Factors affecting molecular packing in mixed lipid monolayers and bilayers

The nature of the molecular interactions and the factors determining molecular packing in mixed phospholipid/glyceride monolayers and bilayers were investigated by monolayer and nuclear magnetic resonance (NMR) techniques. Force-area curves were obtained at various temperatures for monolayers, at the air-water interface, of synthetic lecithins and a phosphatidyl ethanolamine mixed with di- and triglycerides in different molar ratios. The linewidths of peaks in the high resolution NMR spectra of lecithin/glyceride co-dispersions in excess water at different temperatures were used to obtain information about molecular mobilities.It was found that the molecular packing in mixed lipid monolayers and bilayers is determined by the following factors: (1) Whether lipid chains are above or below their melting point (T_C). (2) The difference between experimental temperature and T_C: the larger the difference, the smaller the effect of one component on the other. (3) The degree of similarity of the chains of the components; this influences the degree of cooperativity of chain motions and the degree of mixing of the components. (4) The nature, orientation, mutual interaction and degree of hydration of the polar groups.It is shown that mean molecular area does not always reflect the state of chain motions in lipid films, because of heterogeneity of motion and structure along the molecules. Cooperativity of motion may reduce steric requirements; other effects which are of particular importance for lecithins are interactions of zwitterions, and the influence of polar group hydration.