Messer B M, Cappa C D, Smith J D, Drisdell W S, Schwartz C P, Cohen R C, Saykally R J
Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Phys Chem B. 2005 Nov 24;109(46):21640-6. doi: 10.1021/jp053802v.
The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an "acceptor-only" configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s --> sigma(NH) preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s --> pi(CN) resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the alpha amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.
通过中性和高pH条件下的全电子产额近边X射线吸收精细结构（NEXAFS）光谱，测量了脯氨酸和二甘氨酸水溶液的氮K边光谱。通过与最近报道的甘氨酸水溶液光谱以及分离氨基酸和水合氨基酸簇的计算光谱进行比较，对所有观察到的光谱特征进行了归属。在阴离子甘氨酸光谱中观察到的401.3和402.6 eV处的尖锐前缘共振表明，在高pH条件下，氮末端处于“仅受体”构型，其中两个胺质子均未参与与溶剂的氢键形成。在阴离子脯氨酸的NEXAFS光谱中不存在类似的1s→σ(NH)前缘跃迁，这意味着在阴离子甘氨酸中观察到的仅受体构象是由胺末端围绕甘氨酸C-N键的自由旋转引起的空间屏蔽所致。阴离子二甘氨酸溶液在401.2 eV处表现出展宽的1s→π(CN)共振，在403 eV处表现出宽肩共振，这也表明存在仅受体物种。尽管这种归属不像对甘氨酸那样明确，但它意味着大多数蛋白质的氮末端在高pH条件下能够以仅受体构象存在。还测量了两性离子赖氨酸溶液的NEXAFS光谱，其表现出与阴离子和两性离子甘氨酸相似的特征，这使我们得出结论，α-胺基以仅受体构型存在，而丁铵侧链末端被完全溶剂化。
