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 P½ 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, P½ 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, P½(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 P½). 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.