The existence of acoustic pulse propagation in lipid
monolayers
at the air–water interface is well known. These pulses are
controlled by the thermodynamic state of the lipid membrane. Nevertheless,
the role of acoustic pulses for intra- and inter-cellular communication
is still a matter of debate. Herein, we used the dye di-4-ANEPPDHQ,
which is known to be sensitive to the physical state and transmembrane
potential of membranes, in order to gain insights into compression
waves in lipid-based membrane interfaces. The dye was incorporated
into lipid monolayers made of phosphatidylserine or phosphatidylcholine
at the air–water-interface. A significant blue shift of the
emission spectrum was detected when the state of the monolayer was
changed from the liquid-expanded (LE) to the liquid-condensed (LC)
phase. This “transition sensitivity” of di-4-ANEPPDHQ
was generalized in experiments with the bulk solvent dimethyl sulfoxide
(DMSO). Upon crystallization of solvent, the emission spectrum also
underwent a blue shift. During compression pulses in lipid monolayers,
a significant fluorescence response was only observed when the main
transition is crossed. The optical signature of these wavesin
terms of sign and magnitudewas identical to the response of
di-4-ANEPPDHQ during action potentials in neurons and excitable plant
cells. These findings corroborated the suggestion that action potentials
are nonlinear state changes that propagate in the cell membrane.
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