Cryogenic ion mobility-mass spectrometry for the study of conformations of host-guest complexes

Abstract

Variable-temperature cryogenic ion mobility-mass spectrometry (VT-Cryo-IM-MS) was developed to examine the conformations of host-guest complexes. The VT-Cryo-IM-MS consists of an electrospray ionization source, a quadrupole ion trap, a cryogenic ion drift tube, and a time-of-flight mass spectrometer. The ion drift tube was cooled to 83 K with liquid nitrogen. Two sets of ion funnels are installed at the source and the drift tube to focus the ion beam. We used Li+-encapsulated fullerene (Li+@C60) as a reference ion to examine the ion-mobility resolution of the VT-Cryo-IM-MS apparatus because this ion has a rigid structure with a single conformer. The temperature dependence of collision cross section (CCS) of Li+@C60 and the resolution of the apparatus were determined at each temperature. We examined the conformations of host-guest complexes of a series of metal ions (M = Ca2+, Na+, Ag+, K+, Rb+, and Cs+) with dibenzo-24-crown-8 (DB24C8) by Cryo-IM-MS. All CCS distributions were fitted with two Gaussian functions which are assigned to closed and open conformers. These closed and open conformers have different distances between the two benzene rings of DB24C8. The abundance ratio of closed conformers for these M(DB24C8) complex ions depends on the guest M ion with an order, Ca2+ < Na+ < Ag+ < Cs+ < Rb+ < K+. The stability of the closed conformer of K+(DB24C8) originates from the benzene ring positional relationship which is quite close to that of the free benzene dimer. This special benzene dimer-like conformation produces the strong π-π interaction between the benzene rings of K+(DB24C8).

Highly Efficient Intramolecular Proton Transfer in p-Aminobenzoic Acid by a Single Ammonia Molecule as a Vehicle

“…Details of the experimental setup were given elsewhere. , PABA·H + ions were generated by a homemade electrospray ionization source. A solution of 1 mM PABA dissolved in acetonitrile was used as the sample solution.…”

Intra-host π–π interactions in crown ether complexes revealed by cryogenic ion mobility-mass spectrometry

“…These results were nearly identical to those of the closed conformer of K + (DB24C8) becoming particularly stable due to the special orientation of benzene rings in the complex. 30 To visualize the p-p interaction between the two aromatic rings in the complexes, the NCI analysis was conducted. The RDG isosurfaces of the closed conformers of K + (DB24C8) and K + (DN24C8) are shown in Fig.…”

“…The coexistence of two conformers, open and closed conformers, were confirmed in the study, which have long and short distances between the two benzene rings in a molecule, respectively. 30 These conformers were also assigned by a UVPD spectrum of M + (DB24C8) at B10 K. 46 For the closed conformer of K + (DB24C8), the relative orientation of the two benzene rings in the complex was found to be quite close to that of the benzene dimer. Therefore, this closed conformer was concluded to be highly stabilized by the strong p-p interaction between the benzene rings due to its special benzene dimer-like conformation.…”

“…Cryogenic IM-MS has also been performed using a drift tube cooled by liquid nitrogen. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Crown ether complexes with metal ions are one of the most common host-guest compounds. The encapsulation structures of crown ether complexes in crystal forms were studied by X-ray diffraction.…”

“…[33][34][35][36] Nuclear magnetic resonance studies of structures of the conformations of the dibenzo-30-crown-10 (DB30C10) complexes with alkali metal ions, M (M = Na and K), M + (DB30C10), in solution also showed the same configuration of DB30C10 as that reported for the crystal. 37,38 The conformation of the crown ether complexes in the gas phase was studied by ultraviolet photodissociation (UVPD) 26,[39][40][41][42][43][44][45][46] and IM-MS. [8][9][10][26][27][28][29][30] Previously, we studied the conformations of M + (DB24C8) (M = Na, K, Rb, and Cs) using cryogenic IM-MS. The coexistence of two conformers, open and closed conformers, were confirmed in the study, which have long and short distances between the two benzene rings in a molecule, respectively.…”