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

Abstract

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. It is interpreted as a local increase in density in the bilayer which results from a shortening of the individual lipid molecule due to formation of rotational isomers (e.g., kinks) without an appropriate expansion of the molecular environment. The final membrane expansion is achieved in consecutive steps with a decrease in turbidity and time constants between 100 μ and several seconds which are maximal in the midpoint of the phospholipid phase transition. These steps represent cooperative processes, namely the molecular interaction leading to the membrane expansion. The rate of kink formation implies that kinks migrate through the membrane by energetic transitions forwarded from lipid to lipid, rather than by hopping of individual lipids thereby carrying a kink.