OSMIUM TETROXIDE FIXATION OF LIPIDS: NATURE OF THE REACTION PRODUCTS

To study the possible reaction mechanisms involved in osmium tetroxide fixation as used in electron microscopy, the products were analyzed which result from the interaction of osmium tetroxide and a number of pure substances. By means of paper chromatography, differences could be observed between the reaction products of 1-decene and oleic acid on the one side and of lecithin on the other. In the case of 1-decene and oleic acid, an intermediate osmic acid ester, as postulated by Criegee's reaction scheme, appears to be the main reaction product. In the case of lecithin, the reaction product is mainly a corresponding dihydroxy compound; here the intermediate osmic ester is readily split with formation of osmium dioxide and dihydroxy lecithin. The presence of a phosphoryl choline group as in lecithin thus appears to influence the course of the reaction with osmium tetroxide. If a polar group as well as double bonds play a part in the reactions, this might explains electron microscopic findings suggesting that during osmium fixation the deposition of osmium dioxide probably occurs at the polar ends of the lipid molecules. Our chromatographic experiments show that osmium dioxide, as a colloidally dispersed hydrate, is negatively charged and capable of Coulomb interaction with the quaternary ammonium group present in lecithin. Combined with the finding that osmium dioxide is readily split off from the osmium ester derivative of lecithin which is formed initially, this evidence strongly favors the hypothesis that osmium dioxide is deposited in the polar strata of lipid micelles.