P-31-NMR AND CALORIMETRIC STUDIES OF THE LOW-TEMPERATURE BEHAVIOR OF 3 F-19-LABELED DIMYRISTOYLPHOSPHATIDYLCHOLINES

The thermotropic phase behavior of three difluorodimyristoylphosphatidylcholines, containing a difluoromethylene group in the 2-acyl chain at either the 4-, 8-, or 12-position, has been studied by differential scanning calorimetry (DSC) and by P-31 nuclear magnetic resonance (NMR) spectroscopy at temperatures at and below their normal gel to liquid crystalline transition. The fluorinated lipids have been found to behave in a manner similar to their protonated counterpart with the exception of the 4,4-difluoro isomer. The 4,4-substituted isomer, 1-myristoyl-2-(4,4-difluoromyristoyl)-sn-glycero-3-phosphocholine(2-[4,4-F2]DMPC), appears by P-31 NMR upon cooling to 10 deg below the main (P(beta'), to L(alpha)) phase transition to rapidly achieve a crystalline subgel phase. The same anomalous behavior of rapid conversion to the crystalline (L(c)) phase is seen by DSC upon cooling the 2-[4,4-F2]DMPC lipid to 5-degrees-C. By both DSC and NMR, the few hours taken for conversion of the 2-[4,4-F2]DMPC to a solid phase is unusually short when compared either to the other fluorolipids or to unsubstituted saturated straight-chain diacylphosphatidylcholines, which generally take a day or more for conversion to their solid forms.