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Journal of Lipid Research, Vol. 8, 215-226, May 1967
Copyright © 1967 by Lipid Research, Inc.

Influence of calcium, cholesterol, and unsaturation on lecithin monolayers

Dinesh O. Shah and Jack H. Schulman

Stanley-Thompson Laboratory, School of Engineering, Columbia University, New York 10027

Surface pressures and potentials of mixed monolayers of dicetyl phosphate-cholesterol, dipalmitoyl lecithin-cholesterol, egg lecithin-cholesterol, and phosphatidic acid-cholesterol were measured. The surface potential is shown to be a more reliable parameter for the study of interactions in monolayers than the surface pressure. Monolayers of dicetyl phosphate-cholesterol follow the additivity rule for area/molecule whereas lecithin-cholesterol monolayers deviate from it. The reverse is true for the additivity rule with regard to surface potential/molecule. Thus, the surface potential indicates that there is no interaction (or complex formation) between lecithin and cholesterol, but that there is ion-dipole interaction between dicetyl phosphate and cholesterol, as well as between phosphatidic acid and cholesterol.

The apparent condensation of mixed monolayers of lecithin when cholesterol is added is explained by a consideration of molecular cavities or vacancies caused by thermal motion of the fatty acyl chains, the size of these cavities being influenced by the length and degree of saturation (especially the proportion of monounsaturation) of the fatty acyl chains and the extent of compression of the monolayer. The cholesterol molecules occupy these cavities and therefore cause no proportional increase in area/molecule in the mixed monolayers. Monolayers are liquefied by the presence of cholesterol as well as of unsaturated fatty acyl chains; in contrast, Ca⁺⁺tends to solidify lecithin monolayers. The available evidence suggests that cholesterol can both impart fluidity to the monolayer and occupy the molecular cavities caused by the fatty acyl chains.

Supplementary key words mixed monolayers • cholesterol • dicetyl phosphate • dipalmitoyl lecithin • egg lecithin • phosphatidic acid • surface pressure • surface potential • liquefaction • solidification • calcium • concept of molecular cavities

Submitted on June 14, 1966
Revised on November 10, 1966
Accepted on January 24, 1967

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