Membrane-structure studies using X-ray standing waves.

The principle and unique features of X-ray standing waves as a means for investigating membrane structure are described in this review. Thus far, X-ray standing waves have been used in structural studies of Langmuir-Blodgett and self-assembled monomolecular lipid films. Most recently, the method has been used in studies of supported membranes hosting the peripheral membrane protein, cytochrome c. Structural rearrangements occurring in membranes and at surfaces driven by temperature and composition changes have been monitored as well. Finally, ion distribution in the diffuse-double layer next to a charged membrane has been determined using this approach. The review addresses the manner in which these and related measurements were made. What is not covered in the review is a critical appraisal of the limitations of the X-ray standing wave method. Such limitations (which are pronounced and introduce significant mensuration ambiguities at short distances on silver mirrors as used in the membrane protein topology study) have just recently come to our attention and will be reported on separately (S Kirchner, Z Yin, J Wang & M Caffrey, in preparation).