  The response kinetics of liquid crystalline phosphatidylcholine bilayer
stacks to rapid, IR-laser induced temperature jumps has been studied by
millisecond time-resolved x-ray diffraction. The system reacts on the fast
temperature change by a discrete bilayer compression normal to its surface and
a lateral bilayer expansion. Since water cannot diffuse from the excess phase
into the interbilayer water region within the 2 ms duration of the laser pulse,
the water layer has to follow the bilayer expansion, by an anomalous thinning.
Structural analysis of a 20 ms diffraction pattern from the intermediate phase
indicates that the bilayer thickness remains within the limits of isothermal
equilibrium values. Both, the intermediate structure and its relaxation into
the original equilibrium L_(alpha)-phase, depend on the visco-elastic
properties of the bilayer/water system. We present an analysis of the
relaxation process by an overdamped one-dimensional oscillation model revealing
the concepts of Hooke's law for phospholipid bilayers on a supramolecular
basis. The results yield a constant bilayer repulsion and viscosity within
Hooke's regime suggesting that the hydrocarbon chains act as a buffer for the
supplied thermal energy. The bilayer compression is a function of the initial
temperature and the temperature amplitude, but is independent of the chain
length.
