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The concentrations of two isomeric lysophosphatidylcholines (LPC), free palmitic acid (FPA) and glycerophosphorycholine (GPC), have been evaluated in multibilayer vesicles (MBV) of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC) at different absorbed doses, by 1H and 31P 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 increasing LPC concentration, the onset temperature of the main transition (MT) is proportionally shifted towards lower temperatures, without alteration of its width. A similar effect is also produced by LPC on the lower transition (LT). On the contrary, on increasing the FPA concentration, the MT width increases proportionally. Simultaneously, the LT is shifted towards higher temperatures and disappears above a certain FPA concentration. The overall effect of LPC and FPA on MT appears as a linear combination of the two effects. As far as LT is concerned, LPC and FPA compete with one another in determining whether it will be present or not and, if present, in determining its onset temperature. No GPC effect on the phase transitions could be detected within the limits of the composition of our irradiated MBV.

Chemical and structural modifications in multilayer liposomes of synthetic phosphatidylcholine induced by gamma irradiation are investigated with different techniques. Fluorescence anisotropy of the DPH probe and differential scanning calorimetry reveal a broadening of the main lipid transition and the disappearance of pretransition. Fluorescence anisotropy is shown to be higher in the irradiated sample and particularly so at low temperatures. NMR and TLC results show that lysolecithin and palmitic acid are formed with a consequent change in bilayer organization. The possibility that these modifications may account for the permeability variations observed in irradiated natural membranes is discussed.