POLYMORPHISM OF A TRIPLE-CHAIN LECITHIN IN 2-DIMENSIONAL AND 3-DIMENSIONAL SYSTEMS

Monolayers and bilayers of two isomeric branched chain phosphatidylcholines at the air/water interface are studied by fluorescence microscopy and by thermodynamic measurements, by differential scanning calorimetry and by X-ray diffraction. The two lipids differ in the position of the branched chain at the glycerol backbone but have in common an extended side branch that essentially makes them a lipid with three tails of almost equal length. Most qualitative features of the isotherms are similar, the exception being a difference of 4 angstrom 2 in the minimum molecular area at high lateral pressures. This indicates a more condensed ordered phase of one system, in agreement with fluorescence microscopic observation of domain shapes. The latter exhibit sharp edges and six main branches as expected for dendritic growth with expression of crystal symmetry. In the other system with a limiting molecular area of 19.5 angstrom 2 per chain, the domain boundary is smooth and a theoretically predicted shape instability is observed. Structural differences are discussed and may predominantly result from the different hydrophilicity of ester and ether linkages.