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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 (CDs) has been studied using conventional (X-band) ESR at 9 GHz and high-field high-frequency (HFHF) ESR at 240 and 170 GHz. 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 beta- and gamma-CDs 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(-1) for this rotational diffusion is unusually low. A fascinating feature of TEMPO derivatives included in beta-CD is the detectable molecular motion at temperatures below 77 K. 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 beta- and gamma-CDs at approximately 260 K 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 gamma-cyclodextrin below 200 K, 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.

SUBMITTER: Dzikovski B

PROVIDER: S-EPMC2743463 | biostudies-literature | 2009 Aug

REPOSITORIES: biostudies-literature

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

Dzikovski Boris B Tipikin Dmitriy D Livshits Vsevolod V Earle Keith K Freed Jack J

Physical chemistry chemical physics : PCCP 20090630 31

The molecular dynamics of spin-labeled compounds included into the solid phase of cyclodextrins (CDs) has been studied using conventional (X-band) ESR at 9 GHz and high-field high-frequency (HFHF) ESR at 240 and 170 GHz. 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- ...[more]

PMID: 19639141

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Project description:An extensive set of electron spin resonance spectra was obtained over a wide range of frequencies (9, 95, 170, and 240 GHz) and temperatures (2 to 32 degrees C) to explore the dynamic modes of nitroxide-labeled T4 lysozyme in solution. A commonly used nitroxide side chain (R1), or a methylated analogue with hindered internal motion (R2), was substituted for the native side chain at solvent-exposed helical sites, 72 or 131. The spectra at all four frequencies were simultaneously fit with the slowly relaxing local structure (SRLS) model. Good fits were achieved at all the temperatures. Two principle dynamic modes are included in the SRLS model, the global tumbling of the protein and the internal motion consisting of backbone fluctuations and side chain isomerizations. Three distinct spectral components were required for R1 and two for R2 to account for the spectra at all temperatures. One is a highly ordered and slow motional component, which is observed in the spectra of both R1 and R2; it may correspond to conformers stabilized by interaction with the protein surface. The fraction of this component decreases with increasing temperature and is more populated in the R2 spectra, possibly arising from stronger interaction of the nitroxide ring with the protein surface due to the additional methyl group. The other two components of R1 and the second component of R2 are characterized by fast anisotropic diffusion and relatively low ordering, most likely corresponding to conformers having little or no interactions with nearby residues. Ficoll of different concentrations was added to increase the solution viscosity, thereby slowing down the global tumbling of the protein. A significant effect of Ficoll on the internal motion of an immobilized component was apparent in R2 but not in R1. The ability of such multifrequency studies to separate the effects of faster internal modes of motion from slower overall motions is clearly demonstrated, and its utility in future studies is considered.

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

Publications

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

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