Temperature-induced changes in lecithin model membranes detected by novel covalent spin-labelled phospholipids

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Abstract
Several spin-labeled phospholipids carrying covalently bound 5-doxylstearic acid (2-(3-carboxydecyl)-2-hexyl-4,4-dimethyl-3-oxazolidinoxyl) were intercalated in liposomes of saturated and unsaturated lecithins. Temperature-induced changes of these liposomes, detected by the spin-labeled phospholipids, were in agreement with the previously described transitions of hydrocarbon chains of host lecithins detected by different probes and different techniques, establishing that spin-labeled phospholipids are sensitivie probes for the detection of temperature-induced changes in lecithin model membranes. In addition to the detection of already-known transitions in lecithin liposomes, the coexistence of 2 distinctly different environments was observed above the characteristic transition temperature. This phenomenon was tentatively attributed to the influence of the lecithin polar group on the fluidity of fatty acyl chains near the polar group. Combined with other results from the literature, the coexistence of 2 environments could be associated with the coexistence of 2 conformational isomers of lecithin, differing in the orientation of the polar head group with respect to the plane of bilayer. These findings were discussed in view of the present state of knowledge regarding temperature-induced changes in model membranes.

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