(1) The structural organization and fluidity of a variety of lipid classes were studied by electron spin resonance spectroscopy of fully hydrated planar films labeled with 3-doxyl-cholestane and 5-doxyl-palmitate and of dispersions labeled with 5-doxyl-palmitate. The phase of each lipid was checked by use of a polarizing microscope.(2) Orientation in a lamellar phase was detected for phosphatidyl-choline, -ethanolamine, -inositol, phosphatidic acid, cardiolipin, ceramide, and sphingomyelin. In the presence of 33 or 50 mol % cholesterol, the orientation was improved in all of these lipids with the exception of cardiolipin and orientation was induced in phosphatidylserine, monoglycerides, sphingosine, and cerebrosides. Cholesterol fluidized the sphingolipids sufficiently to solubilize the spin labels and allow formation of oriented multibilayers by this method.(3) Anisotropic motion of 5-doxyl-palmitate was also seen in films and disperions of 1-monoglycerides and diglycerides and in dispersions of gangliosides, which are known to exist in hexagonal or micellar phases.(4) The contribution of the molecular structures to the stability of the lamellar phase is discussed.

Boggs, J. M., & Hsia, J. C. (1973). Structural Characteristics of Hydrated Glycerol- and Sphingo-lipids. A Spin Label Study. Canadian Journal of Biochemistry, 51(11), 1451–1459.