Unveiling host-guest-solvent interactions in solution by identifying highly unstable host-guest configurations in thermal non-equilibrium gas phase

Abstract

We propose a novel scheme of examining the host-guest-solvent interactions in solution from their gas phase structures. By adopting the permethylated β-cyclodextrin (perm β-CD)-protonated L-Lysine non-covalent complex as a prototypical system, we present the infrared multiple photon dissociation (IRMPD) spectrum of the gas phase complex produced by electrospray ionization technique. In order to elucidate the structure of perm β-CD)/LysH⁺ complex in the gas phase, we carry out quantum chemical calculations to assign the two strong peaks at 3,340 and 3,560 cm^-1 in the IRMPD spectrum, finding that the carboxyl forms loose hydrogen bonding with the perm β-CD, whereas the ammonium group of L-Lysine is away from the perm β-CD unit. By simulating the structures of perm β-CD/H⁺/L-Lysine complex in solution using the supramolecule/continuum model, we find that the extremely unstable gas phase structure corresponds to the most stable conformer in solution.

Figure 1

(a) Theoretical IR spectrum, (b) normal modes, and (c) two views of the structure of Complex I of gas phase perm β-CD–protonated L-Lys. Red sticks represent the positions of experimental IRMPD bands. Hydrogen atoms in permetylated β-cyclodextrin are omitted for clarity. Red: oxygen, blue: nitrogen, gray: carbon, white: hydrogen.

Figure 2

(a) Theoretical IR spectra and (b) structures of lower Gibbs free energy perm β-CD–protonated L-Lys complexes II and III in gas phase.

Figure 3

Two views of the structures (Complex A, B and C) of perm β-CD–protonated L-Lys complexes in solution, based on the gas phase (a) Complex I, (b) Complex II, and (c) Complex III, respectively. Pale blue background represents solvent continuum.

Figure 4

Comparison of perm-CD–LysH⁺ configurations in solution (Complex A, wB97X-D/6-31G*) and in gas phase (Complex I, wB97X-D/6-311G*).