Effect of long-chain acyl-CoAs and acylcarnitines on gel-fluid and lamellar-hexagonal phospholipid phase transitions

The effect of a series of long-chain acyl-CoAs and acylcarnitines has been tested by differential scanning calorimetry on the gel-fluid transition of saturated phosphatidylcholines and of dietai-doylphosphatidylethanolamine (DEPE), and on the lamellar-Hexagonal transition of DEPE. Both series of acylderivatives have similar effects (the acylcarnitines being more potent): a decrease in the gel-fluid transition enthalpy and an increase in the gel-fluid transition width. Mixtures of dipalmitoylphosphatidylcholine with palmitoyl-CoA or palmitoylcarnitine (i.e. when all three hydrocarbon chains are 16C in length) display a peculiar behaviour, in that the main endotherm remains unchanged until a high proportion of palmitoylderivative is present, then it collapses suddenly. The disappearance of the gel-fluid main endotherm in the presence of palmitoylcarnitine is due to the fragmentation of the bilayer below the cooperative unit size of the phospholipid, while the same effect is caused by palmitoyl-CoA through the interaction of the coenzyme A polar moiety with the lipid-water interface, the overall bilayer structure being maintained. The effect of both series of compounds on the lamellar to inverted hexagonal phase transition of DEPE is also similar: they both stabilize the lamellar phase, increase the transition temperature and smear out the transition endotherm. Their behaviour may be rationalized considering that they are compounds with a bulky polar head, relative to their single hydrophobic chain, that would favour a positive curvature in the monolayer, while the inverted hexagonal phase requires a negative curvature.