Jozef K. Moscicki

250-GHz electron spin resonance studies of polarity gradients along the aliphatic chains in phospholipid membranes

Rigid-limit 250-GHz electron spin resonance (FIR-ESR) spectra have been studied for a series of phosphatidylcholine spin labels (n-PC, where n = 5, 7, 10, 12, 16) in pure lipid dispersions of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), as well as dispersions of DPPC containing the peptide gramicidin A (GA) in a 1:1 molar ratio. The enhanced g-tensor resolution of 250-GHz ESR for these spin labels permitted a careful study of the nitroxide g-tensor as a function of spin probe location and membrane composition. In particular, as the spin label is displaced from the polar head group, Azz decreases and gxx increases as they assume values typical of a nonpolar environment, appropriate for the hydrophobic alkyl chains in the case of pure lipid dispersions. The field shifts of spectral features due to changes in gxx are an order of magnitude larger than those from changes in Azz. The magnetic tensor parameters measured in the presence of GA were characteristic of a polar environment and showed only a very weak dependence of Azz and gxx on label position. These results demonstrate the significant influence of GA on the local polarity along the lipid molecule, and may reflect increased penetration of water into the alkyl chain region of the lipid in the presence of GA. The spectra from the pure lipid dispersions also exhibit a broad background signal that is most significant for 7-, 10-, and 12-PC, and is more pronounced in DPPC than in POPC. It is attributed to spin probe aggregation yielding spin exchange narrowing. The addition of GA to DPPC essentially suppressed the broad background signal observed in pure DPPC dispersions.

Multifrequency Electron Spin Resonance Study of the Dynamics of Spin Labeled T4 Lysozyme

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.

A 250 GHz ESR study of o-terphenyl: Dynamic cage effects above Tc

Three nitroxide spin probes of different sizes and geometrical shape were used in a 250 GHz ESR study of the probe rotational dynamics in the fragile glass former ortho-terphenyl (OTP) over a wide temperature range from 380 to 180 K. Comparative studies at 9.5 GHz have also been performed. Perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide (MOTA), and 3,3-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL) are, respectively, comparable in size to and larger than the OTP host molecule, whereas Perdeuterated 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) is substantially smaller. The sensitivity of 250 GHz ESR to the details of the rotational tumbling for T≳Tc (where Tc is the crossover temperature) was exploited to show that the relaxation is fit by a model that is characteristic of a homogeneous liquid. A nonlinear least-squares analysis shows that below the melting point, Tm, CSL, and MOTA dynamics are well-described by a model of dynamic cage relaxation proposed by Polimeno and Free...

Diffusion coefficients in anisotropic fluids by ESR imaging of concentration profiles

We report on the use of ESR imaging for determining the translational diffusion constants of typical ESR spin probes in ordered and isotropic solvents. A discussion is given for a Fourier deconvolution method for determining the correct concentration profile if there is more than one hyperfine line in the spectrum of the radical as well as a spatial dependence of the spectrometer sensitivity. A simple but approximate subtraction deconvolution method is also presented. The diffusion constants for 4‐oxo‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPONE) in the nematic and isotropic phases of the liquid crystal p‐pentylbenzylidine‐p‐butylanaline (5,4) were determined. In the isotropic phase the diffusion coefficient is isotropic (D=2.5×10−6 cm2 s−1, at 50 °C), while in the nematic phase it was found to be mildly anisotropic. The diffusion coefficients for motion perpendicular and parallel to the director axis in the nematic phase are respectively, D⊥ =9.0×10−7 cm2 s−1, D∥ =6.4×10−7 cm2 s−1 at 27 °C. This is inte...

High-power 95 GHz pulsed electron spin resonance spectrometer

High-field/high-frequency electron spin resonance (ESR) offers improved sensitivity and resolution compared to ESR at conventional fields and frequencies. However, most high-field/high-frequency ESR spectrometers suffer from limited mm-wave power, thereby requiring long mm-wave pulses. This precludes their use when relaxation times are short, e.g., in fluid samples. Low mm-wave power is also a major factor limiting the achievable spectral coverage and thereby the multiplex advantage of Fourier transform ESR (FTESR) experiments. High-power pulses are needed to perform two-dimensional (2D) FTESR experiments, which can unravel the dynamics of a spin system in great detail, making it an excellent tool for studying spin and molecular dynamics. We report on the design and implementation of a high-power, high-bandwidth, pulsed ESR spectrometer operating at 95 GHz. One of the principal design goals was the ability to investigate dynamic processes in aqueous samples at physiological temperatures with the intent to...

Dynamics of phosphatidylcholine-cholesterol mixed model membranes in the liquid crystalline state

The effects of cholesterol on the dynamics of cholestane spin probe (CSL) in various phosphatidylcholine-cholesterol mixed model membranes are examined. The lateral diffusion, D of CSL in DMPC/POPC/cholesterol ternary mixtures, is measured utilizing an improved dynamic imaging electron spin resonance method. It shows a factor of two decrease at 10 mol % and 25 degrees C, whereas it shows only a 40% decrease at 50 mol % and 50 degrees C. A comparison with results in POPC/cholesterol mixtures, which show a stronger effect of cholesterol on D, indicates that acyl chain unsaturation leads to stronger self association of cholesterol in PC model membranes. An S2CSL dependence of the activation energy for D, has been confirmed for the DMPC/POPC/cholesterol mixtures. Here SCSL is the order parameter for CSL. A similar correlation of R perpendicular, the perpendicular component of the rotational diffusion coefficient, with SCSL, which is true for all three mixtures (DMPC/cholesterol, POPC/cholesterol, and DMPC/POPC/cholesterol) we have studied, is also found. These are associated with the effects of enhanced local ordering on the free volume needed for translation and reorientation. Such correlations of dynamic properties D and R perpendicular with the thermodynamic quantity S, as well as the consistent interpretations of the effect of acyl chain unsaturation on the dynamics in terms of the activity coefficients, strongly emphasize the interrelation between the dynamic structure and the thermodynamics of the PC/cholesterol mixtures.

Dielectric Relaxation in Macromolecular Liquid Crystals

The synthesis of polymeric materials incorporating mesogenic (liquid-crystalline) units in the late 1970s, which was pioneered independently by Finkelmann, Ringsdorf and Wendorff in Mainz,1 and by Shibaev and Plate in Moscow,2 has been received with great interest by the polymer and liquid crystal communities around the world. Hybridization of two distinctively different classes of substances, i.e. liquid crystals and polymers, produced bulk properties of new materials that were a melange of properties of constituent substances. The applied scientists found them appealing for their multifunctional character, especially of copolymers, which enable ‘engineering’ of desired properties of a material conferred by different units. While backbone chains provide polymer characteristics like viscoelasticity and good film- or fiber-forming properties, mesogenic units enhance and facilitate imposition of the orientational order of the material by external fields (electric, magnetic, flow gradient or anchoring field at a substrate), conferring at the same time the anisotropy of many physical properties intrinsic to liquid crystals. Further modification of a material can be made by incorporating into a copolymer chain other functional groups with desired properties. The new materials also attracted the attention of scientists studying fundamental properties of polymers and liquid crystals, since they brought into view completely new problems associated with molecular interactions on the inter- and intramolecular level. It is well known that the most characteristic properties of polymeric materials like fibers, plastics and rubbers depend directly on molecular mobility in these polymers. Similarly, molecular dynamics in liquid crystals direct their applications in contemporary electronic devices. Not surprisingly, therefore, study of correlations between the molecular dynamics and bulk properties is advancing our knowledge and enhancing the technological exploitation of these materials.

Translational diffusion in a smectic‐A phase by electron spin resonance imaging: The free‐volume model

The method of dynamic imaging of diffusion (DID)‐ESR (electron spin resonance) has been utilized to study the anisotropy of translational diffusion of spin probes in the smectic A phase of a eutectic liquid crystal, S2. In particular, the nearly spherical perdeuterated‐TEMPONE (PDT) and the rigid and elongated cholestane spin label (CSL) molecules were studied. Whereas D⊥ (the coefficient of diffusion perpendicular to the nematic director) showed simple Arrhenius dependence for both probes, diffusion parallel to the director displayed two different temperature regimes with a changeover of D∥ at t*≊26–27 °C. The regime above (below) t* is characterized by weak (strong) translational ordering. For CSL the ratio D⊥/D∥<1 above t* which indicates nematiclike behavior, but below t* the behavior is more smecticlike, i.e., D⊥/D∥≳1; for PDT D⊥/D∥≳1 over the whole temperature range. A free volume model is developed to interpret the activation energies associated with D⊥ and D∥ (i.e., E⊥ and E∥) in terms of the orie...

Dynamic imaging of diffusion by ESR

Abstract A comprehensive analysis of the accuracy and reliability of dynamic imaging of diffusion by ESR is presented. The importance of analyzing the data in Fourier space is emphasized, and a new method which enables the determination of the important Fourier modes while also providing a test of the reliability of the measurement of the diffusion coefficient, D x , is presented. It is shown that values of D x ∼ 10 −9 cm −2 s −1 can be measured in about one hour to 10–20% accuracy, whereas for D x ∼ 10 −7 cm 2 s −1 the error should be below 1%. These statements are applicable for experiments in which there is unrestricted diffusion, and an initial Gaussian concentration profile. Systematic error resulting from non-Gaussian concentration profiles is shown to be relatively unimportant. However, the finite sweep time through a spectrum is found to yield a systematic error analogous to a Doppler shift, which tends to cancel only for the case of unrestricted diffusion. The experimental geometry in which there is diffusion from a reflective boundary requires special adjustments to align the gradient-on and -off spectra. A convenient and reliable method to accomplish this is presented. Nevertheless, there are inherently greater sources of error for this geometry, and this is confirmed by the experimental results. Utilizing the reflective boundary geometry, the longitudinal and transverse diffusion coefficients for PD-TEMPONE ( 15 N labeled) in the nematic phase of MBBA at 20°C are 3.7 × 10 −7 and 2.5 × 10 −7 cm 2 s −1 , respectively. Their ratio of 1.5 is consistent with results for other probe molecules in MBBA.

Response to “Comment on ‘A 250 GHz ESR study of o-terphenyl dynamic cage effects above TC’ ” [J. Chem. Phys. 109, 10523 (1998)]

We address the points raised by Giordano and Leporini (GL) and show that accounting properly for the nonexponential decay of the rotational correlation function leads to improved agreement with the Stokes–Einstein–Debye (SED) relation above the crossover temperature TC for those probes 3,3′-dimethyloxazolidinyl-N-oxy-2′,3-5α-cholestane (CSL), and perdeuterated 2,2′,6,6′-tetramethyl-4-methyl aminopiperidinyl-N-oxide) (MOTA) that are well-coupled to the viscous modes of o-terphenyl (OTP) when the average relaxation rate 16〈τ〉 is plotted versus 1/T. On the other hand, 2,2′,6,6′-tetramethyl-4-piperidine-N-oxide (PDT) shows simple Arrhenius behavior in this regime, because of weak coupling to the solvent cage, inconsistent with SED, which was clearly shown in our paper. We also suggest that the difference in chemical structure of the PDT probe, studied by us, compared to 2,2′,6,6′-tetramethylpiperidine-N-oxyl (TEMPO), studied by GL, accounts for the difference in the low-temperature relaxation behavior of the ...