure 2 Unidirectional motion produced by nanoscale rotors. a, In the chemically driven system created by Kelly et al.6, the ratchet design (top) is modified so that a link can be made between the brake and one of the spokes. Making the link (1 → 2) pulls the rotor into a strained, high-energy position. Thermal energy is now sufficient to carry the spoke past the brake, allowing the rotor to regain its favoured situation as in 3. Cleavage of the link restores the original position, except that the rotor has moved through 120°. b, The light-powered system created by Koumura et al.7, viewed down the axis of a central carbon–carbon double bond that links two identical halves of an alkene molecule. The double bond is trying to establish a planar structure — that is, to make the two halves line up. But they are prevented from doing so by bulky substituent groups, resulting in a twisted geometry with four distinct states. The barrier between cis and trans forms is high, but may be overcome by ultraviolet irradiation. Asymmetric substituents (triangles) cause energy differences between the two cis and two trans isomers, such that cis-2 is more stable than cis-1, whereas trans-1 is more stable than trans-2. At appropriate temperatures, the thermally driven cis-1 → cis-2 and trans-2 → trans-1 conversions take place irreversibly. The net result is continuous rotation in the presence of ultraviolet light.

the less stable isomer of each pair is followed by thermal decay into the more stable isomer, resulting in overall rotation (Fig. 2b). The authors have followed this cycle stepwise by performing the photochemical conversions at low temperatures, which slows the trans–trans and cis–cis interconversions so that each of the four species can be observed in sequence. However, at higher temperatures there seems no reason why the system should not rotate continuously under ultraviolet irradiation, and so be seen as a genuine, light-powered molecular motor. Many problems remain before the principles embodied in these molecular motors can be incorporated in real, useful machines. The rotors must be coupled to their surroundings, and the self-assembly processes needed for nanoscale construction still need to be worked out. Nonetheless, the results of these two groups provide further confirmation that molecular-scale engineering is slowly edging towards reality. The difficulties should not be underestimated; molecular design is still an uncertain business, and chemical synthesis can be lengthy and painstaking. Indeed, an engineer may be amazed that the ‘partial motor’ of Kelly et al., containing only 78 atoms, © 1999 Macmillan Magazines Ltd

required over four years to construct. It should not be forgotten, though, that synthesis is a massively parallel manufacturing process; each mole of product corresponds to `6 2 1023 discrete ‘nanodevices’. Both groups have probably made at least 1019 units of their respective systems, and could scale up by several orders of magnitude if required. The bottom-up approach is certainly challenging, but may ultimately have economics on its side. ■ Anthony P. Davis is in the Department of Chemistry, Trinity College, Dublin 2, Ireland. e-mail: [email protected] 1. Feynman, R. Eng. Sci. 23, 22 (1960). 2. Philp, D. & Stoddart, J. F. Angew. Chem. Int. Edn Engl. 35, 1155–1196 (1996). 3. Balzani, V., Gomez-Lopez, M. & Stoddart, J. F. Acc. Chem. Res. 31, 405–414 (1998). 4. Sauvage, J. P. Acc. Chem. Res. 31, 611–619 (1998). 5. Mao, C. D., Sun, W. Q., Shen, Z. Y. & Seeman, N. C. Nature 397, 144–146 (1999). 6. Kelly, T. R., De Silva, H. & Silva, R. A. Nature 401, 150–152 (1999). 7. Koumura, N., Zijlstra, R. W., van Delden, R. A., Harada, N. & Feringa, B. L. Nature 401, 152–155 (1999). 8. Kelly, T. R., Tellitu, I. & Sestelo, J. P. Angew. Chem. Int. Edn Engl. 36, 1866–1868 (1997). 9. Davis, A. P. Angew. Chem. Int. Edn Engl. 37, 909–910 (1998). 10. Noji, H., Yasuda, R., Yoshida, M. & Kinosita, K. Nature 386, 299–302 (1997). 11. Walker, J. E. Angew. Chem. Int. Edn Engl. 37, 2309–2319 (1998).

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