[Spin label study of the surface of lipid bilayers].

The surface of liposomes and natural membranes has been studied by the method of "spin label--spin probe". Various nitroxyl radicals have been attached to membranes either dueto covalent binding with NH2-groups of phosphatidylethanolamine or due to hydrophobic interactions. Addition of paramagnetics of different charge signs (potassium ferricyanide and dibenzenechromium iodide) to spin labeled membranes results in a decrease of the EPR line intensity without marked broadening. Since the paramagnetic is attached to the membrane surface near the label it broadens the label's EPR spectrum so that it can not be observed. In this case one can observe only the spectrum of labels which have no paramagnetics in the vicinity, the nitroxyl part of radicals being inaccessible to direct paramagnetic impacts. The constants of binding of paramagnetics to the membrane surface have been determined from these experiments. All results (including the assymetry of label rotation) can be explained most simply by assuming that the polar lipid groups are oriented parallel to the membranes surface.