Caffrey M, Wang J
Department of Chemistry and Chemical Physics Program, Ohio State University, Columbus 43210-1173.
Faraday Discuss. 1992(94):283-93. doi: 10.1039/fd9929400283.
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.
20世纪60年代中期开发的X射线驻波（XSW）方法，当时被用于以亚埃分辨率确定硅和锗晶体内部及表面重原子的位置。层状合成微结构的出现，主要用作宽带通X射线单色器，开创了使用XSW研究长度尺度为几十埃的生物相关结构的新时代。最初的测量是在模型膜朗缪尔-布洛杰特（LB）膜上进行的，目的是确立XSW方法在以亚埃分辨率确定此类系统中重原子位置以及跟踪重原子层在热致转变过程中移动情况方面的实用性。最近的测量表明，在距离XSW产生表面近1000埃处，XSW定义明确。因此，XSW的有效探测距离也处于这个长度尺度，且分辨率不受影响。除了对有序系统的上述测量外，XSW方法还被用于“直接”描绘膜/水界面处的离子分布。最近的结果表明，通过适当调节磷脂膜旁边水相的pH值，可以可逆地建立扩散双层。本文讨论了这种新的表面技术应用于膜结构和界面现象研究的优缺点。
