Dearden D V, Liang Y, Nicoll J B, Kellersberger K A
Department of Chemistry & Biochemistry, C100 Benson Science Building, Brigham Young University, Provo, UT 84602-5700, USA.
J Mass Spectrom. 2001 Sep;36(9):989-97. doi: 10.1002/jms.215.
The application of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry to the quantitative study of molecular recognition in the gas phase is reviewed. Because most quantitative measurements are dependent on accurate determination of the pressure of a neutral reagent, methods for accurate pressure measurement in FTICR, including gauge calibration using a reaction with known rate constants (the traditional method), exothermic proton transfer rate measurement (often the best method when accurate neutral pressures in the trapping cell are desired), and linewidth measurement (a little-used, but generally applicable method) are discussed. The use of rate constant measurements in molecular recognition is illustrated with examples employing natural abundance isotopic labeling to study self-exchange and 2 : 1 ligand:metal complex formation kinetics in crown ether-alkali cation systems. Self-exchange rates do not correlate with alkali cation/crown cavity size relationships, whereas 2 : 1 complex formation kinetics correlate strongly with size relationships. The use of exchange equilibrium constant measurements to characterize molecular recognition is illustrated by alkali cation exchanges between 18-crown-6 and the isomers of dicyclohexano-18-crown-6. These experiments show that the alkyl-substituted ligand binds alkali cations better than unsubstituted 18-crown-6 in the gas phase, in accordance with expectations based on the higher polarizability of the alkyl-substituted ligand. Further, the metal binding thermochemistry differs for the two dicyclohexano-18-crown-6 isomers, with the bowl-shaped cis-syn-cis isomer binding all the alkali cations more strongly than the cis-anti-cis isomer. The measurement of entropies and enthalpies associated with one of the most subtle forms of molecular recognition, enantiomeric discrimination, is illustrated by studies of the discrimination between enantiomers of chiral amines by dimethyldiketopyridino-18-crown-6. This chiral ligand binds chiral primary ammonium cations that have the opposite absolute configuration at their stereocenter more strongly than the enantiomer with the same absolute configuration. Gas-phase studies show that this enantiomeric discrimination is enthalpic in origin, likely related to more favorable pi-pi stacking for the preferred enantiomer. Entropy disfavors binding of the preferred enantiomer.
本文综述了傅里叶变换离子回旋共振（FTICR）质谱在气相中分子识别定量研究中的应用。由于大多数定量测量依赖于对中性试剂压力的精确测定，因此讨论了FTICR中精确压力测量的方法，包括使用具有已知速率常数的反应进行压力计校准（传统方法）、放热质子转移速率测量（当需要精确测定捕获池中中性压力时通常是最佳方法）以及线宽测量（一种较少使用但普遍适用的方法）。通过使用天然丰度同位素标记研究冠醚 - 碱金属阳离子体系中的自交换和2:1配体:金属配合物形成动力学的示例，说明了速率常数测量在分子识别中的应用。自交换速率与碱金属阳离子/冠醚腔尺寸关系不相关，而2:1配合物形成动力学与尺寸关系密切相关。通过18 - 冠 - 6与二环己基 - 18 - 冠 - 6异构体之间的碱金属阳离子交换，说明了使用交换平衡常数测量来表征分子识别。这些实验表明，在气相中，烷基取代的配体比未取代的18 - 冠 - 6更好地结合碱金属阳离子，这与基于烷基取代配体更高极化率的预期一致。此外，两种二环己基 - 18 - 冠 - 6异构体的金属结合热化学不同，碗状的顺 - 顺 - 顺异构体比顺 - 反 - 顺异构体更强烈地结合所有碱金属阳离子。通过二甲基二酮吡啶基 - 18 - 冠 - 6对手性胺对映体的区分研究，说明了与最微妙的分子识别形式之一，即对映体识别相关的熵和焓的测量。这种手性配体与在其立体中心具有相反绝对构型的手性伯铵阳离子的结合比具有相同绝对构型的对映体更强。气相研究表明，这种对映体识别起源于焓，可能与优选对映体更有利的π - π堆积有关。熵不利于优选对映体的结合。
