Russian Chemical Bulletin, International Edition, Vol. 54, No. 5, pp. 1169—1177, May, 2005 1169 Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1139—1147, May, 2005. 1066-5285/05/5405-1169 © 2005 Springer Science+Business Media, Inc. Inclusion complexes of spin-labeled 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 Photochemistry Center, Russian Academy of Sciences, 7a ul. Novatorov, 119421 Moscow, Russian Federation. Fax: +7 (095) 936 1255. E-mail: livsh@photonics.ru 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. Key words: EPR spectroscopy, spin probes, cyclodextrins, guest—host complexes, indoles, rotation dynamics, semiempirical quantum-chemical calculations. Cyclodextrins (CDs) are cyclic glucose oligomers. Due to the presence of a hydrophobic cavity in the molecule they can form guest—host complexes with a variety of organic compounds. 1—6 There are several reasons for re- cent considerable interest in these complexes. First, for many hydrophobic compounds binding to CDs dramati- cally increases the solubility in water, thus making pos- sible delivery of various pharmaceuticals 7 or extraction of biologically important hydrophobic molecules, such as cholesterol, from biomembranes. 8 Second, inclusion into the CD cavity causes significant changes in the photophysical properties and reactivities of guest mol- ecules. 1—6,9—12 For instance, the quantum yield of fluo- rescence of aromatic hydrocarbons and heterocyclic com- pounds increases 9 and naphthalene in ternary complexes containing two guest molecules (naphthalene and cyclo- hexane) exhibits room-temperature phosphorescence. 10 In the presence of CDs, catalytic hydrolysis of esters is accelerated 11 and some short-lived radicals are stabilized. 12 Inclusion complexes of CDs with various classes of compounds have been the subject of numerous studies by various methods including EPR spectroscopy (see re- views 1—7,9 and studies 12—18 ). The use of spin-labeled guest molecules is of particular interest, because their EPR spec- tra allow not only the thermodynamic binding parameters and local environment polarity but also the dynamics of guest molecules inside the cavity to be determined. This is important for understanding of the character of guest—host interaction, the relaxation mechanisms of electronic excitation, and the reactivity of guest molecules. One of the important classes of guest molecules com- prises the derivatives of an aromatic heterocycle indole. Indole, a constituent of tryptophan amino acid, fluo- resces, which allows using spin-labeled indole derivatives as double (optical and spin) molecular probes. The aim of the present study was to carry out an EPR study of inclu- sion complexes formed by two spin-labeled indole deriva- tives having the same molecular weight but different