X-ray scattering from labeled membranes

We present a new method for the determination of structural parameters in biological membranes. Recording the continuous scattering of heavy-atom labeled membranes and applying elementary Fourier methods we obtain the scattering of the heavy-atom distribution alone. The details of this distribution are explored by developing a simple model and testing for cases relevant to biological membranes. We find that the intensity distribution is highly sensitive to many key parameters. The increased signal from heavy-atom labeling and the use of an improved x-ray system make it possible to record patterns from dilute membrane suspensions. Thus determination of these parameters is possible in the same environment where many membrane biochemical studies are performed. Application of the method is made to a model lipid bilayer membrane, dipalmitoyl phosphatidylcholine by labeling with UO2++ ions. We determine the precise distance between UO2++ layers on either side of the membrane as well as the width of the label on each side. This determination permits estimation of phosphate separation across single labeled bilayers in an aqueous suspension.