Internal and interfacial structure of membranes studied using X-ray standing waves

The X-ray standing wave (XSW) method developed in the mid-Sixties was used then to determine the position of heavy atoms in and on crystals of silicon and germanium with sub-Angström resolution. The advent of layered synthetic microstructures, used primarily as wide-bandpass X-ray monochromators, heralded a new era in the use of XSW to study biologically relevant structures with a length scale of the order of tens of Angströms. The original measurements were performed on model membrane Langmuir-Blodgett (LB) films and served to establish the utility of the XSW approach in determining heavy-atom location in such systems with sub-Angström resolution and in tracking the heavy-atom layer as it moves during a thermotropic transition. Recent measurements show that the XSW is well defined at close to 1000 A from the XSW generating surface. Thus, the useful probing distance of XSW is of this length scale also without a compromise in resolution. In addition to the above measurements on well ordered systems the XSW method is being used to profile ion distribution 'directly' at the membrane/aqueous interface. Recent results show that the diffuse double layer can be established reversibly by suitably adjusting the pH of the aqueous phase next to a phospholipid membrane. The advantages and disadvantages of this new surface technique as applied to the study of membrane structure and interfacial phenomena are discussed.