Subgel Studies of Dimyristoylphosphatidylcholine Bilayers.
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When bilayers of some saturated PCs are stored for three or more days at ~273 K, a lamellar subgel phase has been detected at temperatures below the pre-transition by differential scanning calorimetry (DSC). However, the subgel phase and corresponding subtransition (subgel to planar gel) for dimyristoylphosphatidylcholine (DMPC), a 14-carbon acyl chain phospholipid, has not been clearly characterized. Using novel supercooling techniques, new and accurate data characterizing the subgel formation and subtransition of DMPC was obtained through differential scanning calorimetry (DSC) and fluorescence spectroscopy with 1,6-diphenyl-1,3,5-hexatriene (DPH). It was discovered that the formation of the DMPC subgel phase requires incubation at temperatures of 268 K or lower for two hours or more. Kinetics of the subgel formation tells us that it is a very complex process and demonstrates that the planar gel phase is merely metastable below the subtransition, and not the thermodynamically stable phase. The subgel growth of DMPC is proven to be the dehydration of the head group region and the subtransition is a process in which poorly hydrated DMPC becomes hydrated.