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Abstract:
The kinetics of the main phase transition of dimyristoylphosphatidyl choline (DMPC) unilamellar vesicles were investigated in the time range from microseconds to seconds. Iodine laser-temperature jump (ILTJ) experiments showed three discrete relaxation phenomena. Time resolved cryo-electron microscopy (CEM) was applied to produce images of intermediate states typical for the relaxation times of lipid vesicles in the micro- to millisecond time window. A careful measurement of the rate of temperature decrease observed during the production of vitrified lamellae of aqueous samples on a copper grid was performed. The best conditions resulted in average rates of cooling of 3 × 104 K/s. By comparing the images from CEM of DMPC vesicle samples vitrified above, at, and below the phase transition temperature a structural model was designed, which explains the temperature jump relaxation times in the micro- to millisecond time range by the formation and disappearance of coexisting clusters of crystalline, intermediate, and fluid lipid areas inside the DMPC bilayers.
