THERMOTROPIC BEHAVIOR OF GAMMA-IRRADIATED DIACYLPHOSPHATIDYLCHOLINE MULTIBILAYER VESICLES - ROLE OF SINGLE RADIOLYSIS PRODUCTS

The concentrations of two isomeric lysophosphati-dylcholines (LPC), fr ee palmitic acid (FPA) and glycerophos-phorycholine (GPC), have been e valuated in multibilayer vesicles (MBV) of 1,2-dipalmitoyl-sn-glycero- 3-phosphoryl-choline (DPPC) at different absorbed doses, by H-1 and P- 31 nmr. No evidence of cross-linking of lipid molecules has been found . The presence of radiolysis products of DPPC is by itself capable of justifying the thermotropic behaviour of irradiated MBV, as it appears with differential scanning calorimetry analysis. In fact, on increasi ng LPC concentration, the onset temperature of the main transition (MT ) is proportionally shifted towards lower temperatures, without altera tion of its width. A similar effect is also produced by LPC on the low er transition (LT). On the contrary, on increasing the FPA concentrati on, the MT width increases proportionally. Simultaneously, the LT is s hifted towards higher temperatures and disappears above a certain FPA concentration. The overall effect of LPC and FPA on MT appears as a li near combination of the two effects. As far as LT is concerned, LPC an d FPA compete with one another in determining whether it will be prese nt or not and, if present, in determining its onset temperature. No GP C effect on the phase transitions could be detected within the limits of the composition of our irradiated MBV.