Multifrequency ESR study of spin-labeled molecules in inclusion compounds with cyclodextrins

Abstract

The molecular dynamics of spin labeled compounds included into the solid phase of cyclodextrins (CD) has been studied using conventional (X-Band) ESR at 9 GHz and high-field high-frequency (HFHF) ESR at 240 and 170GHz. The patterns of axial rotation at these higher frequencies are clear just by inspection of the spectrum, unlike the case for 9 GHz spectra. That is HFHF ESR is sensitive to molecular motion about the diffusion axis collinear with the X, Y or Z-direction of the magnetic g-and A-tensors of the nitroxide moiety (referred to respectively as X, Y or Z-rotation). For doxyl stearic acids (Z-rotation) and TEMPOyl caprylate (X-rotation) included in β-and γ-CD's we were able to determine the rate of molecular motion and the corresponding potential barriers. We emphasize that determining the rate of Z-rotation by ESR is feasible only using HFHF ESR. For the X-rotation case we suggest that the motion of the nitroxide moiety consists of fast small-angle librations about the magnetic X-axis superimposed by rotational diffusion about the same axis. The potential barrier of 1.7 Kcal/mol for this rotational diffusion is unusually low. A fascinating feature of TEMPO derivatives included in β-CD is the detectable molecular motion at temperatures below 77K. For the other CD -spin probe systems, we used multifrequency analysis to assign the conformations of spin-labeled molecules. A dramatic spectral change for 16-sasl in β-and γ-CDs at 260K corresponds to a tilting of the position of the nitroxide moiety on the rotating molecule relative to the long diffusion axis, while for TEMPO derivatives in γ cyclodextrin below 200K, we observe a rapid transition from fast to very slow rotational motion. More complex features are best studied by means of multifrequency ESR experiments. The visual clarity and the simplicity of analysis of the ESR spectra shown in this work should provide a benchmark for future studies of molecular motion by HFHF ESR.

Azaadamantyl nitroxide spin label: complexation with β-cyclodextrin and electron spin relaxation

“…Cyclodextrins are polymers of D-glucopyranose with hydrophobic cavities that form inclusion complexes with a wide range of small molecules including nitroxides [16, 17, 18] and adamantyl groups [19, 20, 21]. Complexation of 3 with β-cyclodextrin was examined as a method of decreasing mobility.…”

Spin-label saturation-recovery EPR at W-band: Applications to eye lens lipid membranes

“…This easy approach cannot be applied to analyze the EPR spectra at W-band. Only with some restrictions can the ordering potential and rotational diffusion coefficient be obtained by computer simulation of W-band EPR spectra [30–33]. These simulations were not performed here.…”

Interaction of the β amyloid – Aβ(25–35) – peptide with zwitterionic and negatively charged vesicles with and without cholesterol

“…Electron Paramagnetic Resonance (EPR) spectroscopy of nitroxide spin labels has been successfully utilized to study the polarity(Marsh, 2001, 2002; Marsh and Toniolo, 2008; Dzikovski et al, 2009) and fluidity (Severcan and Cannistraro, 1988; Peric et al, 2005; Alves et al, 2008; D’Errico et al, 2008) of lipid membranes. D’Errico and colleagues(D’Errico et al, 2008), using phospholipids spin labeled at two different positions on the hydrocarbon chain, have investigated the interaction between Aβ(25–35) and negatively charged and zwitterionic phospholipid bilayers with and without cholesterol by EPR spectroscopy.…”