Paramagnetic Spin Relaxation Research Articles

Filtering the NMR Spectra of Mixtures by Coordination to Paramagnetic Cu2.

The paramagnetic spin relaxation (PSR) filter allows the selective NMR signal suppression of components in mixtures according to their complexation ability to a paramagnetic ion. It relies on the faster relaxation of nuclei in paramagnetic environments and thus is complementary to classical diffusion and relaxation filters. So far, the PSR filter has established Gd3+ as the sole PSR agent, restricting the paramagnetic filtering repertoire. Herein, we present Cu2+ as a robust PSR agent with characteristic filtering properties. While Gd3+ depends on unspecific ion-pair interactions with anionic components, Cu2+ stands out for filtering species via ordered coordination complexes. An evaluation of the paramagnetic effect of Cu2+ over more than 50 small molecules and polymers has unveiled different sensitivities to Cu2+ (especially high for pyridines, diamines, polyamines, and amino alcohols) and precise filtering conditions for mixtures (1H, COSY, and HMQC) that were challenged with a test bed of commercial drugs. The advantage of integrating Cu2+ and Gd3+ for the stepwise PSR filtering of complex mixtures is also shown.

Electron paramagnetic resonance characterization and electron spin relaxation of manganate ion in glassy alkaline LiCl solution and doped into Cs2SO4

Manganate ion, MnO42−, has important roles in catalysis and potential roles in water treatment. EPR spectra of MnO42− in a glassy alkaline solution of concentrated LiCl at X-band and Q-band at 80 K exhibit g1 = 1.9776 ± 0.001, g2 = 1.9677 ± 0.001, g3 = 1.9560 ± 0.001 and A1 = 182 ± 9, A2 = 275 ± 15, and A3 = 400 ± 15 MHz. In Cs2SO4 the spectra were simulated with 1.908 ± 0.001, g2 = 1.909 ± 0.001, g3 = 1.937 ± 0.001 and A1 = 90 ± 20, A2 = 100 ± 20, and A3 = 400 ± 15 MHz. Simulations required large distributions in A values which suggests that hyperfine splittings are sensitive to differences in geometry. Continuous wave spectra are observable at 80 K in glassy alkaline LiCl, but only up to about 20 K in Cs2SO4. In glassy alkaline LiCl electron spin relaxation was measured at X-band using spin echo and inversion recovery from 4.2 to 60 K. Tm is 4.6 μs at 4.2 K and decreases at higher temperatures as it becomes driven by T1. T1 decreases from ca. 34 ms at 4.2 K to ca. 240 ns at 60 K. Tm and T1 in Cs2SO4 are too short to measure by electron spin echo. The distorted tetrahedral geometry of MnO42− results in faster relaxation than for other 3d1 spin systems that have square pyramidal (C4v) or distorted octahedral geometries.

Filtering the NMR Spectra of Complex Mixtures through Polymer-Mediated Paramagnetic Spin Relaxation.

A polymer-mediated paramagnetic spin relaxation (PSR) filter is presented for the selective suppression of signals from polymer-interacting species in the 1D and 2D NMR spectra of mixtures. The combined use of Gd3+ and a polymer with a high transverse relaxation enhancement (R2p , which gives a measure of the Gd3+ -complexing ability) results in the suppression of signals from any polymer-interacting component in mixtures, irrespective of their R2p . By using poly(acrylic acid) (PAA) as a model system, we demonstrate selective filtering of the signals of typical low-R2p species (insensitive to Gd3+ ), such as molecular/polymeric cations and non-ionic polymers, which, through PAA recognition (electrostatic/hydrogen-bonding interactions), become exposed to the paramagnetic effect of Gd3+ , while leaving non-PAA-interacting species unaffected. Typical suppression conditions involve PAA (approximately equimolar amount with respect to the species to be filtered) accompanied by sub-mm concentrations of Gd3+ and T2 -filters ≤100 ms. Overall, by exploiting the PSR principles and the recognition properties of polymers, selective NMR filtrations that are not attainable by diffusion, relaxation, or direct PSR filters, can be achieved.

Fast NMR Screening of Macromolecular Complexes by a Paramagnetic Spin Relaxation Filter.

The paramagnetic spin relaxation filter is described for the rapid NMR screening of intermolecular interactions between ligands and macromolecular anionic receptors with large transverse relaxation enhancements (R2p). The addition of micromolar concentrations of Gd3+ to the mixture produces the immediate broadening/suppression of the NMR signals of interacting species while leaving unaffected those of noncompetitive binders (one-dimensional and two-dimensional experiments). The method is highly sensitive, unveiling interactions that are too weak to generate changes in chemical shifts or relaxation times. It is operationally very simple and hence, it is amenable to ready implementation by nonspecialists. Examples of application such as detecting the formation of interpolymer complexes, cyclodextrin host–guest interactions, and the screening of DNA ligands are included that demonstrate the reliability and broad applicability of the method.

Predicting PSR Filters by Transverse Relaxation Enhancements

The paramagnetic spin relaxation (PSR) filter allows the suppression of the NMR resonances of individual components in mixtures according to their Gd(3+)-complexing ability. The difficulty in predicting this property hampers, however, the widespread application of this filter. Herein we describe that the PSR filter is dominated by the transverse relaxation enhancement (R(2p)) experienced by nuclei in the presence of Gd(3+), so that R(2p) represents a reliable predictive tool of suppression in the 1D and 2D PSR filter of complex mixtures. The robustness of R(2p) as a predictive tool in PSR filters has been demonstrated at different magnetic fields and for the (1)H, (13)C, COSY, and HMQC filtering of commercial multicomponent compositions, including beverages and drugs.

Fast-field-cycling relaxometry enhanced by Dynamic Nuclear Polarization

Fast-field-cycling (FFC) NMR relaxometry experiments enhanced by in-situ Dynamic Nuclear Polarization (DNP) were performed for 1H and 13C nuclear spins with a setup based on a commercial electronically switching FFC relaxometer and a recently-built Alderman–Grant type microwave resonator for 2GHz. DNP-enhanced 1H relaxation dispersion profiles were compared to reference measurements and literature data in order to prove the reliability of DNP-enhanced relaxometry data. The method was then used to investigate the paramagnetic nuclear spin relaxation of 13C in a benzene-13C6,D6 solution of nitroxide radicals. Dispersion profiles of good quality were obtained within 2h of measurement time from a comparatively small sample of 60μl. As a prospect for future applications, DNP experiments with a high-molecular weight Poly(butadiene-1,4) melt and BDPA radical were carried out at 2GHz and 9.7GHz microwave frequency, showing solid effect DNP enhancements. In-situ hyperpolarization by DNP may provide extended possibilities for FFC relaxometry, e.g. by allowing enhanced detection of dilute or insensitive nuclear spins, additional selectivity or faster measurements of small samples.

Membrane Interaction of Disease-Related Dynorphin A Variants

The membrane interaction properties of two single-residue variants, R6W and L5S, of the 17-amino acid neuropeptide dynorphin A (DynA) were studied by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy. Corresponding gene mutations have recently been discovered in humans and causatively linked to a neurodegenerative disorder. The peptides were investigated in buffer and in isotropic solutions of q = 0.3 bicelles with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or DMPC (0.8) and 1,2-dimyristoyl-sn-glycero-3-phospho(1'-rac-glycerol) (DMPG) (0.2). The CD results and the NMR secondary chemical shifts show that R6W-DynA has a small α-helical fraction in buffer, which increases in the presence of bicelles, while L5S-DynA is mainly unstructured under all conditions studied here. R6W-DynA has an almost complete association with zwitterionic bicelles (∼90%, as probed by NMR diffusion experiments), similar to the behavior of wtDynA, while L5S-DynA has a weaker association (∼50%). For all peptides, the level of bicelle association is increased in negatively charged bicelles. The L5A-DynA peptide adopts a very shallow position in the headgroup region of the bicelle bilayer, as studied by paramagnetic spin relaxation enhancement experiments using paramagnetic probes. Similarly, the results show that R6W-DynA is more deeply buried in the bilayer, with only the C-terminal residues exposed to solvent, again more similar to the case of wild-type DynA. We suggest that the results presented here may explain the differences in cell toxicity of these disease-related neuropeptide variants.

Structural Characterization of Hsp12, the Heat Shock Protein from Saccharomyces cerevisiae, in Aqueous Solution Where It Is Intrinsically Disordered and in Detergent Micelles Where It Is Locally α-Helical

Hsp12 (heat shock protein 12) belongs to the small heat shock protein family, partially characterized as a stress response, stationary phase entry, late embryonic abundant-like protein located at the plasma membrane to protect membrane from desiccation. Here, we report the structural characterization of Hsp12 by NMR and biophysical techniques. The protein was labeled uniformly with nitrogen-15 and carbon-13 so that its conformation could be determined in detail both in aqueous solution and in two membrane-mimetic environments, SDS and dodecylphosphocholine (DPC) micelles. Secondary structural elements determined from assigned chemical shifts indicated that Hsp12 is dynamically disordered in aqueous solution, whereas it gains four helical stretches in the presence of SDS micelles and a single helix in presence of DPC. These conclusions were reinforced by circular dichroism spectra of the protein in all three environments. The lack of long range interactions in NOESY spectra indicated that the helices present in SDS micelles do not pack together. R(1) and R(2), relaxation and heteronuclear NOE measurements showed that the protein is disordered in aqueous solution but becomes more ordered in presence of detergent micelles. NMR spectra collected in presence of paramagnetic spin relaxation agents (5DSA, 16DSA, and Gd(DTPA-BMA)) indicated that the amphipathic α-helices of Hsp12 in SDS micelles lie on the membrane surface. These observations are in agreement with studies suggesting that Hsp12 functions to protect the membrane from desiccation.

Extended conformations in alanine peptides

Spectroscopic evidence for the presence of local order in unfolded proteins, including polyproline II (PII) structure, now appears incontrovertible. The data supporting this order relies on analysis of short chain peptides. The dimensions of unfolded chains nevertheless conform to random coils. We have re-examined the dimensional properties of short chains using paramagnetic proton spin relaxation measurements to evaluate intermediate range distances (r) within alanine peptides linked to short proline arms.(1) Two peptides were employed in our studies,, OO-T∗-PPPA∗PPPA∗-OO and OO-T∗-PPPAAAA∗-OO, where O is Ornithine, T∗ is Toac (2,2,6,6-Tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), A∗ is 15N labelled alanine. Mesostate based Monte-Carlo sampling calculations (2) were carried out to interpret the relaxation data. The conclusion of the study is that over 90% occupation of extended conformations, i.e. PII and β mesostates, is required to reproduce the experimentally observed distance averaging. Compact structures including αR, αL and turns are clearly present but are not dominant in the conformational ensemble. Analysis of the conformation of other side chains will be presented.(1) Chen, K et al. Ang. Chem. Int. Ed. 2007 46, 9036.(2) Gong, H, Fleming, PH, Rose, GD. Proc. Natl. Acad. Sci. USA 2005, 102, 16227.

Comment on “Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies” [J. Chem. Phys. 127, 124705 (2007)

In the recent study of Cu(2+)-doped BaF(2) Hoffmann et al. demonstrated using electron paramagnetic resonance spectroscopy that the Cu(2+) impurity experiences an off-center shift from the substitutional position and ascribe this distortion to the Jahn-Teller effect. In this comment, we propose that the off-center distortion observed for BaF(2):Cu(2+) is driven by pseudo-Jahn-Teller interactions which are qualitatively different and in competition with the usual Jahn-Teller effect. It is also stressed that off-center phenomena in a given host lattice are not necessarily enhanced by decreasing the ionic radius of the impurity.

Iron chelates: a challenge to chemists and Mössbauer spectroscopists

The speciation of iron in aqueous solutions containing Fe3 + and selected chelates such as EDTA, EDDA, CDTA and HEDTA has been studied using transmission 57Fe Mossbauer spectrometry in frozen solutions. The protonation of various complexes as well as binuclear complex formation could be detected as a function of pH. Autoreduction of Fe3 + to Fe2 + was observed in several cases. Reaction with hydrogen peroxide proved to be rather different for the four ligands, while the dihapto complex [XFe(η 2-O2)]3 − had surprisingly identical Mossbauer parameters for X = EDTA, CDTA or HEDTA. Paramagnetic spin relaxation observed in the Mossbauer spectra was found to be strongly influenced by the identity of the chelating ligand, despite the basically spin-spin origin of the phenomenon.

Paramagnetic NMR Relaxation in Polymeric Matrixes: Sensitivity Enhancement and Selective Suppression of Embedded Species (1H and 13C PSR Filter)

A study of the practical applications of the addition of paramagnetic spin relaxation (PSR) ions to a variety of polymers (PLL, PAA, PGA, PVP, and polysaccharides such as hyaluronic acid, chitosan, mannan, and dextran) in solution (D2O and DMSO-d6) is described. Use of Gd(III), Cu(II), and Mn(II) allows a reduction of up to 500% in the 1H longitudinal relaxation times (T1), and so in the time necessary for recording quantitative NMR spectra (sensitivity enhancement) neither an increase of the spectral line width nor chemical shift changes resulted from addition of any of the PSR agents tested. Selective suppression of the 1H and 13C NMR signals of certain components (low MW molecules and polymers) in the spectrum of a mixture was attained thanks to their different sensitivity [transverse relaxation times (T2)] to Gd(III) (PSR filter). Illustration of this strategy with block copolymers (PGA-g-PEG) and mixtures of polymers and low MW molecules (i.e., lactose-hyaluronic acid, dextran-PAA, PVP-glutamic acid) in 1D and 2D NMR experiments (COSY and HMQC) is presented. In those mixtures where PSR and CPMG filters alone failed in the suppression of certain components (i.e., PVP-mannan-hyaluronic acid) due to their similarity of 1H T2 values and sensitivities to Gd(III), use of the PSR filter in combination with CPMG sequences (PSR-CPMG filter) successfully resulted in the sequential suppression of the components (hyaluronic acid first and then mannan).

Molecular structure and dynamics of off-center Cu2+ ions and strongly coupled Cu2+–Cu2+ pairs in BaF2 crystals: Electron paramagnetic resonance and electron spin relaxation studies

X-band and Q-band electron paramagnetic resonance (EPR) spectra of Cu(2+) in BaF(2) crystal were recorded in the temperature range of 4.2-200 K. Spin-Hamiltonian parameters of single Cu(2+) complexes and of Cu(2+)-Cu(2+) pairs were derived and discussed. A special attention was paid to the dimeric species. Their molecular ground state configuration was found as having antiferromagnetic intradimer coupling with the singlet-triplet splitting J=-35 cm(-1). The zero-field splitting being D=0.0365 cm(-1) at 4.2 K increases with temperature as an effect of thermal population of excited dimer configurations. Electron spin echo (ESE) method was used for measurements of electron spin lattice and phase relaxation. The spin-lattice relaxation data show that except for coupling to the host lattice phonons the Cu(2+) ions are involved in local mode motions with energy of 82 cm(-1). Phase relaxation (ESE dephasing) of single Cu(2+) ions is due to spin diffusion at low temperatures. This relaxation is hampered for temperatures higher than 30 K due to the triplet state population of neighboring Cu(2+)-Cu(2+) dimers, which disturb dipolar coupling between Cu(2+) ions. For higher temperatures the relaxation is dominated by Raman T(1) processes. Fourier transform ESE spectrum displays dipolar Cu-F splitting which allowed determination of the off-center shift of Cu(2+) as delta(s)=0.132 nm. The dynamical effects observed in EPR spectra and in electron spin relaxation both for single Cu(2+) ions and Cu(2+)-Cu(2+) pairs are discussed as due to jumps between six off-center positions in the crystal unit cell and jumps between various dimer configurations.

Managing the solvent water polarization to obtain improved NMR spectra of large molecular structures

In large molecular structures, the magnetization of all hydrogen atoms in the solute is strongly coupled to the water magnetization through chemical exchange between solvent water and labile protons of macromolecular components, and through dipole-dipole interactions and the associated "spin diffusion" due to slow molecular tumbling. In NMR experiments with such systems, the extent of the water polarization is thus of utmost importance. This paper presents a formalism that describes the propagation of the water polarization during the course of different NMR experiments, and then compares the results of model calculations for optimized water polarization with experimental data. It thus demonstrates that NMR spectra of large molecular structures can be improved with the use of paramagnetic spin relaxation agents which selectively enhance the relaxation of water protons, so that a substantial gain in signal-to-noise can be achieved. The presently proposed use of a relaxation agent can also replace the water flip-back pulses when working with structures larger than about 30 kDa. This may be a valid alternative in situations where flip-back pulses are difficult to introduce into the overall experimental scheme, or where they would interfere with other requirements of the NMR experiment.

Mössbauer spectroscopic evidence for iron(III) complexation and reduction in acidic aqueous solutions of indole-3-butyric acid

Mossbauer spectroscopic studies were carried out in acidic (pH 2.3) 57FeIII nitrate containing aqueous solutions of indole-3-butyric acid (IBA), rapidly frozen in liquid nitrogen at various periods of time after mixing the reagents. The data obtained show that in solution in the presence of IBA, iron(III) forms a complex with a dimeric structure characterised by a quadrupole doublet, whereas without IBA under similar conditions iron(III) exhibits a broad spectral feature due to a slow paramagnetic spin relaxation which, at liquid nitrogen temperature, results in a large anomalous line broadening (or, at T = 4.2 K, in a hyperfine magnetic splitting). The spectra of 57FeIII+IBA solutions, kept at ambient temperature under aerobic conditions for increasing periods of time before freezing, contained a gradually increasing contribution of a component with a higher quadrupole splitting. The Mossbauer parameters for that component are typical for iron(II) aquo complexes, thus showing that under these conditions gradual reduction of iron(III) occurs, so that the majority (85%) of dissolved iron(III) is reduced within 2 days. The Mossbauer parameters for the iron(III)-IBA complex in aqueous solution and in the solid state (separated from the solution by filtration) were found to be similar, which may indicate that the dissolved and solid complexes have the same composition and/or iron(III) coordination environment. For the solid complex, the data of elemental analysis suggest the following composition of the dimer: [L2Fe-(OH)2-FeL2] (where L is indole-3-butyrate). This structure is also in agreement with the data of infrared spectroscopic study of the complex reported earlier, with the side-chain carboxylic group in indole-3-butyrate as a bidentate ligand. The Mossbauer parameters for the solid 57FeIII-IBA complex at T = 80 K and its acetone solution rapidly frozen in liquid nitrogen were virtually identical, which indicates that the complex retains its structure upon dissolution in acetone.

A low-field paramagnetic nuclear spin relaxation theory

A low-field theory of paramagnetically enhanced proton spin-lattice relaxation is developed resulting in closed analytical expressions. The theory describes paramagnetically enhanced water proton spin-lattice relaxation in complexes of transition metal ions with electron spin quantum number S=1. In the low-field regime the electron spin system is dominated by a static or permanent zero-field splitting interaction which is much larger than the Zeeman interaction. The electron spin is thus quantized in the molecular fixed principal frame of the static ZFS-interaction rather than in the laboratory fixed frame. In the molecular fixed frame the zero-field splitting Hamiltonian is characterized by an axial parameter (D) and a rhombic parameter (E). It is shown how the relative magnitude of D and E strongly influence the magnitude of the paramagnetically enhanced proton spin lattice relaxation rate. In describing electron spin relaxation in the molecular fixed frame, we consider a transient ZFS-interaction and a reorientation modulated Zeeman interaction as the two main relaxation mechanisms. Then, using Redfield theory we derive expressions for the relevant electron spin relaxation rates in terms of spectral densities. In contrast to the Zeeman or high field regime, the electron spin system in the low field limit does not have a permanent magnetic moment. The lack of this magnetic moment implies that electron spin-lattice relaxation processes do not influence the paramagnetically enhanced proton spin-lattice relaxation rates. Instead three spin–spin relaxation rates determine the enhanced proton spin-lattice relaxation profile. We express the electron spin–spin relaxation rates in terms of spectral densities determined by the mean square value of the transient zero-field splitting interaction, its characteristic correlation time, and the reorientational correlation time of rank 1.

Mössbauer effect and magnetization studies of mixed valence solid solution NpOH·NpH 2− x

By dissolving Np metal in a mixed solution of NaCl, MgCl 2 and Na 2SO 4 in water, a new compound NpOH precipitates together with NpH 2− x forming a solid solution (double salt) NpOH·NpH 2− x . 237Np Mössbauer spectroscopy, d.c. magnetization and X-ray studies of the precipitate are reported. NpOH crystallizes in the CaF 2 ( Fm3m) structure with a lattice parameter of 5.375 Å. The Mössbauer absorption spectrum of NpOH is very well resolved from that of NpH 2− x (single absorption line) owing to their large difference in Mössbauer isomer shift (IS; 28.1 mm s −1). The Mössbauer studies indicate that the Np ion in NpOH is tetravalent whereas in NpH 2− x it is trivalent. Both behave paramagnetically down to 2.5 K; however, NpOH exhibits paramagnetic spin relaxation. The Mössbauer spectra can be fitted within a relaxation model assuming an isolated Kramers doublet and, even better, assuming a quartet ground state ( T 8 (2) of 4 I 9 2 in cubic symmetry). The saturation hyperfine field derived is 350(50) T. NpH 2− x in the solid solution exhibits a positive IS relative to NpH 2.0 as a result of volume contraction. The IS of the neptunium dihydride system depends on the hydrogen concentration. From this result we derive for NpH 2− x stabilized in the solid solution the value x≈0.4.

Superconductivity and magnetism in Y1−x Ca x Sr2GaCu2O7−δ

Samples of Y1−x Ca x Sr2GaCu2O7−δ (x=0, 0.4) doped with57Fe, prepared under various oxygen pressures, have been studied by magnetometry and Mossbauer spectroscopy. Most of the iron ions (Fe3+) enter the Ga(Cu(1)) site. For thex=0 sample, the Mossbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of Cu,T N=370 K. The iron ions in the Ga site display magnetic order only at low temperatures. At temperatures above 90 K, these iron ions display a pure quadrupole doublet Mossbauer spectrum. The samplex=0.4 also displays magnetic order of the Cu(2) site,T N≈370 K. A sharp drop in the hyperfine field is observed atT N, probably associated with a first-order phase transition or two-dimensional ordering. The iron nuclei in the Ga site display paramagnetic long spin relaxation time phenomena at 4.2 K. Thex=0.4 sample prepared under 110 atm oxygen pressure, displays superconductivity,T c≈50 K. The Mossbauer spectra give evidence of the presence of two phases. One displays magnetic order, the other is paramagnetic, the last is probably associated with the superconducting phase.

Magnetism and superconductivity in Y1−χCaχSr2GaCu2O7−δ

Samples of Y 1−χ Ca χ Sr 2 GaCu 2 O 7−δ ( χ =0, 0.4), prepared under various oxygen pressures, have been studied by magnetometry. Samples doped with 57 Fe have been studied also by Mössbauer spectroscopy. The majority of the iron ions (Fe 3+ ) enters the Ga (Cu(1)) site, a minority enters the Cu(2) site. For the χ =0 sample, the Mössbauer spectra of the iron nuclei in the Cu(2) sublattice display magnetic order of the Cu with T N=370 K. The moments order along the c -axis. The iron in the Ga site displays magnetic order only at low temperatures, probably induced from the Cu(2) site. At temperatures above 90 K, these iron ions display a single pure quadrupole doublet Mössbauer spectrum, with a splitting of 2.0 mm/s. The sample χ =0.4 prepared under ambient pressure of oxygen (APO) also displays magnetic order of the Cu(2) site, with T N ≈370 K. The iron nuclei in the Ga site display paramagnetic long spin relaxation times phenomena at 4.2 K. The χ =0.4 sample, prepared under 110 atm oxygen pressure (HPO), displays superconductivity according to the magnetometry measurements ( T c≈50 K). The Mössbauer spectra give evidence of the presence of two phases in this sample. One displays magnetic order like the APO sample, the other is paramagnetic. The latter is probably associated with the superconducting phase.

Nucleation and medium Range Order in Silicate Liquids: Inferences From NMR Spectroscopy

ABSTRACTNMR spectroscopy is beginning to provide quantitative information on short- and medium-range structure in silicate glasses and liquids, and on molecular-scale dynamics in the liquids. Data on coordination numbers, second and higher neighbor connectivities, and larger scale heterogeneities, as determined by NMR at ambient and high temperatures, are potentially very useful in models of nucleation and growth of immiscible liquid and crystalline phases. New techniques such as dynamic angle spinning (DAS), high temperature magic angle spinning (MAS) are especially promising. Detailed studies of paramagnetic nuclear spin relaxation can characterize compositional heterogeneity in glasses to distance scales of at least several nanometers.