Inclusion complexes of spin-labeled indoles with cyclodextrins in aqueous solutions

Abstract

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.

Electron Spin Resonance.: Part One: A Diagnostic Method in the Biomedical Sciences

“…On the basis of ESR spectral simulation, it was possible to determine the orientation of the indole ring in relation to the plane of the host molecule and the rotational diffusion coefficients of the complexes, which corresponded to the hydrodynamic volume of one gamma-CD molecule. In contrast to the complexes with gamma-CD the rotational correlation times, T, of the complexes with beta-CD correspond to a hydrodynamic volume which much exceeds the volume of a single beta-CD molecule 157 . The synthesis is reported of two isomers of permethylated beta-cyclodextrin, labelled with paramagnetic TEMPO moieties attached on the same rim of the cavity, whose host-guest properties were subsequently investigated by EPR spectroscopy 158 .…”

ESR, STESR, DFT, and MD Study of the Dynamical Structure of Cucurbituril[7]–Spin Probe Guest–Host Complexes

“…From these figures, it can be observed that, generally, there is a satisfactory agreement between the experimental and simulated ESR spectra. However, as was emphasized earlier, 24,25,34 the parameters of the molecular rotation of the spin probes, rotational frequencies, and angular amplitudes cannot be determined unambiguously using the A zz ′ (or S) values, since the same A zz ′ or S values can be obtained by varying the slow rotation frequency in the full solid angle (4π) or by varying the angular amplitude in the fast librations model. In particular, it is difficult to correctly and unambiguously choose between these two limiting models of molecular motion for the often-occurring case of high S values (S ≥ 0.9) from the simulations of the conventional first-harmonic ESR spectra because of the large inhomogeneous broadening.…”

“…Sample capillaries were positioned along the symmetry axis of the standard 4 mm quartz ESR sample tube that contained mineral oil for thermal stability. The root-meansquare microwave magnetic field ⟨B 1 2 ⟩ 1/2 measurement for a "point" sample of aqueous peroxylamine disulfonate with known T 1 and T 2 24 values gave a maximum value of 0.51 G. The low-power ESR spectra were recorded at a power of 26 dB (H 1 ≅ 0.0255 G) and a small modulation amplitude, which rules out spectral distortion. The STESR spectra, i.e., the ESR absorption spectra detected on the second harmonics of modulation in the 90°phase with a modulation field (V 2 ′), were recorded under partial microwave saturation, essentially as described in ref 27.…”

Electron spin resonance

“…In contrast to the complexes with gamma-CD the rotational correlation times, T, of the complexes with beta-CD correspond to a hydrodynamic volume which much exceeds the volume of a single beta-CD molecule. 39 The synthesis is reported of two isomers of permethylated beta-cyclodextrin, labelled with paramagnetic TEM-PO moieties attached on the same rim of the cavity, whose host-guest properties were subsequently investigated by ESR spectroscopy. 40 It is known that the ESR spectra of cyclodextrins (CD) labelled with TEMPO are sensitive to the incorporation of large guest molecules.…”