PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 650 August 20, 2003   by Phillip F. Schewe, Ben Stein, and
James Riordon

GIANT HELIUM MOLECULES, containing only two atoms but assuming a
size as large as a small virus, have been created by researchers at
the �cole Normale Sup�rieure 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 �cole Normale Sup�rieure,
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.  (L�onard
et al., Physical Review Letters,15 August 2003; contact Allard Mosk,
a.p.mosk{at}utwente.nl or J�r�mie L�onard, Leonard{at}lkb.ens.fr).

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,
gclwong{at}uiuc.edu) 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
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