Acyl chain interactions and the modulation of phase changes in glycerolipids

Fluorescence polarization measurements with 1,6-diphenyl-1,3,5-hexatriene and differential scanning calorimetry (DSC) were used to monitor phase transitions and order in the liquid state in sonicated dispersions of mono-, di- and triacylglycerols. Residual order in melted glycerolipids was indicated when the structural order parameter, S, assumed non-zero values at temperatures, t greater than or equal to tf, the DSC-determined fusion temperature. Residual order was observed with cis unsaturated di- and triacylglycerols but not with corresponding trans unsaturated or with saturated compounds. The reduced fluidity was attributed to adjacent binding of fatty acids to the glycerol molecule and the resulting interactions between fatty acyl moieties and packing effects. Lipids were considered as in an isotropic liquid or highly fluid state when diphenylhexatriene fluorescence anisotropy, rs, was equal to or less than 0.08, corresponding to S = 0. Temperatures, t0.08, for transition from the fluid state upon cooling were noted when rs = 0.08, and delta t = t0.08-tf was then taken as a measure of residual order. Tri-, 1,2-di and 1,3-dioleoylglycerol delta t values were 75, 60.9 and 13.6 degrees C, respectively. Tri-, 1,3-di- and monolinoleoylglycerol delta t values were 86, 30 and 41 degrees C, respectively. Restrictions in mobility when observed are attributable to interactions between adjacent acyl chains. Double bond location in the hydrocarbon chain affected ordering in the liquid state as simple triacylglycerol esters of cis 18:1 delta 6, trans 18:1 delta 6 and cis 24:1 delta 15 exhibited t = 37, 14 and 18 degrees C, respectively.