The Binding Behavior of Cyclodextrins toward a Nitroxide Spin Probe in the Presence of Different Alcohols As Studied by EPR

Abstract

Stability constants, rates of association and dissociation, and thermodynamic and activation parameters for the formation of inclusion complexes between the radical guest, N-benzyl- tert-butyl- d 9-nitroxide and beta- or 2,6- O-dimethyl-beta-cyclodextrin (CDs), have been determined by EPR spectroscopy in water in the presence of 14 different alcohols, differing in size and lipophilicity. In all cases, it was found that addition of alcohol, depending on its structure and concentration, causes a reduction of the stability of the paramagnetic complex. Global analysis of EPR data allowed us to explain the CDs binding behavior: we discarded the formation of a ternary complex, where alcohol and radical guest are coincluded into CD cavity, while data were found more consistent with the formation of a binary complex alcohol:CD competing with the monitored complex nitroxide:CD. Both kinetic and thermodynamic analysis of the experimental results have revealed that the presence of alcohols affects to a larger extent the dissociation rather then the association of radical probe and CD and that the former process is of greater importance in determining the stability of the complex, this confirming the reliability of the competition model proposed. This competition has been used for the indirect determination of the stability constants of complexes between CD and examined alcohols. By using a similar approach, we showed EPR spectroscopy can be considered a rapid and accurate technique to investigate the CDs binding behavior toward different nonradical guest.

Using Cyclodextrins to Encapsulate Oxygen-Centered and Carbon-Centered Radical Adducts: The Case of DMPO, PBN, and MNP Spin Traps

“…The same authors detected “multimodal” inclusion: in the case of a nitroxide with several functional groups, different complexes were generated, depending on the group that became the guest . The choice of an interesting nitroxide led to the detection, with excellent resolution due to d -enrichment, of different ESR spectra for the “in” and “out” nitroxides (inside and outside the CD host), with assignment based on the a N values . More recently, it was demonstrated that pulse ESR methods such as electron spin−echo envelope modulation (ESEEM) and double electron−electron resonance (DEER) can lead to additional structural information compared to continuous wave (CW) ESR …”

“…7 The choice of an interesting nitroxide led to the detection, with excellent resolution due to D-enrichment, of different ESR spectra for the "in" and "out" nitroxides (inside and outside the CD host), with assignment based on the a N values. 8 More recently, it was demonstrated that pulse ESR methods such as electron spin-echo envelope modulation (ESEEM) and double electron-electron resonance (DEER) can lead to additional structural information compared to continuous wave (CW) ESR. 9 NMR studies have added important details on the relative location of guests in R-, β-, and γ-CDs.…”

Improving Spin Probe Methodologies to Investigate Supramolecular Assemblies

“…As a first example, the EPR method was employed for studying the effect of aliphatic alcohols on the kinetic of complexation by CDs . In this work we also found that substitution of the tert ‐butyl hydrogens of BTBN1 by deuterium atoms significantly improve the spectral resolution of EPR lines and, thus, the accuracy in measuring the kinetic rate constants.…”

Electron spin resonance

“…Stability constants of complexes between CD and the various alcohols were determined. 102 The ternary inclusion complexes of two spin-labeled pyrrolidineand piperidine-containing indole derivatives and two hydrocarbons, benzene and cyclohexane, with gand b-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.…”