Influence of Ca2+ and ethanol on the aggregation and thermal phase behaviour of L-dihexadecylphosphatidylcholine liposomes.
The influence of Ca2+ and ethanol on vesicle aggregation and thermal phase behaviour of the diether lipid 1,2-dihexadecylphosphatidylcholine (DHPC) was studied by light absorbance and DSC. At temperatures below the pretransition the ethanol-injected vesicles of L-DHPC were rapidly aggregated by Ca2+. Upon raising the cation concentration a biphasic increase in aggregation saturating at an approximate [Ca2+]/[lipid] ratio of 1.5:1 was observed. Further increase in [Ca2+] up to [Ca2+]/[lipid] stoichiometries exceeding 2.5:1 led to the loss of aggregation. Removal of ethanol by dialysis abolished Ca(2+)-induced aggregation. Ethanol-injected vesicles of the ester-linked L-dipalmitoylphosphatidylcholine (L-DPPC) or the racemic DL-DHPC were not aggregated by Ca2+ thus indicating the importance of the absence of ester carbonyls as well as the stereochemical configuration of the lipid in determining the mode of interaction of DHPC with Ca2+. Differential scanning calorimetry of multilamellar liposomes of L-DHPC showed an increase by 8 degrees in the pretransition temperature Tp in the presence of 250 mM ethanol. Both with and without ethanol, increasing concentrations of Ca2+ corresponding to [Ca2+]/[lipid] ratios of 1:1 to 20:1 caused a gradual decrease in Tp and finally the disappearance of the pretransition. Concomitantly a slight elevation in Tm occurred. No principal differences were observed in the thermal phase behaviour of the L-isomer and racemic DL-DHPC.
Kõiv
A
A
Department of Medical Chemistry, University of Helsinki, Finland.
Kinnunen
P K
PK
D002118:Calcium; D002152:Calorimetry, Differential Scanning / Q000379:methods; D002412:Cations; D000431:Ethanol; D007700:Kinetics; D008081:Liposomes; D008274:Magnesium; D008345:Manganese; D010742:Phospholipid Ethers / Q000737:chemistry*; D012964:Sodium
C052298:1,2-dihexadecyl-sn-glycero-3-phosphocholine; D002412:Cations; D008081:Liposomes; D010742:Phospholipid Ethers; D000431:Ethanol; D008345:Manganese; D012964:Sodium; D008274:Magnesium; D002118:Calcium