Number 650, August 20, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
Giant Helium Molecules
Giant helium molecules, containing only two atoms but assuming a size
as large as a small virus, have been created by researchers at the École
Normale Supérieure in Paris. At sizes ranging from 10 to 100
nanometers, these helium molecules are the largest diatomic (two-atom)
molecules ever created by a factor of 5 (and comparable to the size
of viruses, which vary in length from 5-300 nm). What's more, helium
is an inert gas that does not normally form molecules. To observe the
new giant molecular states, one needs to start from an ultracold gas
of atoms. At the École Normale Supérieure, researchers
trap a cloud of helium atoms with magnetic fields. Each helium atom
is in a long-lived "metastable" state and carries nearly 20
eV of internal energy, which is more than 10 billion times its average
energy of motion. In the confines of a magnetic trap, the hottest He
atoms evaporate and the colder atoms remain, lowering the temperature
of the cloud to 10 microkelvins (millionths of a degree above absolute
zero). Then, a laser pairs up He atoms through a process called "photoassociation,"
in which light of a precise color changes the state of the atoms so
that they attract each other more strongly. This attraction comes about
through light-induced "dipoles" (momentary separations of
positive and negative charge in each He) to cause the atoms to bind
to each other. To detect the molecules, the researchers record a temperature
rise in the cloud that results from the successful absorption of the
laser light. In a typical experiment, one percent of the atoms absorbs
the light, corresponding to the formation of about 100,000 molecules.
In each of the molecules, the atoms are sufficiently far apart that
they resist destructive "auto-ionization" effects, in which
an electron jumps from one atom to the other and breaks apart the molecule.
In fact, the atoms are so distant from each other that the researchers
had to account for the finite speed of light: each atom of the pair
sees the other the way it was a femtosecond earlier. The researchers
had to include this "retardation" effect in their calculations
to get agreement with the measured data. The molecules last for an average
of 50 nanoseconds--a remarkably long time due to the huge amounts of
internal energy in each He atom. In precisely measuring the forces that
bind the molecule, the researchers can obtain detailed information about
the helium atom. In addition, the metastable helium molecule can sensitively
test the accuracy of calculations in quantum chemistry, the application
of quantum mechanics to chemical systems such as molecules. (Léonard
et al., Physical Review Letters,15 August 2003; contact
Allard Mosk, or Jérémie
Léonard).
Like-Charged Biomolecules Can Attract Each
Other
Like-charged biomolecules can attract each other, in a biophysics phenomenon
that has fascinating analogies to superconductivity. Newly obtained
insights into biomolecular "like-charge attraction" may eventually
help lead to improved treatments for cystic fibrosis, more efficient
gene therapy and better water purification. The like-charge phenomenon
occurs in "polyelectrolytes," molecules such as DNA and many
proteins that possess an electric charge in a water solution. Under
the right conditions, polyelectrolytes of the same type, such as groups
of DNA molecules, can attract each other even though each molecule has
the same sign of electric charge. Since the late 1960s, researchers
have known that like-charge attraction occurs through the actions of
"counterions," small ions also present in the water solution
but having the opposite sign of charge as the biomolecule of interest.
But they have not been able to pin down the exact details of the phenomenon.
To uncover the mechanism behind like-charge attraction, a group of experimenters
(led by Gerard Wong, Univ of Illinois
at Urbana-Champaign, 217-265-5254) found that counterions organize themselves
into columns of charge between the protein rods. Along these 'columns',
the ions are not uniformly distributed, but rather are organized into
frozen "charge density waves."
Remarkably, these tiny ions cause the comparatively huge actin molecule
to twist, by 4 degrees for every building block (monomer) of the protein.
This process has parallels to superconductivity, in which lattice distortions
(phonons) mediate interactions between pairs of like-charged particles
(electrons). In the case of actin, charge particles (ions) mediate attractions
between like-charged distorted lattices (twisted actin helix). (Angelini
et al., Proceedings of the National Academy of Sciences,
July 22, 2003). In the next experiment, they investigated what kinds
of counterions are needed to broker biomolecular attraction. Researchers
have long known that doubly charged (divalent) ions can bring together
actin proteins and viruses, and triply charged (trivalent) ions can
make DNA molecules stick to one another, but monovalent ions cannot
generate these effects. Studying different-sized versions of the molecule
diamine (a dumbbell-shaped molecule with charged NH3 groups as the "ends"
and one or more carbon atoms along the handle) to simulate the transition
between divalent and monovalent ion behavior, they found that the most
effective diamine counterions for causing rodlike M13 viruses to attract
were the smallest ones. These small diamine molecules had a size roughly
equal to the "Gouy- Chapman" length, the distance over which
its electric charge exerts a significant influence. Nestled on the M13
virus surface, one end of the short diamine molecule neutralizes the
virus's negative charge, while the other end supplies a positive charge
that can then draw another M13 virus towards it (Butler
et al., Physical Review Letters, 11 July 2003; also
see Phys. Rev. Focus,
21 July 2003).
In a third experiment, researchers noticed that the like-charge attractions
could cause actin molecules to organize themselves into a novel phase
of liquid crystal (a structure with both liquid-like and solid-like
properties). Adding small amounts of magnesium ions to a solution of
actin rods caused the rods to arrange themselves into a stack of 2-dimensional
rafts (see figure).
This discovery may revise notions of how cells control the actin cytoskeleton..
Previously, researchers assumed that only proteins could do all the
work in assembling this structure, which helps the cell to move, shape
itself and divide. However, this newly discovered phase opens the possibility
that physical interactions--electrostatics, electric charge, and entropy--could
work synergistically with proteins to regulate the cytoskeleton in a
wide range of cellular functions (Wong
et al., Phys. Rev. Lett., 4 July 2003).
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