Structure and Dynamics of Phospholipid Membranes from Nanoseconds to Seconds

Abstract

The “Iodine-Laser Temperature Jump” (ILTJ) technique offers the unique possibility to measure dynamic processes in membrane systems as well as protein structures from nanoseconds to seconds, resulting in relaxation amplitudes which are correlated with the enthalpy changes of these often complicated systems. The new electron microscopic (EM) technique of fast freezing (< 10−4 s) of thin lamellar preparations and the use of a cryo EM allows the determination of structural details without the need of contrast chemicals. ILTJ measurements from 10−9 s –100 s are presented which cover the whole crystalline-fluid transition of unilamellar vesicles (UVs) from dipalmitoylphosphatidylcholine (DPPC) or dimyristoylphosphatidylcholine (DMPC). Especially tailored probe lipids like acridineorangelecithin (AOL) and diphenylhexatrienpalmitoylphosphatidylcholine (DPH PC) were used to confirm the turbidity measurements by time resolved absorption and fluorescence anisotropy changes. Five well separated relaxation signals of increasing cooperativity could be observed. By reconstructing the equilibrium data of the phase transition of membranes from the amplitudes of the kinetic relaxations it could be proved that the dynamic processes represent the whole crystalline-fluid transition. A model is presented which aims to explain the kinetic results on a molecular basis. UVs with incorporated cholesterol, gramicidin A and bacteriorhodopsin showed a strong shift of the slower relaxation amplitudes (100 μs – 20 ms) towards the 10 μs time range. We explain these results by the preference of intermediate states of order of the annular lipids in the surrounding of functional units like bacteriorhodopsin. We therefore call the relaxations τ3 around 10 μs functional important movements FIMs after Frauenfelder.