Direct Determination of Biomembrane Structures

Abstract

In early attempts to reveal the molecular organization of cell membranes, x-ray diffraction was employed to probe multilamellar arrays of phospholipid bilayers [1–4]. Suitable multilayer samples were either built up by dipping a support surface through a Langmuir-Blodgett monolayer or by examination of stacked natural membranes. More recently, other techniques such as epitaxial crystallization on a substrate have been devised to orient phospholipid multilayers on an electron microscope grid to be studied by electron diffraction [5]. Studies of such lipid and biomembrane lamellar arrays are necessary because of the difficulty in crystallizing phospholipid samples suitable for x-ray diffraction. For example, the study of polydispersity in acyl chains (length, unsaturation), the principal factor controlling bilayer fluidity, has been frustrated since the least amount of impurity impedes the growth of suitable single crystals [6]. Needless to say, inclusion of other important bilayer lipids, such as cholesterol, that also control membrane properties [7], was also excluded from single crystal studies. Only after suitable single crystals were obtained, have the structures of representative diacyl phospholipids (phosphatidylethanolamine [8], phosphatidylcholine [9], phosphatidyl-N,N-dimethylethanolamine [10], phosphatidylglycerol [11] and phosphatidic acid [12]) been determined, to reveal preferred molecular conformations and packing arrangements of polar headgroups and diacyl glycerol moieties.