Lipid concentration affects the kinetic stability of dielaidoylphosphatidylethanolamine bilayers

The bilayer to hexagonal phase transition temperature (Th) of dielaidoylphosphatidylethanolamine is 65.5 degrees C as measured by DSC heating scans at lipid concentrations below 100 mg/ml and at scan rates ranging from 1.7 to 45 degrees C/h. However, at lipid concentrations above 100 mg/ml and at scan rates of 1-3 degrees C/h the measured Th decreases below 65.5 degrees C. At a lipid concentration of 500 mg/ml and a heating scan of 1.2 degrees C/h the transition to the hexagonal phase occurs at 62.7 degrees C. However, this same sample scanned at a rate of 34 degrees C/h has a transition temperature of 64.6 degrees C. Thus a combination of high lipid concentration and slow scan rate is required to significantly lower the hexagonal phase transition temperature below 65 degrees C. These results demonstrate that the rate of conversion of the bilayer to the hexagonal phase is dependent on the concentration of the lipid suspension even under conditions of full hydration. Furthermore, a 100 mg/ml suspension of this lipid which has a Th of 64.3 degrees C at a scan rate of 3.2 degrees C/h has a lower hexagonal phase transition temperature of 62.8 degrees C after pelleting the lipid with low-speed centrifugation but retaining the same amount of solvent in the supernatant above the pellet. Pelleting of the lipid also has a marked effect on the isothermal rate of conversion of the bilayer to hexagonal phase as observed by 31P NMR. The conversion is highly temperature-dependent and is orders of magnitude more rapid for the pelleted sample than for the suspension.(ABSTRACT TRUNCATED AT 250 WORDS)