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). 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-
), 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-
unit has a hairpin structure with two α he-
). 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-
(suppl. 1), 1 (2001).
zyme. Although biochemical confirmation is
among all those involved in the treatment
5. C. Lipinski, J. Pharmacol. Toxicol. Methods 44
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.
, 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
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
lectrons 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
). 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.
) 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
). 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, M
F, is monitored through the resistance, which depends on the relative ori-
Waterfront Place, Pittsburgh, PA 15222, USA. E-mail:
entation of M
F and the pinned-layer magnetization, M
P. The resistance continuously varies from
low to high resistance as M
F and M
P 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
). 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.
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
). 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-
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
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.
) 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
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
). After that false start, Büttner et al
) that an aminyl radical canindeed be stabilized by metal coordination.
arbon- 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.
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 (2
). Stable organ-
also have established functions in oxidation
reactions (see the figure, bottom right panel)
amino acids such as tryptophan or histidine
), photosynthesis (7
), and DNA synthe-
The author is at the Institut für Anorganische Chemie,
have recently been discussed in connection
). 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,
) and photosystem II of photo-
can activate and control these reactive species
Northern Illinois University, De Kalb, IL 60115, USA.
). Aminyl radicals (NR •
through electron transfer. Aminyl radicals
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