TEMPERATURE-DEPENDENT INTERMEDIATE STRUCTURES DURING THE MAIN PHASE-TRANSITION OF DIMYRISTOYL PHOSPHATIDYLCHOLINE VESICLES A COMBINED IODINE LASER-TEMPERATURE JUMP AND TIME-RESOLVED CRYOELECTRON MICROSCOPY STUDY

The kinetics of the main phase transition of dimyristoylphosphatidyl c holine (DMPC) unilamellar vesicles were investigated in the time range from microseconds to seconds. Iodine laser-temperature jump (ILTJ) ex periments showed three discrete relaxation phenomena. Time resolved cr yo-electron microscopy (GEM) was applied to produce images of intermed iate states typical for the relaxation times of lipid vesicles in the micro- to millisecond time window, A careful measurement of the rate o f temperature decrease observed during the production of vitrified lam ellae of aqueous samples on a copper grid was performed. The best cond itions resulted in average rates of cooling of 3 x 10(4) K/s. By compa ring the images from CEM of DMPC vesicle samples vitrified above, at, and below the phase transition temperature a structural model was desi gned, which explains the temperature jump relaxation times in the micr o- to millisecond time range by the formation and disappearance of coe xisting clusters of crystalline, intermediate, and fluid lipid areas i nside the DMPC bilayers.