This work presents a systematic study on the host-guest interactions between the macrocyclic host molecule cucurbit[7]uril (CB[7]) and amino acids (AAs) including three basic AAs (Lys, Arg, and His) and three aromatic AAs (Phe, Tyr, and Trp) to elucidate the origin of the high selectivity of CB[7] toward AA residues in proteins. Complex formation between CB[7] and each AA was examined in solution (by isothermal titration calorimetry and NMR) as well as in the gas phase (by ion mobility mass spectrometry and collision-induced dissociation), and the results were further combined with computational investigations. Generally, the aromatic AAs show higher binding affinities than the basic AAs in buffer solutions with various pH values. On the contrary, the gas-phase stabilities of the basic AA complex ions are higher than those of the aromatic AA complex ions, suggesting that the direct ion-dipole interactions between the charged side chains of the basic AAs and the polar carbonyl groups of CB[7] predominate in the absence of water. The ion-dipole interactions are less significant in water, since the original interactions of the guests with water are lost upon complex formation. In contrast, the transfer of the hydrophobic groups from the bulk into the hydrophobic CB[7] cavity suffers less from the desolvation penalty, resulting in higher binding affinities in water. Therefore, initial guest solvation is another key factor which should be considered when designing high-affinity host-guest systems, in addition to the contribution from the release of high-energy water molecules from the CB[7] cavity (J. Am. Chem. Soc. 2012, 134, 15318-15323).
The investigation of ion structures based on a combination of ion mobility mass spectrometry (IM-MS) experiments and theoretical collision cross section (CCS) calculations has become important to many fields of research. However, the accuracy of current CCS calculations for ions in nitrogen drift gas limits the information content of many experiments. In particular, few studies have evaluated and attempted to improve the theoretical tools for CCS calculation in nitrogen drift gas. In this study, based on high-quality experimental measurements and theoretical modeling, a comprehensive evaluation of various aspects of CCS calculations in nitrogen drift gas is performed. It is shown that the modification of the ion-nitrogen van der Waals (vdW) interaction potential enables accurate CCS predictions of 29 small ions with ca. 3% maximum relative error. The present method exhibits no apparent systematic bias with respect to ion CCS (size) and dipole moment, suggesting that the method adequately describes the long-range interactions between the ions and the buffer gas. However, the method shows limitations in reproducing experimental CCS at low temperatures (<150 K) and for macromolecular ions, and calculations for these cases should be complemented by CCS calculation methods in helium drift gas. This study presents an accurate and well-characterized CCS calculation method for ions in nitrogen drift gas that is expected to become an important tool for ion structural characterization and molecular identification. The experimental values reported here also provide a foundation for future studies aiming at developing more efficient computational tools.
d-/l-Isoleucine chirality could be differentiated using IRMPD spectroscopy, ion-mobility mass spectrometry, and DFT calculations for the permethylated β-cyclodextrin non-covalent complex.
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