Monolayers of mitochondrial cardiolipin and cerebronic acid and their equimolar mixture

Abstract

Biological membranes are essentially organized mixtures of oriented film-forming compounds. Thus the study of monolayers of mitochondrial cardiolipin and cerebronic acid alone and in mixtures should shed light on the structure and function of biomembranes. Mitochondrial cardiolipin forms a monolayer which shows essentially no compression-expansion-recompression hysteresis up to 24 dyn/cm and reaches a maximum pressure of 29 dyn/cm. Electron micrographs of the collapsed film reveal disks unlike any collapse structures heretofore observed. The precursors of the disks may well be liposomes formed by double-layer folds of the monolayer moving down into the aqueous substrate. Cerebronic acid forms a rigid fatty-acid type monolayer whose isotherm rises to a stable maximum pressure of 68 dyn/cm. Such stability is most unusual for a film of this type. Electron micrographs of the “collapsing” film show many tall ridges normal to the direction of compression but no fallen ridges (flat ribbons or platelets) like those that have dominated the micrographs of similar films. Isotherms for an equimolar mixture of cardiolipin and cerebronic acid show slight interference (mixing) at moderate pressures, squeezing out of considerable cardiolipin near its collapse pressure, and moderate interference at high pressures where cerebronic acid dominates. Electron micrographs suggest considerable mixing of the collapsed structures.