NOVEL THERMAL PHASE-TRANSITION BEHAVIOR OF PHOSPHATIDYLCHOLINE ANALOGS CONTAINING 1,2,4-BUTANETRIOL AS THEIR BACKBONE

The sn-glycerol moiety in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was replaced by the rac-1,2,4-butanetriol residue, and the aque ous dispersions of the resulting DPPC analogs, viz. rac-but-4-yl-[2-(t rimethylammonium)ethyl]phosphate (1,2-bPC) and rac-but-4-yl-[2-(trimet hylammonium)ethyl]phosphate (1,3-bPC), were characterized by different ial scanning calorimetry (DSC) and fluorescence polarization technique . Also, the corresponding (3R)-isomer of 1,3-bPC (1,3-(3R)bPC) was pre pared and characterized by DSC. While the thermal phase transition pro perties of 1,2-bPC were similar to that of racemic DPPC, 1,3-bPC in id entical conditions showed an abnormal property by exhibiting a metasta ble phase behavior at about 15 degrees C. This abnormal property was a ssociated only with the racemic mixture and was completely absent in i ts 3R-isomer. 1,3-(3R)bPC, unlike DPPC, showed only two high enthalpy transitions at about 29.7 degrees C (Delta H, 7.45 kcal/mol) and 35.3 degrees C (Delta H, 6.93 kcal/mol). These results clearly demonstrate that an insertion of one additional methylene residue between the glyc erol C1 and C2 carbons in DPPC markedly alters its thermal phase trans itional properties. whereas these properties remain virtually unchange d if a similar chemical change is introduced between the glycerol C2 a nd C3 carbon atoms.