Honda M, Baba Y, Shimoyama I, Sekiguchi T
Quantum Beam Science Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata-Shirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan.
Rev Sci Instrum. 2015 Mar;86(3):035103. doi: 10.1063/1.4913653.
X-ray absorption fine structure (XAFS) measurements are widely used for the analysis of electronic structure. Generally, XAFS in the soft X-ray region is measured under vacuum, but chemical structures under vacuum are typically different from those under operando conditions, where chemical species exhibit their function. Here, we developed an XAFS measurement instrument, as a step toward operando fluorescent, which yields XAFS measurement using synchrotron radiation in the soft X-ray region. We applied this method to analyze the local electronic structure of the sulfur atoms in L-cysteine in different pH solutions. In water at pH 7, the hydrogen atom does not dissociate from the thiol (-SH) group in L-cysteine, which forms a structure surrounded by and interacting with water molecules. The XAFS spectrum of L-cysteine in solution was altered by changing the pH. At pH 9, the hydrogen atom dissociated and a thiolate anion was formed. Although the -SH group was oxidized to SO4 (2-) when L-cysteine was adsorbed on a metal surface and dried, no oxidation was observed in solution. This may be because the water molecules were densely packed and protected the -SH group from oxidation. Our results show that this instrument aimed toward operando fluorescence XAFS measurements in the soft X-ray region is useful for structural analysis of sulfur atoms in organic molecules in air and in solution. The instrument will be applied to the structural analysis of materials containing elements that have absorption edges in soft X-ray region, such as phosphorus and alkali metals (potassium and cesium). It will be also particularly useful for the analysis of samples that are difficult to handle under vacuum and materials that have specific functions in solution.
X射线吸收精细结构（XAFS）测量被广泛用于电子结构分析。一般来说，软X射线区域的XAFS是在真空条件下测量的，但真空条件下的化学结构通常与实际操作条件下的不同，在实际操作条件下化学物质发挥其功能。在此，我们开发了一种XAFS测量仪器，作为迈向实际操作荧光的一步，该仪器利用软X射线区域的同步辐射进行XAFS测量。我们应用此方法分析了不同pH值溶液中L-半胱氨酸中硫原子的局部电子结构。在pH值为7的水中，氢原子不会从L-半胱氨酸的硫醇（-SH）基团上解离，该基团形成了一个被水分子包围并与水分子相互作用的结构。通过改变pH值，溶液中L-半胱氨酸的XAFS光谱发生了变化。在pH值为9时，氢原子解离并形成硫醇盐阴离子。尽管当L-半胱氨酸吸附在金属表面并干燥时，-SH基团被氧化为SO4（2-），但在溶液中未观察到氧化现象。这可能是因为水分子紧密堆积，保护了-SH基团不被氧化。我们的结果表明，这种旨在进行软X射线区域实际操作荧光XAFS测量的仪器对于分析空气和溶液中有机分子中的硫原子结构很有用。该仪器将应用于含有在软X射线区域有吸收边的元素（如磷和碱金属（钾和铯））的材料的结构分析。它对于分析在真空下难以处理的样品以及在溶液中有特定功能的材料也将特别有用。
