Structure and molecular dynamics of the ternary complexes of cyclodextrins with spin-labeled indoles and hydrocarbons in the solid phase. ESR spectroscopy and quantum chemical calculations

Abstract

The ternary inclusion complexes of two spin-labeled pyrrolidine-and piperidine-containing indole derivatives (1 and 2, respectively) and two hydrocarbons, benzene and cyclohexane, with γ-and β-cyclodextrins (CD) (altogether eight complexes) were prepared and studied by ESR in the solid phase over a broad temperature range. For most ternary complexes, the hydrophobicity of the NO group environment is much higher than for binary complexes devoid of hydrocarbons. The rotational mobility of both spin-labeled indoles in the ternary complexes of γ-CD is much greater than in binary complexes of γ-CD, which is due to transition to the stoichiometry 2γ-CD-1(2)-C₆H₆(C₆H₁₂) from 1: 1 stoichiometry. The guest rotational mobility in the complexes with either of the CD is higher for 2 than for 1. The saturation transfer ESR spectra show that the rotational mobility of 1 in γ-and β-CD in the presence of C₆H₆ or C₆H₁₂ has a character of fast librations in an angular cone, whose amplitude increases with temperature, whereas for radical 2, the rotation occurs in the full solid angle. The structures and energies of all complexes were calculated for different modes of inclusion of guest molecules using the PM3 method with the standard set of parameters. The calculation results are in qualitative agreement with experimental data. The results demonstrate the possibility of substantial modification of the molecular dynamics and hydrophobicity of the environment of “functional” guest molecules by introducing a second regulatory guest molecule into the CD cavity.