Monolayer coupling in phosphatidylserine bilayers: distinct phase transitions induced by magnesium interacting with one or both monolayers.
We have investigated the thermotropic behavior of phospatidylserine bilayers interacting with Mg2+ either on one side or both sides, using differential scanning calorimetry. Large unilamellar vesicles (LUV) of phosphatidylserine exposed to Mg2+ on the external side only displayed an upward shift of the gel-liquid transition temperature (Tm) of about 6-8 degrees C relative to the Tm of LUV in Na+. Mg2+ was shown not to enter the vesicle interior, by means of fluorescence measurements on encapsulated 8-hydroxyquinoline-5-sulfonate. Multilamellar vesicles prepared in the presence of Mg2+, or vesicles prepared by Mg2+-induced fusion of small unilamellar vesicles, had Tm values that were shifted upward by about 16-17 C degrees. When the latter preparation was treated with EDTA to produce vesicles with Mg2+ inside and Na+ outside, the Tm was found to be shifted again by only 6-8 degrees C. These observations indicate that the monolayer interacting with Na+ fluidizes the monolayer interacting with Mg2+, and that the latter tends to solidify the former. The two monolayers thus appear to be coupled, possibly by hydrocarbon chain interdigitation.
Düzgüneş
N
N
Cancer Research Institute, University of California, San Francisco 94143-0128.
Newton
C
C
Fisher
K
K
Fedor
J
J
Papahadjopoulos
D
D
D000818:Animals; D002152:Calorimetry, Differential Scanning; D002417:Cattle; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005614:Freeze Fracturing; D066298:In Vitro Techniques; D008051:Lipid Bilayers; D008274:Magnesium; D010718:Phosphatidylserines; D012964:Sodium
D008051:Lipid Bilayers; D010718:Phosphatidylserines; D012964:Sodium; D008274:Magnesium