Edwards David C, Myneni Satish C B
Department of Chemistry, Princeton University, Frick Laboratory, Princeton, New Jersey 08544, USA.
J Phys Chem A. 2006 Oct 26;110(42):11809-18. doi: 10.1021/jp0611976.
X-ray absorption spectroscopy (XAS) is widely used to explore the coordination environments and structures of metal complexes in aqueous solutions and disordered phases. Although soft-XAS studies on gaseous phases, solid phases and their interfaces have shown that XAS is a versatile tool in studying the functional group composition of organic molecules, the application of XAS to studying aqueous solutions is seriously limited because of experimental difficulties. In this report, using a modified synchrotron endstation geometry, we show how soft-XAS was used to study the changes in electronic states of reactive functional groups in a bacterial macromolecule, desferrioxamine B (desB, a hydroxamate siderophore) and its structural analogue (acetohydroxamic acid (aHa)). We collected C, N, and O near edge X-ray absorption fine structure (NEXAFS) spectra of these molecules in aqueous solutions and complemented their spectral interpretation with calculated X-ray spectra of "hydrated" aHa. The experimental spectra of desB are similar to those for aHa at the C, N, and O K-edges. In addition, the electronic transitions of amide and hydroxamate functional groups in the macromolecule can be distinguished from the N spectra. Small energy differences in the pi*(C=O)NO and the transitions at the C- and N-edges of aHa and desB indicate that the substituent attached to N in desB ((CH2)n) determines the electron density in the (C=O)NO core. As the solution pH increased, the pi*(C=O)NO transition of the hydroxamate group of these two molecules exhibit energy shifts at the C-, N-, and O-edges, which are consistent with increased electron delocalization in the (C=O)NO core of aHa (and desB), predicted from the calculations. The spectra of the aHa(H2O)3- anion also provide evidence for partial N-deprotonation at pH values usually attributed to an O-acid. These results indicate that soft-XAS is well suited for studying the electronic states of different functional groups in aqueous organic macromolecules.
X射线吸收光谱法（XAS）被广泛用于探究金属配合物在水溶液和无序相中 的配位环境及结构。尽管对气相、固相及其界面的软XAS研究表明，XAS是研究有机分子官能团组成的通用工具，但由于实验困难，XAS在研究水溶液方面的应用受到严重限制。在本报告中，我们使用经过改进的同步加速器终端站几何结构，展示了如何利用软XAS研究细菌大分子去铁胺B（desB，一种异羟肟酸铁载体）及其结构类似物（乙酰氧肟酸（aHa））中反应性官能团电子态的变化。我们收集了这些分子在水溶液中的C、N和O近边X射线吸收精细结构（NEXAFS）光谱，并用“水合”aHa的计算X射线光谱对其光谱解释进行补充。desB的实验光谱在C、N和O K边与aHa的光谱相似。此外，大分子中酰胺和异羟肟酸官能团的电子跃迁可以从N光谱中区分出来。aHa和desB在π*（C=O）NO以及C边和N边跃迁中的小能量差异表明，desB中与N相连的取代基（（CH2）n）决定了（C=O）NO核心中的电子密度。随着溶液pH值升高，这两种分子异羟肟酸基团的π*（C=O）NO跃迁在C边、N边和O边都出现了能量位移，这与计算预测的aHa（和desB）（C=O）NO核心中电子离域增加一致。aHa（H2O）3-阴离子的光谱也为通常归因于O-酸的pH值下的部分N去质子化提供了证据。这些结果表明，软XAS非常适合研究水性有机大分子中不同官能团的电子态。
