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 contain ing 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)-C 6 H 6 (C 6 H 12 ) 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 C 6 H 6 or C 6 H 12 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 possi bility of substantial modification of the molecular dynamics and hydrophobicity of the envi ronment of "functional" guest molecules by introducing a second regulatory guest molecule into the CD cavity.Studies of the inclusion complexes of cyclodextrins (CD) have aroused considerable interest in recent years. The solubility of guest molecules may increase by several orders of magnitude upon inclusion into the CD cavity; simultaneously, their physical and chemical properties considerably change. 1-10 The inclusion complexes with CD have been the subjects of numerous studies carried out by various methods (see, for example, Refs 1-7); however, important aspects of their structure and espe cially the dynamics of guest molecules in the CD cavi ties 8-12 remain obscure. The use of spin labeled guest molecules is a promising approach to the study of the structure and molecular dynamics of the inclusion com plexes. The ESR spectra of spin labels are highly sensitive to rotational dynamics over a broad range of correlation times and to the environment polarity and intermolecular contacts. 13a Previously, 14,15 we studied the structure and molecular dynamics of the binary complexes of spin la beled indole analogs containing piperidine and pyrrolidine radical fragments with β and γ CD in an aqueous solu tion and determined the stoichiometry, the binding con stants, the polarity of the environment, and the accessi bility of the reporter NO group of guest molecules for water soluble paramagnetic ions. In the solid phase where the same complexes form a dilute solid solution in the CD, the molecular dynamics of guest molecules in the β and γ CD cavities have been studied ov...

Ternary inclusion complexes of cyclodextrins with spin-labeled indoles and hydrocarbons. The formation of 2: 1: 2 complexes and partial dissociation of spin-labeled guests in the presence of a hydrocarbon liquid phase

“…1 The resulting temperature dependences are fitted satis factorily with straigt lines in the Arrhenius coordinates (Fig. 6).…”

“…These complexes are obtained with the use of a large excess of CD and hydro carbon. 1 Since the ESR lineshapes of the complexes are indicative of a high magnetic dilution (the absence of dipole-dipole interaction between the spin labeled guests), the sample can be considered as the solid phase of the CD-hydrocarbon binary complexes doped with ter nary complexes containing spin labeled indoles.…”

“…1 The kinetics of variation of the ESR spectra in the presence of the hydrocarbon liquid phase was studied as follows: a specified amount (10 µL) of C 6 H 6 or C 6 H 12 was added into a tube with a finely dispersed powder of the initial ternary com plex and, after 2 min, the ESR spectra were recorded at 295 K. In subsequent experiments, the liquid hydrocarbon was removed by evaporation in an argon flow.…”

“…The structures of both the host mol ecule (β or γ CD) and the two guest molecules were varied in this study. Two derivatives containing pyrrol idine N oxyl (1) and piperidine N oxyl (2) radical rings were used as spin labeled indoles and benzene and cyclo hexane were used as hydrocarbons. It was found 1 that all four complexes with γ CD and one complex with β CD exhibit two types of ESR signals corresponding to spin labeled guests with different rotational mobilities, whereas for two other complexes with β CD (2β CD-2-C 6 H 6 and 2β CD-2-C 6 H 12 ), only one ESR signal correspond ing to complexes with higher mobility of spin labeled indole is recorded.…”

“…1 To this end, the influence of the hydrocarbon liquid phase on the ESR spectra of spin labeled indoles in the β(γ) CD-1(2)-C 6 H 6 (C 6 H 12 ) ter nary complexes was studied under steady state and ki netic conditions and quantum chemical calculations for the complexes of different composition and stoichiometry were carried out.…”