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-Angstrom resolution. The advent of l ayered synthetic microstructures, used primarily as wide-bandpass X-ra y monochromators, heralded a new era in the use of XSW to study biolog ically relevant structures with a length scale of the order of tens of Angstroms. The original measurements were performed on model membrane Langmuir-Blodgett (LB) films and served to establish the utility of t he XSW approach in determining heavy-atom location in such systems wit h sub-Angstrom resolution and in tracking the heavy-atom layer as it m oves during a thermotropic transition. Recent measurements show that t he XSW is well defined at close to 1000 angstrom from the XSW generati ng surface. Thus, the useful probing distance of XSW is of this length scale also without a compromise in resolution. In addition to the abo ve measurements on well ordered systems the XSW method is being used t o profile ion distribution 'directly' at the membrane/aqueous interfac e. Recent results show that the diffuse double layer can be establishe d 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 i nterfacial phenomena are discussed.