Thermotropic Behavior of Cardiolipin and Dimyristoylphosphatidylcholine Bilayers in the Presence and Absence of Calcium

Abstract

Cardiolipin is an unusual lipid in that it is comprised of two diacylglycerols linked by a glycerol bridge. It is in found primarily in membranes in which electron transport is coupled to phosphorylation. There it plays a critical role in effective functioning of the electron transport chain. The mechanism by which cardiolipin plays this role may be through effects on bilayer properties as well as through direct interactions with protein complexes. In this study we investigate the influence of increasing the composition of tetramyristoyl cardiolipin (TMCL) on the thermotropic behavior of dimyristoyl phosphatidylcholine (DMPC) bilayers using differential scanning calorimetry (DSC). DSC experiments were performed using a MicroCal VP-DSC microcalorimeter. Lipid dispersions of DMPC/TMCL were prepared in which TMCL constituted 0, 5,10, 20, 50 and 100 mole percent of the total lipid. In these studies the Tm increased from 25 oC to 40 oC as the TMCL composition increased. Ca++ has been shown to have a plethora of in vivo effects on biological membranes and to induce alterations in lipid phase behavior of model membranes. Therefore we further investigated the thermotropic behavior of DMPC/TMCL bilayers in the presence of Ca++. Above 20 mole percent TMCL a distinct phase separation was induced. The effect of Ca++ on cardiolipin/PC membranes was also investigated using 6-dodecanoyl-2-dimethylaminonaphthalene (LAURDAN). These data are consistent with an alteration of the hydration of the bilayer in the headgroup region in the presence of Ca++ and suggest that Ca++ induces tighter packing.