Paramagnetic Solute Research Articles

Solution NMR of Battery Electrolytes: Assessing and Mitigating Spectral Broadening Caused by Transition Metal Dissolution.

NMR spectroscopy is a powerful tool that is commonly used to assess the degradation of lithium-ion battery electrolyte solutions. However, dissolution of paramagnetic Ni2+ and Mn2+ ions from cathode materials may affect the NMR spectra of the electrolyte solution, with the unpaired electron spins in these paramagnetic solutes inducing rapid nuclear relaxation and spectral broadening (and often peak shifts). This work establishes how dissolved Ni2+ and Mn2+ in LiPF6 electrolyte solutions affect 1H, 19F, and 31P NMR spectra of pristine and degraded electrolyte solutions, including whether the peaks from degradation species are at risk of being lost and whether the spectral broadening can be mitigated. Mn2+ is shown to cause far greater peak broadening than Ni2+, with the effect of Mn2+ observable at just 10 μM. Generally, 19F peaks from PF6 - degradation species are most affected by the presence of the paramagnetic metals, followed by 31P and 1H peaks. Surprisingly, when NMR solvents are added to acquire the spectra, the degree of broadening is heavily solvent-dependent, following the trend of solvent donor number (increased broadening with lower solvent donicity). Severe spectral broadening is shown to occur whether Mn is introduced via the salt Mn(TFSI)2 or is dissolved from LiMn2O4. We show that the weak 19F and 31P peaks in spectra of electrolyte samples containing micromolar levels of dissolved Mn2+ are broadened to an extent that they are no longer visible, but this broadening can be minimized by diluting electrolyte samples with a suitably coordinating NMR solvent. Li3PO4 addition to the sample is also shown to return 19F and 31P spectral resolution by precipitating Mn2+ out of electrolyte samples, although this method consumes any HF in the electrolyte solution.

Depth Distribution of Spin-Labeled Liponitroxides within Lipid Bilayers: A Combined EPR and Molecular Dynamics Approach

The distribution in an egg–phosphatidylcholine bilayer of a series of spin-labeled nitroxides, potentially useful as targeted antioxidants, has been investigated using molecular dynamics (MD) simulations. The in silico method has been tested at first for a series of n-doxyl-phosphocholine-doped bilayers, with the doxyl moiety located at different positions (n) of the lipid chain, in analogy to electron paramagnetic resonance (EPR) spin labeling and other MD studies. As a result, a novel calibration curve has been obtained, suitable to determine the absolute membrane penetration depth of any paramagnetic solute from EPR measurements. A second series of MD simulations was then carried out on the newly synthesized series of liponitroxides (NOXs) recently tested as antioxidants against the lipid peroxidation of polyunsaturated fatty acids in membranes: their penetration depths, as determined by EPR in phosphatidylcholine liposomes, were correlated with their antioxidant efficacy. In these NOXs, a glycerol moi...

Magnetic Resonance Imaging of the Effect of Zeolite on Lithium Uptake in Poplar

The use of a zeolite (clinoptilolite) to protect poplar plants from lithium-contaminated soil has been studied using magnetic resonance imaging. Lithium was used as a model contaminant as it could be tracked directly using specific nuclear magnetic resonance probes, rather than relying on relaxation time effects on protons due to paramagnetic solutes. The sorption of lithium to the zeolite was investigated both in static and dynamic systems; lithium was found to sorb readily to the zeolite over time. Poplar plants were grown in soil microcosms consisting of either sand or sand and zeolite with nutrients provided through the use of Hoagland's solution as the pore fluid. Both one-dimensional profiles of lithium concentration along poplar stems and direct lithium imaging of stem cross-sections were employed to reveal the uptake of the contaminant into the plant structure, showing significantly less lithium present in plants grown in sand and zeolite than those grown in sand alone. Evidence of structural features involved in the uptake of lithium was also obtained.

Picosecond water dynamics adjacent to charged paramagnetic ions measured by magnetic relaxation dispersion

Measurements of water-proton spin-lattice relaxation rate constants as a function of magnetic field strength [magnetic relaxation dispersion (MRD)] in aqueous solutions of paramagnetic solutes reveal a peak in the MRD profile. These previously unobserved peaks require that the time correlation functions describing the water-proton-electron dipolar coupling have a periodic contribution. In aqueous solutions of iron(III) ion the peak corresponds to a frequency of 8.7 cm-1, which the authors ascribe to the motion of water participating in the second coordination sphere of the triply charged solute ion. Similar peaks of weaker intensity in the same time range are observed for aqueous solutions of chromium(III) chloride as well as for ion pairs formed by ammonium ion with trioxalatochromate(III) ion. The widths of the dispersion peaks are consistent with a lifetime for the periodic motion in the range of 5 ps or longer.

Proton electron double resonance imaging (PEDRI) of the isolated beating rat heart.

Proton electron double resonance imaging (PEDRI) is a double resonance technique where proton MRI is performed with irradiation of a paramagnetic solute. A low-field PEDRI system was developed at 20.1 mT suitable for imaging free radicals in biological samples. With a new small dual resonator, PEDRI was applied to image nitroxide free radicals in isolated beating rat hearts. Experiments with phantoms showed maximum image enhancement factors (IEF) of 42 or 28 with TEMPONE radical concentrations of 2-3 mM at EPR irradiation powers of 12W or 6W, respectively. In the latter case, image resolution better than 0.5 mm and radical sensitivity of 5 microM was obtained. For isolated heart studies, EPR irradiation power of 6W provided optimal compromise of modest sample heating with good SNR. Only a small increase in temperature of about 1 degrees C was observed, while cardiac function remained within 10% of control values. With infusion of 3 mM TEMPONE an IEF of 15 was observed enabling 2D or 3D images to be obtained in 27 sec or 4.5 min, respectively. These images visualized the change in radical distribution within the heart during infusion and clearance. Thus, PEDRI enables rapid and high-quality imaging of free radical uptake and clearance in perfused hearts and provides a useful technique for studying cardiac radical metabolism.

Water-Proton Relaxation by a Noncovalent Albumin-Binding Gadolinium Chelate: An NMRD Study of a Potential Blood Pool Agent

Magnetic dipolar interactions between solvent water protons and paramagnetic solutes, such as chelated Gd3+ ions, provide an efficient means for increasing both the longitudinal (1/T1) and transverse (1/T2) relaxation rates of solvent protons. For small paramagnetic chelate complexes, interactions by direct water coordination to the metal ion (inner sphere) and diffusion in the outer sphere environment of the metal complexes are the two main contributors to relaxation enhancement. At magnetic resonance imaging (MRI) fields and physiologic conditions, these contributions are additive and generally comparable in magnitude, whereas for macromolecular complexes, inner sphere contributions tend to dominate relaxation. The inner sphere contribution to 1/T1 is given by: 1/T1pis =q[Gd3+]/55.5(T1M + τM). Here, q is the number of coordinated inner sphere water molecules, T1M is the longitudinal relaxation time of the coordinated water protons, and τM is their lifetime on the metal ion. For coordinated Gd3+ ions with q = 1, both T1M and τM are of the order of microseconds. 1/T1 is a complicated function of the strength of the interaction, the magnitude of the applied MRI field B0, and an overall correlation time, τC. τC is given by τC−1 = τR−1 + τS−1 + τM−1, the inverse of the sums of the reciprocals of the orientational relaxation time of the complex τR, the electronic relaxation time of the paramagnetic metal ion τS, and τM, respectively. For Gd3+ ions, τS can also be a function of B0, often characterizable by another correlation time τV. In any event, the enhancement is maximized for kinetically labile complexes, when exchange of the inner sphere water is rapid (but not too rapid), such that τC << τM << T1M ( 1 Outhred RK George EP A nuclear magnetic resonance study of hydrated systems using the frequency dependence of the relaxation processes. Biophys J. 1973; 13: 83-96 Abstract Full Text PDF PubMed Scopus (30) Google Scholar , 2 Koenig SH Brown III, RD Relaxation of solvent protons by paramagnetic ions: dependence on magnetic field and chemical environment—implications for NMR imaging. Magn Reson Med. 1984; 1: 478-495 Crossref PubMed Scopus (187) Google Scholar , 3 Bertini L Luchinat C NMR of paramagnetic molecules in biological systems. Benjamin/Cummins, Menlo Park, Calif1986 Google Scholar ).

Magnetic and electrochemical investigations on anions derived from oligoketones containing fluorenone and benzophenone units. An approach to the design of stable multiradical organic materials

The mono- and poly-anions of some mono-, di- and tetra-ketones containing fluorenone and benzophenone moieties have been studied by NMR and cyclic voltammetry. Alkali metal anion radicals of fluorenone, generated by electron transfer from an alkali metal naphthalene radical anion exhibit markedly lower molar paramagnetic solvent shifts than those generated by direct reduction with an alkali metal. Evidence is provided for the involvement of hyperfine interactions between the naphthalene and the paramagnetic solute. The dianion of the diketone 9 is paramagnetic as are the dianions of the tetraketones 6 and 7. The NMR data, combined with those obtained by cyclic voltammetry, indicate that polyketones possessing fluorenone moieties connected through isophthaloyl ‘spacers’ are promising systems for the preparation of high-spin organics and electrophores.

Paramagnetically induced nuclear magnetic resonance relaxation in solutions containing S⩾1 ions: A molecular-frame theoretical and physical model

The enhancement of nuclear magnetic resonance (NMR) relaxation rates produced by paramagnetic solutes is physically rather different for electron spin S=1/2 paramagnetic species than for S⩾1 species due to the presence of zero-field splitting interactions in the electron spin Hamiltonians of the latter. When the zfs energy is larger than the electronic Zeeman energy, the electron spin precessional motion is spatially quantized with respect to the molecule-fixed principal axis system (PAS) of the zfs tensor rather than along the external laboratory magnetic field. An analytical theory of the orthorhombic zfs limit has been derived in which the motion of the electron spin variables is described in the zfs-PAS and that of the nuclear spin variables in the laboratory coordinate frame. The resulting theoretical expressions are simple in form and suggest a physically transparent interpretation of the experiment. The NMR relaxation enhancement R1p results from additive contributions, R1x, R1y, and R1z, arising from the molecular-frame Cartesian components of the time-dependent electron spin magnetic moment operator μr(t). Each Cartesian component R1r depends on the dipolar power density at the nuclear Larmor frequency that is produced by the corresponding Cartesian component of μr(t). The theory displays the dependence of the relaxation enhancement on the variables of molecular structure in a very simple and physically transparent form: R1r∝r−6[1+P2(cos θr)], where r is the interspin distance and cos θr is the direction cosine of the interspin vector with the rth principal axis of the zfs tensor. New experimental data are presented for the model S=1 complex [trans-Ni(II)(acac)2(H2O)2] (acac=acetylacetonato) in dioxane solvent. The magnetic field dependence of the proton T1 of the axial water ligands has been measured over the range 0.15–1.5 T, the lower end of which corresponds to the zfs limit. The experimental data have been analyzed using the new analytical theory for the zfs-limit regime in conjunction with spin dynamics simulations in the intermediate regime. Dipolar density power plots are presented as graphical devices which clearly exhibit the physical information in the experiment, and which permit a rapid differentiation of the sensitive and insensitive parameters of theory. The data analysis depends strongly on the zfs parameter |E| and on the electron spin relaxation time τS,z along the zfs-PAS z-axis, but only very weakly on the other parameters of theory. A fit of the data to theory provided the values |E|=1.8±0.1 cm−1 and τS,z=8.0±0.3 ps.

Spin conversion of positronium in paramagnetic solutions studied by age-momentum-correlation method

Correlated measurement of the lifetime and of the lineshape of the 511 keV annihilation radiation of positrons (age-momentum correlation, AMOC) has become a powerful tool for investigating reactions of positron or positronium in condensed matter as a function of time. The beam-based β + gDE AMOC method installed at the Stuttgart relativistic positron beam facility (E e+ kin 4MeV) offers substantial advantages over the conventional γγΔE coincidence technique resulting in a fast data taking due to the unity detection efficiency of the β + scintillator which provides the start signal for the lifetime measurement. In this paper the application of AMOC to positron-ium chemistry is illustrated by the study of spin conversion of positronium in the system methanol/HTEMPO as a function of HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl, a paramagnetic solute) concentration. The same technique can be applied to other systems and other reactions in positronium chemistry, e.g., oxidation or complex formation

Variable-pressure dynamic NMR studies: effects of paramagnetic metal ions on NMR parameters in nonexchanging systems

The possible effects of paramagnetic relaxation on the apparent volumes of activation for exchange reactions in solution, as measured by NMR at high pressures, are considered. Two model paramagnetic systems that do not undergo ligand exchange on the NMR time scale were examined: tri(acetylacetonato)chromium(III) in various perdeuterated solvents, and tris(ethylenediamine)nickel(II) ion in ethylenediamine solvent. No pressure dependence was discernible up to 200 MPa for the chemical shifts of 1H (exemplifying nuclei of spin 1/2) in the Cr(III) complex, or of solvent 14N (representing quadrupolar nuclei) in the Ni(II)–ethylenediamine case. The line widths Δv1/2, however, were significantly dependent on pressure. For 1H in the Cr complex, the increase of Δv1/2 with pressure was less than expected from the theory of scalar interactions, and was small enough to imply that any contribution from this source to the observed volume of activation in exchanging systems may be neglected. For 14N in liquid ethylenediamine, the increase of Δv1/2 with pressure was significantly greater when a paramagnetic solute was present. Thus, before the observed Δv1/2 at a pressure P (in MPa) measured by the NMR of a quadrupolar nucleus can be used to obtain a chemical exchange rate for a paramagnetic solute, it should be reduced by an amount Δv1/20 exp{θ(P − 0.1)(ΔVV‡/RT + κ)}, where ΔVV‡ is the activation volume for viscous flow and κ the compressibility of the solvent, Δv1/20 is the linewidth at 0.1 MPa in the absence of chemical exchange, and θ is a scaling factor between 0 and 1. The factors Δv1/20 and θ(ΔVV‡/RT + κ) are obtainable from measurements with a chemically equivalent, nonexchanging, paramagnetic solute.

Magnetic impurities in superconductors: A theory with different predictions.

The proclivity of paramagnetic solutes to degrade the transition temperature of a traditional singlet-pairing superconductor is studied anew. ${\mathit{T}}_{\mathit{c}}$ degradation is proportional to the (conduction electron) spin-disorder scattering rate, 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$, caused by the solute spins. Accordingly the ${\mathit{T}}_{\mathit{c}}$ loss increases with solute concentration. The initial slope (versus 1/${\mathrm{\ensuremath{\tau}}}_{\mathit{s}}$) is found to depend on the superconductor and therefore is not the universal constant proposed by Abrikosov and Gor'kov. Instead the decrease is inversely proportional to \ensuremath{\lambda}, the electron-phonon coupling constant. Consequently a weak superconductor is doubly jeopardized by paramagnetic impurities: Its superconductivity is easily suppressed not only because ${\mathit{T}}_{\mathit{c}}$ is small to begin with, but also because the initial slope is steeper. Another unexpected consequence of the theory involves potential scattering which, acting alone, does not significantly influence ${\mathit{T}}_{\mathit{c}}$ (as surmised by Anderson). Nevertheless, the ${\mathit{T}}_{\mathit{c}}$ reduction caused by exchange scattering will be partially suppressed when the overall mean free path becomes smaller than the coherence length. This compensation has been demonstrated experimentally by comparing the influence of magnetic impurities in a pure host superconductor with that in a similar host having (also) nonmagnetic solutes. Such observed recovery of ${\mathit{T}}_{\mathit{c}}$, expected from this study, contradicts prior theories for magnetic solutes.

Nuclear magnetic resonance relaxation enhancements produced by paramagnetic solutes: Effects of rhombicity in the zero field splitting tensor with the S=2 spin system as an example

Effects due to the nonuniaxial part of the zero field splitting (ZFS) tensor on NMR relaxation enhancements produced by paramagnetic species in solution (the NMR PRE) has been studied theoretically and experimentally in the ZFS limit, i.e., in the limit where the ZFS energy is large compared to the Zeeman energy. In the ZFS limit, the precessional motion of the electron spin is quantized with respect to molecule-fixed coordinate axes. The uniaxial part of the ZFS tensor induces precessional motion in the transverse (x,y) components of the electron spin vector S, and x,y anisotropy in the ZFS tensor (i.e., a nonzero ZFS parameter E) induces precessional motion in the z component of S. The NMR-PRE phenomenon is particularly sensitive to the motion of Sz and hence also to ZFS anisotropy in the xy plane. Mathematical expressions have been derived which describe the motion of the spin vector evolving under the influence of a general rhombic ZFS Hamiltonian and the influence of this motion on the NMR PRE in the ZFS limit. It is shown that oscillations in Sz occur at the transition frequencies of the S spin system; the frequencies and amplitudes of the precessional components of Sz can be calculated by diagonalizing the general ZFS Hamiltonian. These motions and their consequences with respect to the behavior of the NMR PRE are described in detail for the S=2 spin system. A parametrization of NMR-PRE data is proposed which gives a clear criterion for the conditions under which rhombic parts of the ZFS tensor significantly affect the relaxation enhancements produced by an S=2 spin system. This criterion is of considerable practical importance for the analysis of NMR-PRE data, since it defines conditions under which data may be analyzed without the need for independent experimental information concerning the magnitude of the ZFS tensor.

Nuclear spin relaxation due to paramagnetic species in solution: Effect of anisotropy in the zero field splitting tensor

The NMR (nuclear magnetic resonance) paramagnetic relaxation enhancement (NMR-PRE) that is produced by paramagnetic solutes in solution has been investigated theoretically with respect to the influence of zero field splitting (zfs) interactions in the electron spin Hamiltonian, in particular with respect to the effects of anisotropy in the zfs tensor. These effects are a physical consequence of the influence of the zfs on the motion of the electron spin vector S̄. When the zfs energy is large compared to the Zeeman energy (the zfs limit), the precessional motion of S̄ is quantized in the molecule-fixed coordinate system that diagonalizes the zfs tensor. The uniaxial portion of the zfs tensor influences the NMR-PRE primarily through its influence on the quantization axes of S̄; the characteristic behavior of the NMR-PRE under the influence of a uniaxial zfs has been described in detail previously. Anisotropy in the zfs tensor induces oscillatory motion in Sz. This motion has a profound influence on the NMR-PRE, the major part of which normally arises from low frequency components of the local magnetic field that are associated with Sz, rather than from the rapidly precessing local fields that are associated with the transverse components S±. For this reason, the NMR-PRE is a sensitive function of zfs anisotropy, which acts to lower the NMR-PRE below the value that occurs in the uniaxial situation. The magnitude of this effect depends on the ratio (E/D) of the anisotropic and uniaxial zfs parameters, on the reduced dipolar correlation time, and on the location of the nuclear spin in the molecular coordinate frame. A second physical effect of zfs anisotropy on the NMR-PRE arises from a resonance between the electron spin precessional motion in the transverse plane with the precessional motion that is perpendicular to the transverse plane (the latter due to zfs anisotropy). Resonance of these motions, which occurs spin energy levels crossings, gives rise to low frequency transverse components of S̄ which result in a resonant increase in the NMR-PRE within a restricted range of E/D ratios.

The Kinetics of the Spin-Conversion of Positronium Measured by β+γ∆E Age-Momentum Correlation

Age-momentum correlation (AMOC) measurements provide time-resolved information on the momentum distribution of annihilating electron— positron pairs through correlated determination of Doppler broadening (SE) of the 511 keV annihilation radiation line and positron age. In the β+ ∆E AMOC technique implemented at the Stuttgart pelletron accelerator the start signals for the age measurements are generated when the relativistic positrons pass through a fast plastic scintillator, which has a detection efficiency close to unity. Beam-based β+ ∆E AMOC measurements thus combine high fl+ γ6,E triple-coincidence rates with low background. From a practical point of view this constitutes a major progress compared to conventional (source-based) ∆E AMOC measurements. The AMOC technique permits direct access to a wide variety of time-dependent processes, among them the reaction kinetics in positronium chemistry. As an example, we present a β+g ∆EAMOC investigation of the kinetics of a spin-conversion reaction of positronium in methanol induced by the presence of a nitrosyl free-radical paramagnetic solute (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl , HTEMPO). The AMOC measurements allowed the time-resolved observation of the spin-conversion process, which had been investigated before by uncorrelated positron lifetime and Doppler-broadening nieasurements. It is found that within the concentration range investigated (< 0.1 moll) the spin-conversion reaction rate at room temperature depends linearly on the HTEMPO concentration with a reaction-rate constant kconv = (22.5 ± 0.5) x 10 9 1 moI -1

Characteristic properties of the nuclear magnetic resonance–paramagnetic relaxation enhancement arising from integer and half-integer electron spins

The influence of zero field splitting (zfs) interactions on the magnetic field dispersion profile of the nuclear magnetic resonance–paramagnetic relaxation (NMR–PRE) (i.e., the enhancement of nuclear magnetic relaxation rates that is produced by paramagnetic solute species in solution) has been explored systematically for S=1, 3/2, 2, and 5/2 spin systems using recently developed theory. To facilitate comparison of results for different spin values, the theory was expressed in a reduced form with Larmor frequencies in units of ωD (the uniaxial zfs parameter D in rad s−1), and correlation times and spin relaxation times in units of ωD−1. For S=1, the functional form of the profile can be described in terms of five types of qualitative features. Two of these are characteristic of Zeeman-limit [Solomon, Bloembergen, and Morgan (SBM)] theory and result from the magnetic field dependence of the spin energy level splittings. The remaining three have no analog in Zeeman-limit theory and arise from a change in the quantization axis of the electron spin precessional motion which, in the zfs limit, lies along molecule-fixed coordinate axes, and, in the Zeeman limit, lies along the external field direction. The reduced field dispersion profiles for the integer spin systems S=1 and S=2 were found to be very similar to each other, the principal difference being that the midfield positions of the requantization features (types 2, 3, and 4) are shifted for S=2 relative to S=1, the magnitude and sign of the shift depending on the position of the nuclear spin in the molecular coordinate frame. For half-integer spins, the dispersion profiles exhibit, in addition to the five features characteristic of integer spins, a sixth type of feature, which is centered somewhat to low field of ωSτc=1, where τc is the dipolar correlation time. The type-6 feature results from field-dependent level splitting of the mS=±1/2 Kramers doublet. It is present when ωDτc≥1. These theoretical predictions have been examined by means of reinterpretations of the NMR–PRE data for tris-(acetylacetonato)–metal complexes of V(III) (S=1), Cr(III) (S=3/2), Mo(III) (S=3/2), Mn(III) (S=2), and Fe(III) (S=5/2). As predicted, type-6 features are absent for the integer spin complexes, for which the T1 field dispersion profiles are nearly field independent. The experimental profiles were successfully simulated quantitatively by the generalized theory, but not by Zeeman-limit theory. For the half-integer spin systems, the predicted zfs-related type-6 features appear to be present in the profiles, particularly for Mo(acac)3, for which the data deviate significantly from the Zeeman-limit profile in a manner that is explained by the generalized theory.

Permeability of liposomal membranes to water: results from the magnetic field dependence of T1 of solvent protons in suspensions of vesicles with entrapped paramagnetic ions.

The diffusive permeability to water molecules, Pd, of lipid vesicles with entrapped paramagnetic solute ions can be determined rapidly from analysis of the magnetic field dependence (nuclear magnetic relaxation dispersion, or NMRD profile) of T1 of exterior solvent water protons. Such data yield tau, the mean lifetime of solvent molecules inside the vesicles, from tau = (fT1Para) - T1Ves, where f is the volume fraction of entrapped water, T1Para is the observed T1 corrected for buffer background, and T1Ves is the relaxation time of water protons in the entrapped solution. For small spherical unilamellar vesicles of inner radius R, Pd = R/3 tau, f can be obtained accurately from knowledge of both the concentration of Gd(DTPA)2- in the solution in which the vesicles were formed and the average concentration of ions in the final sample. At low temperatures, in the limit of slow exchange, T1Para becomes independent of field and tau = fT1Para; the observation of a field-independent profile is a control that confirms that no paramagnetic material is external to the vesicles. We have measured T1Para, using a field-cycling relaxometer, for suspensions of POPC (1-palmitoyl-2-oleoyl-lecithin) vesicles with 100-500 mM entrapped Gd(DTPA)2- and membrane concentrations of cholesterol ranging from 0 to 40 mol %. These profiles, which span the field range 0.01-50 MHz proton Larmor frequency, were taken at 5, 15, 25, and 35 degrees C. Concentrations of Gd(DTPA)2- were determined independently by both ICP analyses and NMRD methods. Values for Pd for vesicles with 100 mM Gd(DTPA)2- and outer diameters 100 nm +/- 20%, as determined by quasielastic light scattering, are 63, 47, 24, 16, and 8.7 x 10(-4) cm s-1, at 25 degrees C, for cholesterol concentrations of 0, 10, 20, 30, and 40%, respectively. The corresponding activation enthalpies are 14, 14, 14, 17, and 17 kcal/M. Comparison with 2H NMR studies of deuterated POPC vesicles with no cholesterol at 20 degrees C, and with 10% at 40 degrees C, which yielded the same order parameter for the palmitoyl acyl chains, gives no indication of a correlation between order parameter and permeability.

Water proton relaxation rate enhancements as a function of magnetic field strength and nature and size of paramagnetic solutes

The theoretical expectations for nuclear longitudinal relaxation rate enhancements for water protons sensing paramagnetic metal ions are shown. The cases of S = 1 2 (Cu 2+ and VO 2+), S = 1 (Ni 2+), S = 5 2 (Mn 2+ and Fe 3+), S = 7 2 (Gd 3+) are considered by including some factors which split the S manifold at zero field and the effect of the rotation.

Influence of water on the mobility of small molecules dispersed in a polymeric system.

The rotational mobility of paramagnetic solutes dispersed in partially hydrated macromolecules (proteins, polysaccharides, synthetic polymers) was measured using Electron Spin Resonance. A critical minimum amount of water was observed to be necessary for these molecules to reach a level of mobility of the same order as in dilute solutions. This amount of water depended on the size of the diffusing solute and on the microporosity of the macromolecule. Above this critical moisture range, a progressive increase of the proportion of mobile solute occurred over a hydration range determined by the size of the diffusing solute. At the same time, the rotational diffusivity of the mobile solute increased linearly with water content. The mobilization pattern of spin-labelled side chains of caseinates was observed to be similar to that of the solute. Results are discussed with reference to free volume theory.

Field-cycled proton-electron double-resonance imaging of free radicals in large aqueous samples

We have recently published a new method of imaging free radicals in aqueous solutions called proton-electron double-resonance imaging (PEDRI) ( I ). In this technique a conventional proton NMR image is collected while the EPR resonance of a free radical solute is irradiated. If the EPR irradiation has sufficient power, the NMR signal from those protons being relaxed by the paramagnetic solute is enhanced, and the parts of the sample containing free radical exhibit greater intensity in the final image. Unlike EPR imaging (2) the sample size in PEDRI is not constrained by magnetic field gradient requirements. In this Communication we present the first results of an extension of PEDRI which uses magnetic field cycling, allowing larger samples to be imaged with lower levels of applied radiofrequency power. PEDRI is an imaging version of a dynamic nuclear polarization experiment (35). The enhancement of the NMR signal upon irradiation of the EPR of the solute may be written empirically as

Recent results in muonium solution kinetics

Using Muonium Spin Rotation (MSR) techniques the aqueous solution kinetics of several muonium addition reactions and spin conversion interactions have been studied. The addition reactions show both diffusion and activation-controlled reaction rates with isotope effects between 1 and 3 for diffusion-control and between 7 and 31 for activation-control reactions. Barrier energies are typically ≈15 kJ/mole and ≈30 kJ/mole, respectively, for these processes in water. Spin-conversion interactions involving Ni(aq+2 and Ni(cyclam)+2 complexes showed that spin-conversion of “triplet” Mu by a paramagnetic solute occurs at or near the diffusion-controlled limit while the chemical reaction with the diamagnetic configuration of Ni(cyclam)+2 occured some 100 times slower at kM ⩽ 5×108M−1s−1.