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 tran sition of saturated phosphatidylcholines and of dielaidoylphosphatidyl ethanolamine (DEPE), and on the lamellar-hexagonal transition of DEPE. Both series of acylderivatives have similar effects (the acylcarnitin es being more potent): a decrease in the gel-fluid transition enthalpy and an increase in the gel-fluid transition width. Mixtures of dipalm itoylphosphatidylcholine with palmitoyl-CoA or palmitoylcarnitine (i,e . when all three hydrocarbon chains are 16C in length) display a pecul iar behaviour, in that the main endotherm remains unchanged until a hi gh proportion of palmitoylderivative is present, then it collapses sud denly, The disappearance of the gel-fluid main endotherm in the presen ce of palmitoylcarnitine is due to the fragmentation of the bilayer be low the cooperative unit size of the phospholipid, while the same effe ct is caused by palmitoyl CoA through the interaction of the coenzyme A polar moiety with the lipid-water interface, the overall bilayer str ucture being maintained. The effect of both series of compounds on the lamellar to inverted hexagonal phase transition of DEPE is also simil ar: they both stabilize the lamellar phase, increase the transition te mperature 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 pha se requires a negative curvature.