Year 2005, Month 5 (Section Full Articles, Page 1169)
Inclusion complexes of spinlabeled indoles
with cyclodextrins in aqueous solutions
V. A. Livshits,* B. G. Dzikovski, V. G. Avakyan, E. A. Samardak,
E. Yu. Polyakova, V. Yu. Rudyak, and M. V. Alfimov
Guest-host complexes of β- and γ-cyclodextrins (CDs) with two spin-labeled indole derivatives 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 hydrodynamic 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 Ni2+ 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 energetically 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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