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Advanced laser spectroscopy exposes the
unique organization of water molecules under
model membrane surfaces
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The behavior of water molecules as they
contact biological substances has long puzzled
scientists. The first few layers of interfacial
water can display complex arrangements that
distinctly influence biochemical reactivity and
function. Mapping these interfaces, however,
is extremely difficult because chemical
signatures of surface-bound water are often
swamped by bulk liquid signals. Now, researchers
led by Tahei Tahara from the Riken Institute
have developed a laser spectroscopy technique
that conclusively determines the orientation of
water molecules beneath charged lipid layers—
the primary components of cell membranes.
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Phospholipids are fatty acid molecules that
contain two parts: hydrophobic ‘tails’ made of
long hydrocarbon chains and hydrophilic ‘heads’
comprised of charged phosphate groups
and other organic units. At the air–water interface,
phospholipids spontaneously form into monolayer
films, with their tails extending into the air and
their heads immersed in water. The structure
and orientation of water molecules below such
monolayers has been a matter of controversy.
Some investigators suggest that the partially
positive-charged hydrogen atoms of water
orientate ‘up’ or ‘down’ to align with the lipid
head charge, while others suggest the opposite
outcome.
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Tahara and colleagues resolved this debate by
using an optical technique called heterodyne-
detected vibrational sum frequency generation
(HD-VSFG) spectroscopy, which has extremely
high surface sensitivity. HD-VSFG combines two
laser beams with different frequencies at an
interface to generate a sum-frequency signal;
when vibrations of surface molecules resonate
with the applied laser, the sum-frequency signal
rapidly shoots up—instantly identifying which
chemicals are present. Because this signal
originates from non-linear surface polarization
effects, it contains only contributions from
interfacial species. “HD-VSFG automatically
probes the depths of water layers that are
different from the bulk,” says Tahara.
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Determining the orientation of surface water
required heterodyne detection, a method that
determines the phase of weak signals via interference
with a reference beam. According to Tahara,
performing such measurements required
precisely sensing changes to the signal light’s
optical phase—meaning the researchers had to
control the laser beams with nanometer-scale
accuracy.
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The teams’ experiments on three different lipid
monolayers revealed that the interfacial structures
are governed by the net charge of the heads:
water hydrogen atoms pointed up with anionic
lipid heads, and faced downwards in the presence
of cationic. “This is totally different from the
situation for reactions in aqueous solutions,”
says Tahara, who believes that the results will
shed light on important reactions that take place
at cell membranes, such as enzyme activation.
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