On the phase transition kinetics of phospholipid bilayers. Relaxation experiments with detection of fluorescent anisotropy.

Relaxation experiments were performed on vesicles of dimyristoylphosphatidylcholine in the lipid phase transition region by means of a Joule heating temperature jump technique. The time course of fluorescence anisotropy of the dye 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in the bilayer, was observed. Since the dye always exhibits stationary anisotropy in the time range of observation, its anisotropy represents the order of the bilayer during the entire course of the experiment.

The kinetics of the formation of rotational isomers in the hydrophobic tail region of phospholipid bilayers.

Very fast structural changes in dipalmitoyl phosphatidylcholine molecules forming a vesicular bilayer were investigated by means of a laser temperature-jump technique. After temperature increases of about 1 K within 1 ns, the solution turbidity increases with a time constant of about 4 ns. This time constant exhibited no appreciable temperature dependence and represents a noncooperative process.

Evaluation of cooperativity for phase transitions in two- and three-dimensional systems.

Use of the so-called "cooperative unit" (readily obtainable from the midpoint slope of phase transition curves) is discussed for the determination of cluster sizes and cooperative interaction energies. This quantity has been commonly employed in a rather empirical way since its correct interpretation is known only for some special cases (linear systems, all-or-none transitions). It is shown in the framework of a lattice model (Ising model) that the cooperative unit may be interpreted in terms of correlation functions and that it defines an average cluster corresponding to the patch size as obtained from scattering experiments.

Kinetic investigations on the phase transition of phospholipid bilayers.

Pressure-jump experiments were performed on vesicles and liposomes of dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine following the time course of solution turbidity. For both lipids two relaxation effects were evaluated the time constants of which exhibit clear maxima at the mid-point of the phase transition. The time constants lie for vesicles in the 100 microseconds and 1 ms ranges and for liposomes in the 1 ms and 10 ms ranges.