The physical properties of phospholipids I. Solid state and mesomorphic properties of some 2, 3-diacyl-DL-phosphatidylethanolamines

Abstract
A study has been made of the polymorphic and mesomorphic properties of a series of 2,3-diacyl-DL-phosphatidylethanolamines by means of a variety of physical techniques including light microscopy, infrared and nuclear magnetic resonance spectroscopy, X-ray diffraction techniques and differential thermal analysis. The results are consistent in showing that the phospholipids exhibit polymorphism and that, when a phospholipid is heated, marked transitions of an endothermic character occur many tens of degrees below the published capillary melting point. We show that these transitions are associated with the formation of a liquid crystalline phase. The transition temperature associated with the largest endothermic transition decreases with increase in the amount of unsaturation in the hydrocarbon chain of the fatty acid residues present, and also is found to depend upon the nature of the unsaturation, i.e. whether it is cis or trans in character. The X-ray long spacing decreases markedly at this temperature and the infrared spectra show, at this temperature, a considerable change from a 'crystalline' type to 'liquid' type spectrum. Nuclear magnetic resonance measurements show a considerable reduction in the line width and second moment at this temperature, consistent with the hydrocarbon chains undergoing considerable molecular motion. Pretreatment of the crystalline material and the presence of water are shown to have a marked influence upon the transitions. The implications of this liquid crystalline transition to the interpretation and use of infrared spectra for characterizing phospholipids, and also upon the rigidity of monolayers and bilayers prepared with phospholipids, are discussed, and an example of the latter is shown by comparison of force-area curves of two isomeric phospholipids. The more general implications of the work to biological membranes are also mentioned.

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