Studies of lipid phase transitions in some model membranes using differential scanning calorimetry

Vaughan, David J. (1976) Studies of lipid phase transitions in some model membranes using differential scanning calorimetry. Masters thesis, Memorial University of Newfoundland.

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Abstract

Phosphatidylcholines (PC's) and phosphatidylethanolamines (PE's) have been shown, in general, to comprise the major lipids of most biological membranes. Therefore, a systematic study using differential scanning calorimetry of DPPC, DPPE and their N-methylated intermediates as well as their diether analogues was undertaken in order to characterize the phase behaviour of such model systems with a view to understanding the more complex behaviour observed in real membranes. It was observed that the lipid phase transition temperature (Tc) decreased as methylation of the PE head group increased. The transition enthalpies for these systems of single lipids showed no systematic variation with methylation. -- It has been noted that saturated lecithins (as well as DPPG) display a pretransition occuring several degrees before the main hydrocarbon chain transition on DSC thermograms. The introduction of "foreign" molecules (even ones of very similar structure) in lecithin bilayers disrupts the long range order in the gel phase and either modifies or abolishes the pretransitional endotherm, depending upon the concentration of the perturbing molecule. Certain water soluble additives have no effect on the pretransition up to 20 mole %. However, cooling thermograms of such systems display a minor exotherm occuring ∿12° K below the main exotherm which is absent in pure lecithin systems. -- The tricyclic antidepressant, desipramine, has been shown to affect lipid phase behaviour. When included at equimolar concentrations with single lipids it lowers the gel to liquid crystalline phase transition temperature and generally increases the enthalpy of the phase change. The extent of lowering of the phase transition temperature was found to be dependent on the head group structure of the phospholipids. PE's were affected greater than PC's, in both a diester and diether series of lipids. Binary mixtures of lipids were also analyzed calorimetricaily and when desipramine was included in these mixes so as to be equivalent on a molar basis with one of the lipids it was found to exhibit a preference for the lower melting component.

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