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. Two relaxation processes were detected within the 1-100 ms range with maximal time constants at the midpoint of transition. At least one process was faster than the temperature jump dead time. The relaxation times, especially the maximal relaxation times, depend strongly on the bilayer curvature: larger vesicles imply larger time constants. This observation can explain differences between kinetic results of different laboratories. A comparison between the kinetic findings by means of three different dyes located at different sites along the lipid molecules suggests that the slower steps of the lipid phase transition mechanism involve the entire lipid molecule rather than individual parts of the molecule. The results of this present contribution reconfirm the recently published phase transition mechanism which postulates a series of steps of which the first, the kink formation, is fast and noncooperative and the following ones, representing the expansion of the aggregates, are slower and cooperative.