Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown-6

Abstract

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K(+)•DB18C6•(H2O)n (n = 1-5), in a cold, 22-pole ion trap. We also present for comparison spectra of Rb(+)•DB18C6•(H2O)3 and Cs(+)•DB18C6•(H2O)3 complexes. We determine the number and the structure of conformers by analyzing the spectra with the aid of quantum chemical calculations. The K(+)•DB18C6•(H2O)1 complex has only one conformer under the conditions of our experiment. For K(+)•DB18C6•(H2O)n with n = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb(+)•DB18C6•(H2O)3 and Cs(+)•DB18C6•(H2O)3 each exhibit only one isomer. The difference can be explained by the optimum matching in size between the K(+) ion and the crown cavity; because the K(+) ion can be deeply encapsulated by DB18C6 and the interaction between the K(+) ion and the H2O molecules becomes weak, different kinds of hydration geometries can occur for the K(+)•DB18C6 complex, giving multiple conformations in the experiment. For K(+)•DB18C6•(H2O)n (n = 4 and 5) complexes, only a single isomer is found. This is attributed to a cooperative effect of the H2O molecules on the hydration of K(+)•DB18C6; the H2O molecules form a ring, which is bound on top of the K(+)•DB18C6 complex. According to the stable structure determined in this study, the K(+) ion in the K(+)•DB18C6•(H2O)n complexes tends to be pulled largely out from the crown cavity by the H2O molecules with increasing n. Multiple conformations observed for the K(+) complexes will have an advantage for the effective capture of the K(+) ion over the other alkali metal ions by DB18C6 because of entropic effects on the formation of hydrated complexes.

Microheterogeneity within conformational states of ubiquitin revealed by high resolution trapped ion mobility spectrometry

“…1,2 Though ions transition through various extents of solvation during the ESI process, [3][4][5][6] several studies have shown that ions produced by ESI can retain a memory of their solution conditions [7][8][9][10][11] and in some cases, major features of their solution structure up to seconds after desolvation. [12][13][14][15][16][17][18] Implicated in the preservation of native states are evaporative cooling leading to kinetic trapping on the experimental timescale, 4,5,[19][20][21][22][23][24][25] the relative stability of the native fold, 26 and intramolecular interactions that stabilize dry ions after removal of remnant solvent from the charge sites. 5,22,[27][28][29] In other cases, gaseous protein structures may be entirely different from their solution phase counterparts because the global energy minimum in solution may only represent a local minimum in the gas phase.…”

Photofragment imaging from mass-selected ions using a reflectron mass spectrometer. II: Formation mechanism of MgF+ in the photodissociation of Mg+FCH3 complex

“…In the ultraviolet photodissociation of Mg + FCH 3 by a 4.66-eV photon, CH 3 + was also observed in the mass spectrum of photofragments in addition to MgF + . However, in the present study, we only calculated the potential energy curve which has a dissociation limit of MgF + + CH 3 .…”

“…One of the purposes of these studies is to elucidate the geometrical structures of metal-molecule cluster ions by infrared photodissociation spectroscopy [1][2][3]. Another purpose is to study the electronic excited states of metal-molecule cluster ions by measuring the electronic spectra with scanning the wavelength of a photolysis laser in the ultraviolet and visible regions [4][5][6][7].…”

“…In these structures, positive charge is almost localized on the Mg atom. It is worth noting that several fragment ions were observed in these ultraviolet photodissociation of Mg + XCH 3 . For example, MgF + and CH 3 + fragment ions were predominantly observed in the photodissociation of Mg + FCH 3 complex ions in all states [4,5].…”

UV photodissociation spectroscopy of cryogenically cooled gas phase host–guest complex ions of crown ethers

“…[18][19][20][21][22][23][24][25][26][27] The cold gas phase complexes are generated either by the supersonic expansion technique for neutral complexes [18][19][20][21][22][23][24] or by the electrospray ionization (ESI)/cold ion-trap method for ionic complexes. [18][19][20][21][22][23][24][25][26][27] The cold gas phase complexes are generated either by the supersonic expansion technique for neutral complexes [18][19][20][21][22][23][24] or by the electrospray ionization (ESI)/cold ion-trap method for ionic complexes.…”