ScienceWeek

SCIENCEWEEK

August 1, 2003

Vol. 7 - Number 31B

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CONTENTS:

1. Planetary Science: History of the Inner Solar System
2. Condensed Matter Physics: On Correlated Electrons
3. Chemistry: Dinitrogen to Ammonia
4. Cell Biology: On the Potassium Ion Channel
5. Molecular Biology: On RNA Repair
6. Genomic Medicine: Cardiovascular Disease
7. Glossary: (Explications and notes re starred (*) terms)

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1. PLANETARY SCIENCE: HISTORY OF THE INNER SOLAR SYSTEM

The following points are made by Conel Alexander (Nature 2003
423:691):

1) Astronomers and astrophysicists have made great strides in
identifying the basic stages in the formation of Sun-like stars
and their planetary systems. Stars form by the collapse of high-
density regions, or cores, inside interstellar clouds. A disk of
gas and dust forms around the growing star, and most of the mass
that is accreted by the star passes through this disk. Early on,
jets of partially ionized gas develop along the system's axis of
rotation. The origin of these bipolar jets is hotly debated, but
they may have a lasting influence on any planets that form in the
inner part of the disk.

2) Although this general picture is well established, very little
is known about the precise conditions and processes that occur
in
a disk during these early stages. This is because it has not been
possible to make direct observations of a disk's mid-plane
region, where planets would form. So far, the best (albeit
imperfect) record of what happened in one disk -- the disk from
which our Solar System formed -- is found in meteorites.

3) With a few exceptions, meteorites are fragments from the
asteroid belt that lies between the orbits of Mars and Jupiter.
Their parent asteroids are the last vestiges of the swarm of
*planetesimals from which the terrestrial planets -- Mercury,
Venus, Earth, and Mars -- formed. The oldest and most primitive
meteorites, the chondrites, appear to consist largely of
aggregates of material that formed in the disk before, or at the
same time that, the planetesimals formed.

4) The major constituents of chondrites (50-80% by volume) are
chondrules -- small spheres of silicates, iron metal, and iron
sulphide. Up to a further 5% is made of predominantly silicate
objects that are rich in calcium and aluminium, known as
calcium–aluminium-rich inclusions, or CAIs. Chondrules and CAIs
range in diameter from tens of micrometres to centimetres, and
formed during brief heating episodes at temperatures of 1600 to
2000 K. The chondrules and some CAIs melted, and even partially
vaporized; other CAIs appear to have condensed out of a hot gas
and never melted. When they formed, they contained significant
concentrations of short-lived radionuclides, although these
concentrations are much lower in chondrules than in CAIs. The
shortest-lived of these radionuclides (41Ca) has a half-life of
only approximately 100,000 years, which means that the time
interval between its synthesis and its incorporation into CAIs
must have been, by astronomical standards, very short. A popular
explanation for this short interval is that a nearby supernova
both triggered the collapse of the fledgling Solar System and
seeded it with the short-lived radionuclides.

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2. CONDENSED MATTER PHYSICS: ON CORRELATED ELECTRONS

One sure thing in science is that whenever the prevailing
authorities in a field announce that nearly all problems have
been solved and that everyone ought to pack up and go home, that
is the time you need to bet all your capital that within a short
time an important discovery or technological innovation will
suddenly open an entire reservoir of new problems that make the
field young again. In science, "maturity" in a field is usually
doomed to be ephemeral, and every scientist knows examples of
this in his own domain. An instance was the so-called "maturity"
of solid-state physics in the 1970s, when independent electron
approximations worked well for most semiconductors and metals,
the phase transition problem seemed solved, and the fundamentals
of magnetism, ferroelectricity, and superconductivity appeared to
be known. Within a short time, however, as if to slam the
authorities who had pronounced solid-state physics a closed book,
there came discoveries of a variety of new materials whose
behavior could not be understood at all with traditional ideas.
These materials have in common the apparent dominant role played
by electron-electron interaction effects, and such systems are
categorized under the general rubric of "highly correlated
electron systems". Examples of such systems are transition metal
oxides, including copper oxide high-temperature superconductors,
heavy fermion metals, organic charge transfer compounds, and one-
and two-dimensional electron gas systems. In addition to
intriguing possible technological applications, the behaviors of
these systems appear to present profound challenges in
fundamental physics.

The following points are made by Yoshinori Tokura (Physics Today
2003 July):

1) As with any other quantum particle, an electron exhibits wave-
like and particle-like characteristics. Which aspect
predominates
in a solid depends on how an electron interacts with its
neighbors. According to the *Bloch theorem, for instance, an
electron placed in a periodic lattice behaves like an extended
plane wave. However, when the number of free electrons in a solid
becomes comparable to the number of the constituent atoms and
the
mutual electron-electron interaction becomes strong, electrons
may lose their mobility.

2) The dual nature is most apparent in correlated-electron
systems, such as the transition-metal oxides in which electron
interactions strongly determine electronic properties. In the
transition-metal ions, for example, /d/ electrons experience
competing forces: Coulombic repulsion tends to localize
individual electrons at atomic lattice sites, while hybridization
with the oxygen /p/ electron states tends to delocalize the
electrons. The subtle balance makes many of the transition-metal
oxides excellent resources for studying and taking advantage of
the metal-insulator transition that can accompany dramatic
changes in a system's electronic properties.

3) An electron in a solid has three attributes that determine its
behavior: charge, *spin, and orbital symmetry. One can imagine
an
orbital, which represents the electron's probability-density
distribution, as the shape of an electron cloud in a solid. The
charge, spin, and orbital degrees of freedom -- and their coupled
dynamics -- can produce complex phases such as liquid-like,
crystal-like, and liquid-crystal-like states of electrons, and
phenomena such as electronic phase separation and pattern
formation.

4) The correlation of electrons in a solid produces a rich
variety of states, typically through the interplay between
magnetism and electrical conductance. That interplay has itself
been a long-standing research topic among condensed matter
physicists. But since the discovery of copper-oxide high-
temperature *superconductors in 1986, a more general interest in
the *Mott transition -- the metal-insulator transition in a
correlated-electron system -- has been emerging. The high-
temperature copper oxides are composed of CuO2 sheets that are
separated from each other by ionic "blocking layers". Although it
has one conduction electron (or hole) per Cu site, each CuO2
sheet is originally insulating because of the large electron
correlation. That behavior is typical of the Mott insulator
state, in which all the conduction electrons are tied to the
atomic sites. The superconducting state emerges when holes from
the blocking layers dope the CuO2 layers in a way that alters the
number of conduction electrons and triggers the Mott transition.
Researchers believe that the strong *antiferromagnetic
correlation, which originates in the Mott-insulating CuO2 sheets
and persists into the metallic state, is most responsible for the
mechanism of high-temperature superconductivity.

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3. CHEMISTRY: DINITROGEN TO AMMONIA

The following points are made by G. Jeffery Leigh (Science 2003
301:55):

1) The reactions by which dinitrogen, N2, is converted to ammonia
by *nitrogen-fixing organisms are one of the continuing
mysteries
of chemistry and biology. Whereas the industrial *Haber-Bosch
process uses temperatures as high as 400ºC and pressures of
several hundred atmospheres, microorganisms can reduce dinitrogen
at ambient conditions.

2) Since the first dinitrogen complex was discovered in 1965 and
the basic structure of the active site of conventional molybdenum
nitrogenases was unraveled, efforts have been made to combine
chemistry and biology to explain the mechanism of biological
nitrogen fixation at the atomic level. One problem is that
dinitrogen in a synthetic complex may not approach the reactivity
of dinitrogen at an enzyme site that is receiving a constant
flux
of electrons and protons. This issue may be addressed by
attaching the dinitrogen complex to an electrode, but even then
it is often difficult to define precisely the molecular species
involved in the electrochemical cycle. It should be possible to
mimic the enzyme reaction more accurately if the range of metal
oxidation states during the reduction of dinitrogen in a
synthetic complex can be restricted. Yandulov and Schrock
(Science 2003 301:76) did just that, and their results may
finally allow us to draw realistic and empirically based
chemistry parallels with dinitrogenase reductions.

3) Dinitrogen chemistry in synthetic complexes involves many
different structural arrangements and transition metals. Very
little of the reactivity observed to date is likely to occur at a
nitrogenase active site. The best-defined sequence of
protonation
reactions in a synthetic complex involves molybdenum or tungsten.
This series cannot model the reduction of dinitrogen by
nitrogenase, because the metal is in oxidation state zero when it
binds dinitrogen. Biological reducing agents are probably not
strong enough to bring this about. In addition, a wide range of
oxidation states is exhibited by the single metal center. It is
unlikely that a single metal atom could cover these oxidation
states in nitrogenase while it reduces dinitrogen without
considerable rearrangement.

Related Material:

ON NITROGENASE

The following points are made by Barry E. Smith (Science 2002
297:1654):

1) Given sufficient water, plant growth and therefore
agricultural productivity is usually limited by the amount of
bioavailable (fixed) nitrogen. Biological nitrogen fixation still
contributes about half of the total nitrogen input to global
agriculture, the rest principally coming from nitrogenous
fertilizer produced chemically from the Haber-Bosch synthesis of
ammonia. To produce the hydrogen gas together with the high
temperatures and pressures needed for this chemical process,
about 1% of the world's total annual energy supply has to be
consumed. In marked contrast, a similar chemical process
requiring only atmospheric temperature and pressure is carried
out by nitrogen-fixing bacteria, many of which live in symbiotic
association with legume plants. The secret of their success is
the enzyme nitrogenase, which transforms atmospheric nitrogen gas
(dinitrogen) into ammonia that plants can then use for growth.
Many groups have tried for decades to determine how nitrogenase
catalyzes this essential process.

2) Nitrogenase (2) consists of two essential metalloproteins:
one, the iron (Fe) protein, is a very specific ATP-activated
electron donor to the other, the molybdenum-iron (MoFe) protein.
The MoFe protein contains two unique metallosulfur clusters: the
P cluster [8Fe-7S] and the [Mo:7Fe:9S]:homocitrate iron-
molybdenum (FeMo) cofactor cluster. About a decade ago, the first
and relatively low-resolution (2.8 angstroms) crystal structure
of the MoFe protein was reported (3). At this level of
resolution, there were still some uncertainties about the
structures of the metalloclusters. However, subsequent
improvements in resolution to 2.0 angstroms (4) and then to 1.6
angstroms (5) yielded what seemed to be the accurate structure of
the FeMo cofactor. The FeMo cofactor is bound to the MoFe
protein
through both a cysteine sulfur ligand (binding to the terminal
tetrahedral iron atom) and a histidine ligand (binding to the
molybdenum atom, which also binds to the homocitrate through its
hydroxyl group and one carboxyl group). One of the features of
the structure that has excited considerable interest is the
trigonal nature of the other six iron atoms, which appear to be
coordinately bound to only three atoms instead of the usual four
or more atoms.

3) A high-resolution structure of part of bacterial nitrogenase
reported by Einsle et al (1) yields some surprises about the
biosynthesis and catalytic activity of this crucial
metalloenzyme. Einsle et al. report the structure of the MoFe
protein of bacterial nitrogenase at an improved resolution of
1.16 angstrom (1). This is a major achievement with a protein of
this molecular size (~240,000 daltons).

References (abridged):

1. O. Einsle et al., Science 297, 1696 (2002).

2. B. E. Smith, in Advances in Inorganic Chemistry, A. G. Sykes,
R. Cammack, Eds. (Academic Press, London, 1999), vol. 47, pp.
159-218.

3. J. S. Kim, D. C. Rees, Science 257, 1677 (1992).

4. J. W. Peters et al., Biochemistry 36, 1181 (1997).

5. S. M. Mayer, D. M. Lawson, C. A. Gormal, S. M. Roe, B. E.
Smith, J. Mol. Biol. 292, 871 (1999).

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4. CELL BIOLOGY: ON THE POTASSIUM ION CHANNEL

The following points are made by Greg Miller (Science 2003
300:2020):

1) In a recent report (Nature 2003 423:33), a team of
neuroscientists announced a long-awaited breakthrough -- an
exquisitely detailed portrait of a type of protein crucial for
the generation of nerve impulses. The team, led by Roderick
MacKinnon of Rockefeller University in New York, drew on the new
image, produced by x-ray crystallography, and a number of
physiological studies, to propose a simple model for the workings
of the protein, a *voltage-gated potassium ion channel.

2) Unfortunately, however, the model contradicts the widely
accepted view of how the channel works, which is based on decades
of solid research. And although everyone agrees that deriving
the
new structure was a technical feat, many of MacKinnon's
colleagues say the final product must be flawed. "There are an
enormous number of reasons to think there's something wrong with
the structure," says Richard Horn of Thomas Jefferson University
in Philadelphia.

3) MacKinnon says he too was uneasy about the structure at first.
But he became convinced by a series of physiological studies
that
support the new model. He suggests that those who dismiss the
model out of hand are too attached to an old paradigm to accept
the new evidence. "The eye only sees what the mind already
knows," he says. The controversy has roiled this normally calm
corner of neuroscience and has many laboratories working overtime
on experiments that put the surprising new model to the test.

4) Voltage-gated potassium channels are highly conserved from the
simplest organisms to the most complex. Most have four identical
subunits, each containing six loosely linked helical segments.
In
the conventional view, all six helices are oriented more or less
perpendicularly to the lipid cell membrane. The fifth and sixth
segments (S5 and S6) of each of the four subunits face the center
and together form the pore through which ions pass. The fourth
segment (S4) is widely thought to be the voltage sensor. This
segment contains a number of positively charged amino acids. The
conventional view of how S4 contributes to voltage gating is that
since the interior of a resting neuron is electrically negative
relative to the fluid surrounding it, in that state, the
positively charged S4 segment, mostly surrounded by watery
crevices, is drawn toward the inside of the cell and the channel
is closed. During a nerve impulse, the inside of the neuron
rapidly becomes more positive, exerting less pull and even
repulsing S4's positive charges, allowing the segment to shift
toward the outside of the cell. This movement somehow pulls on
the other segments of the channel to open the pore and allow
positively charged potassium ions to rush out of the neuron,
restoring the interior to its negative resting condition.

5) The MacKinnon group proposes that instead of traveling with
modest piston-like or screw-like movements through the core of
the channel as most researchers had envisioned, the S3 and S4
segments form a "paddle" that moves like a lever arm through the
lipid cell membrane surrounding the channel, sweeping from the
intracellular side to the extracellular side of the membrane.
This movement somehow -- as with the conventional model, the
details are unknown -- pulls on the other segments of the channel
to open the pore.

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5. MOLECULAR BIOLOGY: ON RNA REPAIR

The following points are made by A. Bellacosa and E.G. Moss
(Current Biology 2003 13:R482):

1) Damage to DNA is a significant issue for all cells,
particularly in cancer where DNA repair commonly fails. It is not
widely appreciated that many agents that cause damage to DNA,
such as radiation and certain cancer chemotherapy drugs, also
damage RNA. Given that there is at least as much RNA in a cell as
DNA, wherever DNA is damaged by such agents, RNA is surely
damaged as well. When the damage to RNA is substantial,
*apoptosis is induced, which is the desired affect of anti-cancer
chemotherapy. Nevertheless, RNA has not been the major focus in
investigating how cells cope with such insults. There have been
scattered insights over the years into the significance of RNA to
genotoxic stress. Recent research suggests that the cell has at
least one specific mechanism to repair RNA damage, indicating a
greater investment in the protection of RNA than previously
suspected.

2) Alkylating agents are endogenous and environmental compounds
that cause mutations, tumors, and neurotoxicity. Chemically, they
add alkyl groups, like methyl or ethyl groups, to organic
macromolecules, in particular to the ring nitrogens and oxygens
of bases of nucleic acids. Cells have developed a host of repair
systems to deal with alkylation damage of DNA. These include the
removal of the damaged residues by DNA glycosylases, followed by
replacement of the nucleotide by DNA polymerases using the
opposite strand as template. Another mechanism is the direct
reversal of the methylation damage, which does not require a
template to make the repair. For example, the Ada and Ogt enzymes
of Escherichia coli restore the normal base of an alkylated DNA
by directly removing the offending chemical group in a suicidal
reaction. The E. coli enzyme AlkB has long been known to act in
alkylation damage repair, but until recently its mechanism of
action was not clear, as biochemical assays failed to detect in
the AlkB protein any of the enzymatic activities known to occur
in DNA repair enzymes.

3) In a remarkable display of the power of bioinformatics applied
to genome sequences, Aravind and Koonin (2001) predicted, based
on its relationship to another family of enzymes, that AlkB would
cause hydroxylation of the methyl group on damaged DNA bases,
and
thus directly reverse alkylation damage. Two laboratories
independently confirmed this prediction and showed that, indeed,
AlkB enzymatically demethylates the DNA bases adenine and
cytosine, unlike the suicidal Ada and Ogt proteins, by oxidative
demethylation: the methyl group is converted to an hydroxymethyl
group which then leaves the base as formaldehyde.

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6. GENOMIC MEDICINE: CARDIOVASCULAR DISEASE

The following points are made by Elizabeth G. Nabel (New Engl. J.
Med. 2003 349):

1) Cardiovascular disease, including stroke, is the leading cause
of illness and death in the US. There are an estimated 62
million
people with cardiovascular disease and 50 million people with
hypertension in the US. In 2000, approximately 946,000 deaths
were attributable to cardiovascular disease, accounting for 39
percent of all deaths in the US. Epidemiologic studies and
randomized clinical trials have provided compelling evidence that
coronary heart disease is largely preventable. However, there is
also reason to believe that there is a heritable component to
the
disease. As future genomic discoveries are translated to the care
of patients with cardiovascular disease, it is likely that what
we can do will change. 

2) Our understanding of the mechanism by which single genes can
cause disease, even though such mechanisms are uncommon, has led
to an understanding of the pathophysiological basis of more
common cardiovascular diseases, which clearly are genetically
complex. This point can be illustrated by a description of the
genetic basis of specific diseases. 

3) Low-density *lipoprotein (LDL) is the major cholesterol-
carrying lipoprotein in plasma and is the causal agent in many
forms of coronary heart disease. Four monogenic diseases elevate
plasma levels of LDL by impairing the activity of hepatic LDL
receptors, which normally clear LDL from the plasma. Familial
hypercholesterolemia was the first monogenic disorder shown to
cause elevated plasma cholesterol levels. The primary defect in
familial hypercholesterolemia is a deficit of LDL receptors, and
more than 600 mutations in the LDLR gene have been identified in
patients with this disorder. One in 500 people is *heterozygous
for at least one such mutation, whereas only 1 in a million is
*homozygous at a single locus. Those who are heterozygous produce
half the normal number of LDL receptors, leading to an increase
in plasma LDL levels by a factor of 2 or 3, whereas LDL levels in
those who are homozygous are 6 to 10 times normal levels.
Homozygous persons have severe coronary *atherosclerosis and
usually die in childhood from *myocardial infarction. 

4) Deficiency of lipoprotein transport abolishes transporter
activity, resulting in elevated cholesterol absorption and LDL
synthesis. For example, mutations in the APOB-100 gene, which
encodes apolipoprotein B-100, reduce the binding of
apolipoprotein B-100 to LDL receptors and slow the clearance of
plasma LDL, causing a disorder known as familial ligand-defective
apolipoprotein B-100. One in 1000 people is heterozygous for one
of these mutations; lipid profiles and clinical disease in such
persons are similar to those of persons heterozygous for a
mutation causing familial hypercholesterolemia. 

5) Sitosterolemia, a rare *autosomal disorder, results from loss-
of-function mutations in genes encoding two ATP-binding cassette
(ABC) transporters, ABC G5 and ABC G8, which act in concert to
export cholesterol into the intestinal lumen, thereby diminishing
cholesterol absorption. Autosomal recessive hypercholesterolemia
is extremely rare (prevalence, less than 1 case per 10 million
persons). The molecular cause is the presence of defects in a
putative hepatic adaptor protein, which then fails to clear
plasma LDL with LDL receptors. Mutations in the gene encoding
that protein (ARH) elevate plasma LDL to levels similar to those
seen in homozygous familial hypercholesterolemia. 

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GLOSSARY:

antiferromagnetic: A "ferromagnetic substance" is a material
(e.g., iron) in which there may be a permanent magnetic moment,
and in which the spins of the atoms are aligned parallel to each
other. In general, "paramagnetic substances" and paramagnetic
chemical groups have a capability to be magnetized which is
slightly greater than that of a vacuum and much less than that of
iron. The paramagnetism is due to the presence of permanent
magnetic dipoles caused by unpaired electron spins. In general,
"antiferromagnetism" is the property of certain paramagnetic
materials that exhibit a temperature dependence similar to that
encountered in ferromagnetism: the magnetic susceptibility
increases with increasing temperature up to a certain point (the
Neel temperature) and then falls with increasing temperature, the
material thus becoming paramagnetic.

apoptosis: In general, "apoptosis" is programmed cell death
produced by control mechanisms designed to destroy defective
cells.

atherosclerosis: "Arteriosclerosis" is a generic term for several
diseases in which the arterial wall becomes thickened and loses
elasticity, and "atherosclerosis" is a form of arteriosclerosis
characterized by patchy thickening (atheroma) in the subintimal
layer (i.e., immediately below the innermost layer [intima]) of
medium and large arteries, the thickening capable of reducing or
obstructing blood flow.

autosomal disorder: In general, a genetic disorder involving one
or more chromosomes that are not sex chromosomes.

Bloch theorem: Named after Felix Block (1905-1983). A theorem
concerning the quantum mechanics of crystals, the theorem
interpreted to mean that the wave function for an electron in a
periodic potential is a plane wave modulated by a periodic
function.

Haber-Bosch process: In general, the industrial process for the
direct synthesis of ammonia from N2 and H2 over a catalyst.

heterozygous: Having two different alleles at a specific
autosomal (or X chromosome in a female) gene locus.

homozygous: Having two identical alleles at a specific autosomal
(or X chromosome in a female) gene locus.

lipoprotein: In general, a micellar complex of protein and
lipids.

Mott transition: In general, a discontinuous electronic phase
transition from a metal to an insulator (and vice versa) at 0
degrees kelvin. Named after Nevill Francis Mott (1905-1996),
Nobel Prize in Physics 1977.

myocardial infarction: In general, an area of necrosis of cardiac
tissue resulting from a sudden insufficiency of arterial or
venous blood supply. The most common cause is thrombosis of an
atherosclerotic coronary artery.

nitrogen-fixing organisms: In general, the incorporation of
atmospheric nitrogen (N2) to form nitrogenous organic compounds.
Most of such fixation is produced by nitrogen-fixing prokaryotes,
both symbiotic and free-living bacteria.

planetesimals: "Planetesimals" are bodies with dimensions of
10^(-3) to 10^(3) meters that are believed to form planets by a
process of accretion. The term "accretion" here refers to an
aggregation, an increase in the mass of a body by the addition of
smaller bodies that collide and adhere to it, provided the
relative velocities are low enough for coalescence. As the mass
of the agglomerate increases, so does the rate of accretion, and
this accretion process is believed to generally occur in the form
of a disk. A stellar accretion disk is a swarm of dust grains
that evolve into planetesimals and then planets.

spin: In quantum mechanics, electrons, protons, and neutrons have
an intrinsic angular momentum known as "spin", and a magnetic
moment parallel or antiparallel to that angular momentum. When
electrons are combined together to form an atom or ion, there is
a resultant angular momentum which is a combination of the
intrinsic spin of the electrons and the angular momentum due to
their motion about the nucleus, and this is the "spin" of the
atom or ion. Atoms or ions with non-zero spin are magnetic atoms
or ions. The idea of electron spin was first proposed by Goudsmit
and Uhlenbeck in 1925 to explain the splitting of atomic
spectroscopic emission lines in the presence of a magnetic field.
Elementary particle spin involves a virtual rotation about the
axis of the particle, which means only two spin states are
possible, one clockwise and one counterclockwise.

superconductor: Superconductivity is a property of many metals,
alloys, and chemical compounds at temperatures near absolute
zero, at which temperatures their electrical resistivity
vanishes. High-temperature superconductors were unknown until
1986, but at present there are some known high-temperature
superconductors with critical temperatures greater than 100
kelvins. The accepted theory of ordinary superconductivity is the
Bardeen-Cooper-Schrieffer theory (BCS theory) (1957). At the
present time, a successful theory of high-temperature
superconductivity has not been developed, in spite of a great
deal of effort. Johannes Georg Bednorz (1950- ) and K. Alexander
Mueller (1927- ) shared the Nobel Prize in Physics in 1987 for
their discovery of high-temperature superconductivity in a
ceramic oxide (lanthanum-barium-copper) alloy at 30 degrees
kelvin, at that time the highest superconductivity temperature
ever observed.

voltage-gated: Most ion channels are selective, allowing only
certain ions to pass, and an individual cell has ion channels
with various ion selectivities. The selectivity of an ion channel
can be "gated", the channel effectively opened or closed, and
ion
channels are said to *voltage-gated or *ligand-gated, depending
on how the change in selectivity is provoked.

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