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In the course of a study of natural, hydrogenated, and synthetic phospholipides it has already become clear that some compounds in which the lipide chains are fully saturated do interact with osmium tetroxide and that suitable preparations of such compounds show a regular, layered structure when viewed in the electron microscope. One clear cut example, which seems worthy of early mention, is a synthetic L-a distearoyl phosphatidyl L-serine kindly supplied for structural studies by Dr. Erich Baer. X-ray diffraction studies show that at room temperature, the layer spacing in this material is about 63 A, but at higher temperatures two other polymorphic forms showing smaller layer spacings have been detected. At 70 to 80°C. the layer spacing is reduced to 55 to 50 A, and at 90 to 100°C. there is a further sharp decrease in layer spacing to about 38 A. Unlike the fully saturated synthetic phosphatidyl ethanolamines and phosphatidyl cholines previously studied (1), the phosphatidyl serine does react with osmium tetroxide, though only slowly, requiring 6 to 8 hours in buffered 1 per cent osmium tetroxide to become really blackened. This fixed material shows a 63 A diffracting unit which is unchanged after dehydration and embedding in araldite. Thin sections (Philips or Porter-Blum ultra-

Electron Microscope and X-ray Diffraction Studies of a Saturated Synthetic Phospholipide