Zhang R, Itri R, Caffrey M
Department of Chemistry, The Ohio State University, Columbus 43210, USA.
Biophys J. 1998 Apr;74(4):1924-36. doi: 10.1016/S0006-3495(98)77901-9.
The variable-period x-ray standing wave (XSW) technique is emerging as a powerful tool for studying membrane structure. However, two significant problems arise when the method is used to characterize membranes of thickness dL < 100 A. First, the surface roughness, sigma(r), of the supporting reflecting mirror convolutes with the intrinsic half-width of the marker atom distribution in the membrane, sigma(in), and contributes to an apparent half-width, sigma, which is measured in the XSW experiment. Here we show how the latter terms are related quantitatively [sigma(in) = (sigma2 - sigma(r)2)(1/2)], such that rough mirrors give rise to larger marker atom distribution widths, sigma, and how the required quantity sigma(in) can be determined in the XSW measurement. Second, when the mean position of the marker atom layer, (z), is close to one or both boundaries of the membrane, its distribution function is truncated at the boundary. In such cases, we show why marker atom distribution should be expressed in terms of its first and second moments. We also demonstrate by numerical simulations of realistic samples how the physical parameters, sigma(r), sigma, (z), and dL, affect x-ray reflectivity and fluorescence yield profiles as an aid in their interpretation.
可变周期X射线驻波（XSW）技术正逐渐成为研究膜结构的有力工具。然而，当该方法用于表征厚度dL < 100 Å的膜时，会出现两个重大问题。首先，支撑反射镜的表面粗糙度sigma(r)与膜中标记原子分布的固有半高宽sigma(in)卷积，并导致在XSW实验中测量的表观半高宽sigma。在这里，我们展示了后几项如何定量相关[sigma(in) = (sigma2 - sigma(r)2)(1/2)]，使得粗糙的镜子会导致更大的标记原子分布宽度sigma，以及如何在XSW测量中确定所需的量sigma(in)。其次，当标记原子层的平均位置(z)接近膜的一个或两个边界时，其分布函数在边界处被截断。在这种情况下，我们展示了为什么标记原子分布应该用其一阶和二阶矩来表示。我们还通过对实际样品的数值模拟，展示了物理参数sigma(r)、sigma、(z)和dL如何影响X射线反射率和荧光产率分布，以帮助解释它们。
