Structural requirements for the formation of ordered lipid multibilayers--a spin probe study

Spin probes were intercalated in oriented films formed from a series of lipids to study their ability to form ordered bilayers. Factors found to be important are the size and charge of the headgroup, the number, length, and degree of unsaturation of the acyl chains, the presence of cholesterol, and the type and concentration of ions in the aqueous phase. Unsaturated diglycerides and monoglycerides did not form bilayers; saturated monoglycerides formed well-ordered bilayers above their transition temperatures and cholesterol allowed the formation of ordered bilayers at lower temperatures. Saturated diglycerides did not form bilayers at temperatures at which the probe was dissolved in the system. The addition of calcium ions increased the anisotropy of films formed from a mixture of lecithin, cholesterol, and phosphatidic acid, but disrupted films formed from phosphatidylserine. Caution must be exercised in interpreting electron spin resonance spectra of films since films of tightly packed lipids tend to exclude spin probes and preparations which are clearly lamellar manifest a high degree of disorder within the bilayers.