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cell (6). ATP synthase is a biological rotary most living organisms, including humans.
the first time in many years, there is real motor made up of two major structural do- There is very limited sequence similarity hope of achieving the quantum therapeutic mains, F0 and F1 (see the figure). The F1 do- leap required to make an impact on the glob- main is composed of subunits α3, β3, γ, δ, al TB epidemic. It is therefore of the utmost and ε; the F0 domain includes one a subunit, safety of the compound, as borne out by the phase II clinical trials. Furthermore, the arranged in a symmetrical disk. The F0 and mycobacteria-specific activity of R207910 F1 domains are linked by central stalks (sub- [(3), table S1] may also be the consequence against M. ulcerans—the agent of an emerg- units γ and ε) and peripheral stalks (subunits of limited sequence similarity among bac- ing human disease called Buruli ulcer (9), b and δ). The proton-motive force fuels the terial AtpE proteins. However, those antitu- for which surgery is the only cure—also rotation of the transmembrane disk and the bercular agents that show highly restricted raises expectations for a safer treatment for central stalk, which in turn modulates the nu- activity (such as isoniazid, ethionamide, cleotide affinity in the catalytic β subunit, and pyrazinamide) are all prodrugs requir- leading to the production of ATP. The c sub- References
unit has a hairpin structure with two α he- (7). Although its chemical structure gives 1. C. Dye et al., J. Infect. Dis. 185, 1197 (2002).
2. C. Dye, S. Scheele, P. Dolin, V. Pathania, M. C.
lices and a short connecting loop. The two Raviglione, J. Am. Med. Assoc. 282, 677 (1999).
mutations affect the membrane-spanning α 3. K. Andries et al., Science 307, 223 (2005); published
helices of the ATP synthase c subunit and online 9 December 2004 (10.1126/science.1106753).
4. Global Alliance for TB Drug Development, may restrict binding of R207910 to the en- Tuberculosis 81 (suppl. 1), 1 (2001).
zyme. Although biochemical confirmation is among all those involved in the treatment 5. C. Lipinski, J. Pharmacol. Toxicol. Methods 44, 235
now required, it is possible that the drug im- 6. D. Stock et al., Science 286, 1700 (1999).
pedes assembly of the mobile disk or inter- studies already show that this compound can 7. Y. Zhang, W. R. Jacobs Jr., C. Vilchèze, in Tuberculosis feres with its rotational properties, leading to greatly shorten the duration of therapy, both and the Tubercle Bacillus, S. T. Cole, K. D. Eisenach, D.
N. McMurray, W. R. Jacobs Jr., Eds. (ASM Press, alone and in association with current antitu- Washington, DC, 2005), pp. 115–140.
bercular agents. The DNA gyrase inhibitor 8. E. Nuermberger et al., Am. J. Resp. Crit. Care Med.
169, 421 (2004).
ceptional specificity for mycobacteria. ATP 9. T. S. van der Werf et al., Lancet 362, 1062 (2003).
promise in the same animal models (8). For sive set of dynamical measurements that elu-cidate this effect (see the first figure).
A Ringing Confirmation
transfer? The relative orientation of the elec-tron spins and the magnet determines of Spintronics Theory
whether the spin torque augments or oppos-es the damping torque that forces the mag- net to settle into static equilibrium. Withinthis scenario, two competing models predict Electrons possess both electric charge that this torque differs fundamentally from very distinct behavior when spin transfer re-
the usual torque exerted by magnetic fields.
Traditional electronic devices use The most direct way to test this prediction only charge, but a growing class of elec- nets respond coherently to spin-polarized tronic devices exploits spin. One example is electrons (3). Depending on the strength of the spin-dependent magnetoresistive read- spin-polarized current. On page 228 of this back sensors used in hard disk drives and in issue, Krivorotov et al. (4) present an exten- three different types of dynamical states can emerging nonvolatile magnetic memories.
However, even more ways to use spin are being proposed for new spin-based elec-tronics, or “spintronics” (1).
It has been shown that a current of spin- polarized electrons can change the magneticorientation of a nanometer-scale ferromag- net via an exchange of spin angular momen-tum (2, 3). This effect originates from the way in which ferromagnets align the spin ofconduction electrons along the direction ofmagnetization. In other words, ferromagnetsexert a torque that changes the electron an- Schematic of the “nanopillar” structure used by Krivorotov et al. (4). Electrons polarized by
the pinned ferromagnet exert a torque on the free ferromagnet. At these nanoscale dimensions, action torque on the magnet. Theory predicts spin transfer dominates over the magnetic field produced by the moving electrons, and the largecurrent densities that are necessary to induce a response are easily achieved. Motion of the free The author is with Seagate Research, 1251 layer magnetization, MF, is monitored through the resistance, which depends on the relative ori-
Waterfront Place, Pittsburgh, PA 15222, USA. E-mail: entation of MF and the pinned-layer magnetization, MP. The resistance continuously varies from
low to high resistance as MF and MP go from parallel to antiparallel, respectively.
occur (see the second figure). Reversal oc- before the precession has a chance to die magnetic precession is synchronous with the curs through spatially and temporally coher- down. The amplitude of this oscillation, or current pulse and can quickly wind up to its ent precession of the magnetization. Another “ringing,” increases until the magnet reverses full amplitude in only a few periods (less than model proposes that spin transfer induces its direction (see the second figure, right pan- 1 × 10–9 s). Finally, they demonstrate that a dc incoherent, short-wavelength magnetic os- el). Larger currents drive this switching current can affect the time it takes for the cillations that mimic what would happen if process even faster. This is what Krivorotov et magnetization to settle into static equilibrium the magnet got hot (5, 6). The magnetization (see the second figure, left panel). These data then switches in a stochastic manner akin to Spin transfer also affects the magnetiza- provide clear proof of the spin-torque model tion dynamics below the switching thresh- by demonstrating that spin transfer can con- The experiments of Krivorotov et al. pro- tinuously tune the magnetic damping and in- vide direct evidence for the coherent switch- torque effectively counterbalances damping.
ing process predicted by the spin-torque mod- In this case, the magnet neither switches nor el. When a sufficiently large current pulse is settles back into equilibrium but instead tion induced by spin transfer is already being sent through a nanomagnet, such that the spin rings indefinitely (see the second figure, explored for use as tunable magnetic-based torque opposes the damping, the forces that middle panel). Hence, a dc current can drive microwave oscillators in logic and communi- keep the magnet settled along a particular di- microwave oscillations, which can potential- cations applications (8). Magnetic memory rection are overcome, and the magnet starts to is another application for which spin transfer rotate in response to the driving torque from Krivorotov et al. observe this steady-state seems well suited. In addition to its ability to the electrons. The electrons continually im- switch a magnet between bistable states (that part angular momentum to the magnetization ments (79). Moreover, they show that the is, either a “0” and a “1”), switching withspin transfer is more efficient than with mag-netic fields at nanoscale dimensions.
achieve higher performance and lower cost in solid-state electronics, spin transfer has the potential to replace field-driven switch- References
1. S. A. Wolf et al., Science 294, 1488 (2001).
2. L. Berger, Phys. Rev. B 54, 9353 (1996).
3. J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1
4. I. N. Krivorotov et al., Science 307, 228 (2005).
5. S. Urazhdin et al., Phys. Rev. Lett. 91, 146803 (2003).
Dynamical regimes where spin transfer opposes damping. (Left) When the spin torque is smaller
6. A. Fábián et al., Phys. Rev. Lett. 91, 257209 (2003).
than the damping torque, precession is quickly damped and the magnet settles into static equilibrium 7. S. I. Kiselev et al., Nature 425, 380 (2003).
(dashed arrow). The time scale of the damping can be tuned continuously by the current. (Middle)
8. W. H. Rippard et al., Phys. Rev. Lett. 92, 027201
When the spin torque and the damping torque are effectively equal and opposite over a precessional 9. M. Covington et al., Phys. Rev. B 69, 184406 (2004).
orbit, persistent precession occurs. (Right) When the spin torque is larger than the damping torque, the
precession increases in amplitude until the magnetization completely reverses direction.
where R is an aryl or alkyl) are the most el-ementary class of nitrogen-centered organ- ic radicals. An earlier report of their stabi- Odd Electron on Nitrogen:
lization through metal coordination wasproven erroneous because of intramolecu- A Metal-Stabilized Aminyl Radical lar reduction to an amide (NR –2) ligand (9,
10). After that false start, Büttner et al. now demonstrate (8) that an aminyl radical canindeed be stabilized by metal coordination.
Carbon- and oxygen-centered organic ic radicals with the unpaired (“odd”) elec- The chemical properties of aminyl radi-
tron centered on nitrogen have received less cals are intermediate between those of alkyl cal curiosities or, at best, reactive in- termediates. However, in recent years some been known since the late 19th century. On with R = aryl). Alkyl radicals have essential of these molecules have received widespread page 235 of this issue, Büttner et al. (8) re- biochemical roles, for example as CH2R• in attention beyond chemistry—for example, coenzyme B12–dependent processes (see the as spin carriers in materials science (1) or as figure, top left panel) (3). Aryloxy species reaction sites in biology (27). Stable organ- also have established functions in oxidation reactions (see the figure, bottom right panel) amino acids such as tryptophan or histidine (4, 5), photosynthesis (7), and DNA synthe- The author is at the Institut für Anorganische Chemie, have recently been discussed in connection sis (6). In almost all cases, the radicals are Universität Stuttgart, 70550 Stuttgart, Germany, and with electron transfer in cytochrome c per- accompanied by transition metal ions, which at the Department of Chemistry and Biochemistry, oxidase (6) and photosystem II of photo- can activate and control these reactive species Northern Illinois University, De Kalb, IL 60115, USA.
E-mail: kaim@iac.uni-stuttgart.de synthesis (7). Aminyl radicals (NR • through electron transfer. Aminyl radicals

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