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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.

It has been observed by differential scanning calorimetry that241Am α-particles induce modifications in phosphatidylcholine multilayers in the presence of water excess. Actually, below main transition point the mentioned defects seem to be randomly distributed within the lipid matrix; while above main transition there is some tendency to cluster. Moreover, increasing the irradiation time, the initial temperature of the main transition decreases, while a tail towards higher temperature appears. Such broadening of the lipid main transition seems to indicate the presence in the bilayers of different components than those created by γ-radiation.

By moans of differential calorimetric methods the behaviour of dipalmitoyl phosphatidylcholine and distearoyl phosphatidylcholine multilayers has been studied before and after irradiation with60Co γ-rays. It has been found that with the increase of the radiation absorbed dose an increasing amount of trapped water becomes free water. Moreover, after an absorbed dose of about 7.104Gy, a part of the bound water becomes free water and at the same time it is possible to create a component not mixable with the original lipid matrix.

Nonbilayer structures, as lysophospholipid micelles, can be produced in membranes by gamma irradiation. Interesting information on the gamma-ray effect on the lysophospholipid organization can be obtained by spin-label techniques. Variations of the TEMPO partition are appreciable, particularly in the range of the lipid thermotropic transition, even from relatively low doses. With increasing dose this transition becomes more and more complex and wider, according to the previous thermodinamic analysis.

ESR and DSC techniques have been applied to the study of aqueous dispersions of dipalmitoyl phosphatidylcholine multilayers both unirradiated and γ-irradiated, with TEMPO spin-label added to them. The examen of ESR spectra gave information on the molar fraction (f) of TEMPO bound to the lipid phase. While for the unirradiated samples one observes a sharp variation offat the main transition temperature, in the case of irradiated samples the speed of variation is greatly reduced, and other significant details do appear in thefvs.Tcurves. Moreover, always around the transition zone, the irradiation products, when in low concentration, show quite complete miscibility in the lipid bilayers. When their concentration is sufficiently higher, due to a greater irradiation dose, the products of irradiation lead to the formation of a domain-like organization of precipitates, whose composition and fluidity depend on the actual absorbed dose as well as on the temperature.