KINETICS AND DYNAMICS OF ANNEALING DURING SUB-GEL PHASE-FORMATION IN PHOSPHOLIPID-BILAYERS - A SATURATION-TRANSFER ELECTRON-SPIN-RESONANCE STUDY

The saturation transfer electron spin resonance (STESR) spectra of spi n-labeled phosphatidylcholine have been used to follow the kinetics of conversion from the gel phase to the sub-gel phase in aqueous bilayer s of dipalmitoyl phosphatidylcholine. This is a simple, well-defined m odel system for lipid domain formation in membranes. The integrated in tensity of the STESR spectrum from the chain-labeled lipid first incre ases and then decreases with time of incubation in the gel phase at 0- degrees-C. The first, more rapid phase of the kinetics is attributed t o the conversion of germ nuclei to growth nuclei of the sub-gel phase. The increase in STESR intensity corresponds to the reduction in chain mobility of spin labels located in the gel phase at the boundaries of the growth nuclei and correlates with the increase in the diagnostic STESR line height ratios over this time range. The second, slower phas e of the kinetics is attributed to growth of the domains of the sub-ge l phase. The decrease in STESR intensity over this time regime corresp onds to exclusion of the spin-labeled lipids from the tightly packed s ub-gel phase and correlates quantitatively with calibrations of the sp in label concentration dependence of the STESR intensity in the gel ph ase. The kinetics of formation of the sub-gel phase are consistent wit h the classical model for domain formation and growth. At 0-degrees-C, the half-time for conversion of germ nuclei to growth nuclei is appro ximately 7.7 h and domain growth of the sub-gel phase is characterized by a rate constant of 0.025 h-1. The temperature dependence of the ST ESR spectra from samples annealed at 0-degrees-C suggests that the sub transition takes place via dissolution of sub-gel phase domains, possi bly accompanied by domain fission.