Inclusion complexes of spin labeled pyrrolidine (1) and piperidine containing (2) indole derivatives with β cyclodextrin and γ cyclodextrin (CD) were prepared in the solid phase and studied by ESR in a wide temperature interval. For all complexes and free spin probes in solvents of different polarity, local environment polarities of the NO group of the guest mol ecules were determined from the outer extrema separations in the ESR spectra measured at 77 K. From analysis of the Saturation Transfer (ST) ESR spectra and temperature depen dences of linear ESR spectra of the complexes it follows that both guest molecules in γ CD undergo rapid librations. The libration amplitude of the p orbit axis of the NO group in molecule 2 increases with temperature and reaches about 16° at 333 K. The ESR lineshape of the β CD complexes depends on the spin probe/β CD molar ratio (ρ) even at ρ < 0.01. Lineshape analysis of the spectra recorded at different ρ showed that they consist of two components, one of them corresponding to strong spin spin interaction between guest mol ecules and the other corresponding to almost absence of this interaction. The spectral compo nents can be attributed to microphases of the complexes and to isolated complexes in the β CD matrix, respectively. Simulation of the ST ESR and linear ESR spectra of the magnetically diluted complexes showed that the guest motion inside the CD cavity is better described by rotational jumps rather than Brownian diffusion in an orientation potential. In the temperature range 238-333 K, the rotational frequencies of 1 and 2 are in intervals 1.8•10 7 -6•10 7 s -1 and 4•10 7 -1.3•10 8 s -1 , respectively. The rotation occurs over the whole solid angle. Signifi cant differences in the character of molecular dynamics in the γ CD and β CD complexes can be explained by different stoichiometry, namely, 1 : 1 for the former and 2 : 1 for the latter and by different orientation of guest molecules in the complexes. In both cyclodextrins the rotational mobility of molecules 2 is higher than that of 1 owing to intramolecular conforma tional transitions in the piperidine ring of 2 and steric hindrances produced by the methyl group in 1.
Guest host complexes of β and γ cyclodextrins (CDs) with two spin labeled indole de rivatives having the same molecular weights but different structures were studied by EPR spectroscopy in aqueous solutions and semiempirical quantum chemical calculations of these systems were carried out. In the presence of CD the polarity of the NO group environment decreases and the rotational correlation time (τ) of guest molecules increases. Both indole derivatives form 1 : 1 complexes with γ CD, the binding constants of the complexes being different more than twice. Simulation of EPR spectra made it possible to determine the indole ring orientation relative to the plane of the host molecule (at angles in the range 30-60°) and the rotational diffusion coefficients of the complexes, which corresponded to the hydro dynamic volume of one γ CD molecule. In contrast to the complexes with γ CD the rotational correlation times, τ, of the complexes with β CD correspond to a hydrodynamic volume which much exceeds the volume of a single β CD molecule. The complexes with β CD are also characterized by more hydrophobic environment for guest molecules and absence of spin exchange with Ni 2+ ions in the aqueous solution. There results are consistent with a dimeric structure of β CD in the complex and with the orientation of the long axis of the guest molecule along the dimer axis. The energies and geometric parameters were calculated for all complexes by the PM3 method with a conventional set of parameters. The optimized energeti cally stable structures of the 1 : 1 complexes with γ CD and of the 1 : 2 complexes with β CD are consistent with experimental data.
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