Oxygen diffusion–concentration in phospholipidic model membranes. An electron spin resonance–saturation study

Abstract

Fully hydrated liposomes of dipalmitoyl-phosphatidylcholine have been labelled with 5 (or 7, 10, 12, 16)-doxyl stearic acid at pH 6 and 8, and studied by the continuous-wave e.s.r.-saturation technique. The e.s.r. spectral magnitude depends on the hyperfrequency power P and on both T₁ and T₂ relaxation times. Saturation, i.e. the non-linearity of the spectral magnitude plotted vs.√P can be quantified by a P_½ parameter (power at which the signal is half as great as it would be without saturation). If we assume T₂ is weakly modified by spin exchange between the paramagnetic spin probe and oxygen in its triplet state, P_½ is inversely proportional to T₁ and becomes a sensitive parameter to appreciate the oxygen transport (oxygen diffusion–concentration product) inside the bilayers. According to the DPPC bilayer phase-transition diagrams, P_½(oxygen diffusion–concentration) is related to the thermodynamic state of the membrane. This technique provides further information on a particular property of a radioprotective agent, cysteamine, which seems to inhibit spin–triplet exchange and hence maximizes T₁(minimizes P_½). Since radioprotective agents are known to act by scavenging radiation-induced free radicals and by inhibiting oxygen-dependent free radical processes, such a result may contribute to the elucidation of radioprotecting mechanisms.