The occurrence of lipidic particles in lipid bilayers as seen by 31P NMR and freeze-fracture electron-microscopy

Abstract

A new type of lipid organization is observed in mixtures of phosphatidyl-choline with cardiolipin (in the presence of Ca2+), monoglycosyldiglyceride and phosphatidylethanolamine (in the presence of cholesterol). This phase is characterised by an isotropic 31P NMR signal and is visualised by freeze-fracturing as particles and pits on the fracture faces of the lipid bilayer. As the most favourable model for this phase we propose the inverted micelle sandwiched in between the two monolayers of the lipid bilayer.

Further aspects of the Ca2+-dependent polymorphism of bovine heart cardiolipin

“…In agreement with observations made in other lipid mixtures [15] lipidic particle formation must be the result of the coexistence of both bilayer and hexagonal H H phase preferring lipids. It could be suggested that the presence of phosphatidylcholine interferes with the formation of extended areas of the Ca-cardiolipin (l: l) complex thereby inhibiting hexagonal H H phase formation.…”

“…In mixed dioleoylphosphatidylcholine/ cardiolipin bilayers addition of Ca 2 + from a concentrated solution [15] or via the dialysis method (this study) even at Ca 2+ concentrations as low as 0.2 mM induces the formation of the 'isotropic' structure. In agreement with observations made in other lipid mixtures [15] lipidic particle formation must be the result of the coexistence of both bilayer and hexagonal H H phase preferring lipids.…”

“…Since Ca 2+ addition from a concentrated solution to mixed phosphatidylcholine-cardiolipin systems also results in the appearance of an 'isotropic' structure in association with lipidic particles [15], we thought it of interest to see whether similar structures would be observed when the Ca 2÷ was added via the dialysis method. As shown in Fig.…”

“…Therefore, this hypothesis is strengthened by the observation that in mixtures of cardiolipin with bilayer forming lipids a new non-bilayer lipid structure, the 'lipidic particle' has been observed [14,15]. This structure has been interpreted by us as representing intrabilayer inverted miceUes [14][15][16][17]. Although other models have been proposed [18][19][20] recent evidence [21] strongly supports the inverted micellar model.…”

“…Conceptually, it is difficult to reconcile the presence of extended areas of hexagonal Hll phase with the barrier function of the membrane. Therefore, this hypothesis is strengthened by the observation that in mixtures of cardiolipin with bilayer forming lipids a new non-bilayer lipid structure, the 'lipidic particle' has been observed [14,15]. This structure has been interpreted by us as representing intrabilayer inverted miceUes [14][15][16][17].…”