Flick Tawnya G, Campuzano Iain D G, Bartberger Michael D
†Department of Oral Attribute Sciences, Amgen, Inc., Thousand Oaks, California 91320, United States.
‡Department of Molecular Structure and Characterization, Amgen, Inc., Thousand Oaks, California 91320, United States.
Anal Chem. 2015 Mar 17;87(6):3300-7. doi: 10.1021/ac5043285. Epub 2015 Mar 3.
The chirality of substituents on an amino acid can significantly change its mode of binding to a metal ion, as shown here experimentally by traveling wave ion mobility spectrometry-mass spectrometry (TWIMS-MS) of different proline isomeric molecules complexed with alkali metal ions. Baseline separation of the cis- and trans- forms of both hydroxyproline and fluoroproline was achieved using TWIMS-MS via metal ion cationization (Li(+), Na(+), K(+), and Cs(+)). Density functional theory calculations indicate that differentiation of these diastereomers is a result of the stabilization of differing metal-complexed forms adopted by the diastereomers when cationized by an alkali metal cation, M + X where X = Li, Na, K, and Cs, versus the topologically similar structures of the protonated molecules, M + H. Metal-cationized trans-proline variants exist in a linear salt-bridge form where the metal ion interacts with a deprotonated carboxylic acid and the proton is displaced onto the nitrogen atom of the pyrrolidine ring. In contrast, metal-cationized cis-proline variants adopt a compact structure where the carbonyl of the carboxylic acid, nitrogen atom, and if available, the hydroxyl and fluorine substituent solvate the metal ion. Experimentally, it was observed that the resolution between alkali metal-cationized cis- and trans-proline variants decreases as the size of the metal ion increases. Density functional theory demonstrates that this is due to the decreasing stability of the compact charge-solvated cis-proline structure with increased metal ion radius, likely a result of steric hindrance and/or weaker binding to the larger metal ion. Furthermore, the unique structures adopted by the alkali metal-cationized cis- and trans-proline variants results in these molecules having significantly different quantum mechanically calculated dipole moments, a factor that can be further exploited to improve the diastereomeric resolution when utilizing a drift gas with a higher polarizability constant.
氨基酸上取代基的手性可显著改变其与金属离子的结合模式，此处通过与碱金属离子络合的不同脯氨酸异构体分子的行波离子迁移谱-质谱（TWIMS-MS）实验证明了这一点。使用TWIMS-MS通过金属离子阳离子化（Li(+)、Na(+)、K(+)和Cs(+)）实现了羟脯氨酸和氟脯氨酸顺式和反式异构体的基线分离。密度泛函理论计算表明，这些非对映异构体的区分是由于非对映异构体在被碱金属阳离子阳离子化时（M + X，其中X = Li、Na、K和Cs）所采用的不同金属络合形式的稳定性，与质子化分子拓扑相似的结构M + H相对比。金属阳离子化的反式脯氨酸变体以线性盐桥形式存在，其中金属离子与去质子化的羧酸相互作用，质子转移到吡咯烷环的氮原子上。相比之下，金属阳离子化的顺式脯氨酸变体采用紧凑结构，其中羧酸的羰基、氮原子以及（如有）羟基和氟取代基溶剂化金属离子。实验观察到，随着金属离子尺寸的增加，碱金属阳离子化的顺式和反式脯氨酸变体之间的分辨率降低。密度泛函理论表明，这是由于随着金属离子半径增加，紧凑的电荷溶剂化顺式脯氨酸结构的稳定性降低，这可能是空间位阻和/或与较大金属离子较弱结合的结果。此外，碱金属阳离子化的顺式和反式脯氨酸变体所采用的独特结构导致这些分子具有显著不同的量子力学计算偶极矩，当使用具有较高极化率常数的漂移气体时，这一因素可进一步用于提高非对映异构体分辨率。
