n e w s of t h e w e e k
TWO WAYS TO DRIVE MOLECULAR MOTORS Light- and chemically-powereded sv systems rotate propeller-type molecular units
T
wo ingenious new molecules show that it's feasible to construct molecular machines that are driven by external energy sources. In one designed by Dutch and Japanese chemists, energy from ultraviolet light drives an artificial molecular propeller to rotate 360° in one direction only. And chemists at Boston College, Chestnut Hill, Mass., have built a molecular motor that uses chemical energy to achieve unidirectional intramolecular rotation through 120°. The light-driven propeller rotates around the central carbon-carbon double bond in a chiral, helical alkene [Nature, 401,152 (1999)]. The work was carried out by chemistry professor Ben L. Feringa and coworkers at Stratingh Institute, University of Groningen, and chemistry professor Nobuyuki Harada at the Institute for Chemical Reaction Science, Tohoku University, Sendai. 'To the best of our knowledge, this is
form an isocyanate. Clockwise (but not counterclockwise) rotation brings the isocyanate close enough to react with a hydroxypropyl tether attached to the helicene. The urethane formed by the reaction is initially in a high-energy conformation, and at ambient temperature, the tethered triptycene rotates irreversibly in the clockwise direction to a lower energy conformation. The urethane is then cleaved to complete the 120° rotation of the triptycene.
the first observation of a §' monodirectional, photo- | Triptycene rotates in one direction chemically driven rotation of § ChU a propeller-type unit, and it 1 ChU shows that controllable rota- | tory motion can be produced § NHo by the influx of external ener- | gy," the authors state. | The rotation consists of | (CH 2 ) 3 OH (CH 2 ) 3 OH two photoisomerization steps j Isocyanate Triptycene-helicene and two thermal isomeriza- |" tion steps. The group has < achieved three full cycles of repetitive 360° rotary motion with the system, Feringa tells C&EN. "The present system HoN is primitive," he says. "However, we now have the key com(CH 2 ) 3 OH O "NH ponents and techniques available to build molecular ma(CH2)30X chinery that can perform Urethane complex tasks, although there is a long way to go be'This is the first rationally designed fore we can achieve this." In the same issue prototype of a chemically powered moof Nature (page 150), lecular motor," Kelly says. "We are curBoston College pro- rently working on a system that rotates Helical alkene rotates via four fessor of chemistry continuously." isomerization steps The Boston College researchers point T. Ross Kelly and coworkers describe a out that molecular processes used by "biMe eq < system consisting of ological motors"—such as muscle fibers, r triptycene and heli- flagella, and cilia—to convert chemical Me a x UV light cene units connected energy into coordinated movement reby a single bond that main poorly understood. "Although our Me e q Me a x , functions as an axle. system does not achieve continuous and "The triptycene unit fast rotation, the design principles that we acts as a propeller, have used may prove relevant for a better Trans-1 isomer Cis-1 isomer whereas the helicene understanding of biological and synthetic Irreversible unit functions as a molecular motors producing unidirecthermally driven isomerizations friction brake that re- tional rotary motion," they state. sists easy random roAccording to Anthony P. Davis, profestation but allows sor of organic chemistry at Trinity College, r Me a x ' chemically powered Dublin, Kelly's system demonstrates, in Me eq unidirectional rota- an elegant fashion, how transduction of UV light tion," Kelly notes. chemical energy into directed motion Meeq The chemical en- may be performed for unidirectional rotaMe a x ergy for the motor is tion at the molecular level. "Although it provided by phos- can only perform a single 120° turn, it esTrans-2 isomer gene, which reacts tablishes a principle which may eventualMeax = axial CH3 Cis-2 isomer Meeq = equatorial CH3 with an amino group ly be exploited in a genuine, chemically on the triptycene to driven molecular motor," he tells C&EN.
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SEPTEMBER 13,1999 C&EN
He adds that the photochemically driven system developed by Feringa, Harada, and coworkers is probably less relevant to biology. "However, it is remarkable in that continuous unidirectional rotation does indeed seem to be possible under the right conditions," he says. "It may be difficult to harness this motion, but the team does seem to have the essence of a true, light-powered molecular motor." J. Fraser Stoddart, professor of organic chemistry at the University of California, Los Angeles, remarks that the two Nature articles share much conceptually, although the actual molecules employed by the two groups of researchers differ considerably in their design and construction. 'They both rely on chirality of one sort or another as the fundamental basis for their unidirectionality, and they both draw, not unexpectedly, on thermal energy as one of their sources of power," he says. 'Whereas one relies on a neat piece of chemistry to give it its unidirectional motion, the other is stimulated by the clever use of light. "So far, the design and construction of molecular motors has been largely a curiosity-driven exercise," he continues. "Before long, however, it will become a technologically driven pursuit. The science that has been conceived and carried out by the two groups is science at its very best." Michael Freemantle
producers—Ashland and Fortier Methanol, a joint venture between Methanex Corp. and Cytec Industries—have shut down. Methanex, the world's largest methanol producer, is weighing the future of its own business. For years, the company hewed to a strategy of making only one product—methanol. However, after losing $62 million in the first half of 1999, the company now admits that this approach may not be working. 'We are challenging our current single-product business strategy," says President and Chief Executive Officer Pierre Choquette. Methanex's largest shareholder, Canada's Nova Chemicals, is also doing some decision-making. Nova has for several years held a one-quarter stake in Methanex and had expressed interest in acquiring more, but Jeffrey M. Lipton, Nova's president and CEO, now says the company has changed course and no longer wants to increase this stake because Methanex is not "strategically important."
The methanol industry has two major problems. The short-term problem is that low-cost, big-volume methanol plants will start up this year and next in Qatar, Saudi Arabia, Trinidad, and Iran. Brian Grigsby, methanol analyst at the Houston-based consulting firm Chemical Market Associates notes that in contrast to their U.S. and European counterparts, these remote plants run on a new model—consumption of otherwise unsalable natural gas. 'Twenty years ago," he says, "you built a plant to meet market needs. Today, you build one to use up gas." For the longer term, producers are grappling with the planned phaseout in California—and likely the rest of the U.S.—of the gasoline additive methyl tert-bxxtyl ether, which is based on methanol and consumes roughly 40% of U.S. methanol output. Methanol producers can take some solace in that the process will take years. Michael McCoy
New Zealand sun burns brighter
Peak summertime levels of damaging pounds allows more of that high-energy ultraviolet (UV) radiation reaching New radiation to reach Earth's surface. DraZealand increased about 12% over the matic jumps in UVhave been documentpast decade—a direct result of ozone ed under the Antarctic ozone hole, where fully half of the ozone in the atdepletion. Scientists at New Zealand's National mosphere is destroyed each spring. But Institute of Water & Atmospheric Re- information about long-term trends in search (NIWA) document a strong link UV in locations where ozone depletion between the increases in UV radiation is more subtle has been scarce. "Very few good-quality UV measurethey measured at 45° S and observed decreases in ozone levels during the ments were available before the 1990s," southern hemisphere summer [Science, McKenzie tells C&EN. 'This study uses 2 8 5 , 1709 (1999)]. Responding to a protracted pricing 'This is the best verifi- •——•—^ downturn, methanol producers are cation yet of unequivocal As ozone has dropped, starting to make decisions about the fu- increases in UV due to summertime UV has climbed ture of their business. long-term ozone decline," Georgia Gulf Corp. has decided to exit says Richard McKenzie, UV index3 Ozone, Dobson units the business entirely by the end of the NIWA's project manager 12.5 year. The company idled its Plaquemine, for UV radiation studies. • - 320 12.0 La., methanol plant in December in re- 'The increases pose a sig" \ A • / ~ 310 sponse to low prices but has been supply- nificant increase in envi11.5 300 ing customers with imported product. ronmental risks." 290 11.0 Now, the company will permanently Atmospheric ozone ab280 / ^ ™ ^ ^ y ' V / ^ .^ close the plant and take an after-tax sorbs sunlight with wave10.5 / \ / ~ 270 charge of $8 million to write off its metha- lengths shorter than 320 260 10.0 nol assets. Industry sources say the com- nm—the high-energy UVs/ pany is selling its customer list and termi- B rays that can damage PS i i i i i i i i i i i i i i i i i i i i nals to competitor Lyondell Methanol Co. DNA in living systems and *$$$$$&$$$£ * Georgia Gulf was one of three U.S. accelerate decomposition < #Summer < £ < £Year < #(December-February)
