We present comprehensive measurements of the C (carbon) K edge near-edge X-ray absorption (NEXAFS) spectra of all 20 amino acids commonly occurring in nature. Qualitative trends among the spectra of amino acids with similar chemical character are identified and spectral features are compared with extensive ab initio calculations. The contributions of individual units and substitutional groups have been determined to explore their fingerprinting character using the building block concept. Several such units are found. Two that give particularly clear features in the C 1s NEXAFS spectra are the carboxyl group (which can be clearly identified by a pronounced structure due to the C 1s→π~*c=o transition with maximum at 288.65(5) eV) and modified phenol rings in aromatic amino acids (which give sharp C 1s→π~*c=c structures). The latter transitions are located around 285 eV, and their shape is specific for each aromatic amino acid. Other building blocks, such as the CNH_n group and the CH, CC, CO, CN pair bonds, are also identified, although their characteristic features are less pronounced in the C K edge spectra than the carboxylic and aromatic structures. This study provides the basis for rigorous assignment of the NEXAFS spectra of the amino acids, and will be helpful in developing X-ray absorption spectroscopy for quantitative analysis of proteins.
展开▼
机译：我们目前对自然界中常见的所有20种氨基酸的C（碳）K边缘近边缘X射线吸收（NEXAFS）光谱进行全面测量。确定具有相似化学特征的氨基酸光谱之间的定性趋势，并将光谱特征与大量的从头算进行比较。已经确定了各个单元和取代基团的贡献，以使用构件概念来探索其指纹特征。找到了几个这样的单位。羧基（在C 1s NEXAFS光谱中具有特别清晰的特征的两个基团）可以被羧基修饰（由于C 1s→π〜* c = o跃迁，最大值为288.65（5）eV，因此可以通过明显的结构明确识别）芳香族氨基酸中的苯酚环（产生清晰的C 1s→π〜* c = c结构）。后者的跃迁位于285 eV左右，它们的形状对于每种芳香族氨基酸都是特定的。还确定了其他构件，例如CNH_n基团和CH，CC，CO，CN对键，尽管它们的特征在C K边缘光谱中不如羧酸和芳族结构明显。这项研究为氨基酸的NEXAFS光谱的严格分配提供了基础，并且将有助于开发X射线吸收光谱法对蛋白质进行定量分析。
展开▼
