THE EFFECTS OF THE P-NITROPHENYL ESTERS OF THE EVEN-NUMBERED FATTY-ACIDS FROM CAPROIC (C-6) TO STEARIC (C-18) ON THE MAIN PHASE-TRANSITION OF DIMYRISTOYLPHOSPHATIDYLCHOLINE

Differential scanning calorimetry (DSC) is employed in a study of the effects of the p-nitrophenyl esters of the even numbered fatty acids from C6 (caproic acid) to C18 (stearic acid) on the main phase transition of multilamellar suspensions of dimyristoylphosphatidylcholine (DMPC). Mole fractions in the range of 0.02-0.12 were used. Within this concentration range the observed transitions could be well fit on the basis of a model assuming ideal behavior of the esters in both the gel and liquid crystal phases of the lipid, and independently additive van't Hoff and impurity broadening. As expected on the basis of this model, the transition temperatures decreased linearly with increasing ester concentration and the transition enthalpies were independent of ester concentration. Significant differences between the effects of the various esters were observed, but these did not change with changing ester chain length in any regular fashion.