Soluble Spin Research Articles

Mixed order transition and condensation in an exactly soluble one dimensional spin model

Mixed order phase transitions (MOT), which display discontinuous order parameter and diverging correlation length, appear in several seemingly unrelated settings ranging from equilibrium models with long-range interactions to models far from thermal equilibrium. In a recent paper [], an exactly soluble spin model with long-range interactions that exhibits MOT was introduced and analyzed both by a grand canonical calculation and a renormalization group analysis. The model was shown to form a bridge between two classes of 1D models exhibiting MOT, namely between spin models with inverse distance square interactions and surface depinning models. In this paper, we elaborate on the calculations performed in []. We also analyze the model in the canonical ensemble, which yields a better insight into the mechanism of MOT. In addition, we generalize the model to include Potts and general Ising spins and also consider a broader class of interactions that decay with distance using a power law different from 2.

How in vivo EPR measures and images oxygen.

The partial pressure of oxygen (pO₂) in tissues plays an important role in the pathophysiology of many diseases and influences outcome of cancer therapy, ischemic heart and cerebrovascular disease treatments and wound healing. Over the years a suite of EPR techniques for reliable oxygen measurements has been developed. This is a mini-review of pulse EPR in vivo oxygen imaging methods that utilize soluble spin probes. Recent developments in pulse EPR imaging technology have brought an order of magnitude increase in image acquisition speed, enhancement of sensitivity and considerable improvement in the precision and accuracy of oxygen measurements.

ESR Microscopy for Biological and Biomedical Applications

We report on electron-spin resonance microscopy (ESRM) providing sub-micron resolution (~700nm) with a high spin concentration sample, i.e. lithium phthalocyanine (LiPc) crystal. For biomedical applications of our ESRM, we have imaged samples containing rat basophilic leukemia (RBL) cells as well as cancerous tissue samples with a resolution of several microns using a water soluble spin probe, Trityl_OX063_d24. Phantom samples with the nitroxide spin label, (15)N PDT, were also imaged to demonstrate that nitroxides, which are commonly used as spin labels, may also be used for ESRM applications. ESRM tissue imaging would therefore be valuable for diagnostic or therapeutic purposes. Also, ESRM can be used to study the motility or the metabolism of cells in various environments. With further modification and/or improvement of imaging probe and spectrometer instrumentation sub-micron biological images should be obtainable, thereby providing a useful tool for various biomedical applications.

Structural Analysis of Large Protein Complexes Using Solvent Paramagnetic Relaxation Enhancements

The solvent helps out: Conventional protocols for structural analysis of protein complexes often fail if only sparse experimental data are available. NMR-spectroscopy-derived solvent paramagnetic relaxation enhancements (PREs) from a soluble spin label (picture) significantly improve the structural quality of a 150 kDa protein complex. This general protocol is an efficient approach for structural analysis of large protein complexes in solution.

On the surface properties of oleate micelles and oleic acid/oleate vesicles studied by spin labeling

Dilute aqueous systems composed of sodium oleate micelles and sodium oleate/oleic acid vesicles were investigated as a function of pH by electron spin resonance spectroscopy with TEMPO-stearate TEMPO-stearamide as well as with a positively charged water soluble spin label, TEMPO-choline. The dynamics of the three TEMPO-spin labels were found to be sensitive to changes in the interfacial region of the aggregates as a function of pH. The results obtained are consistent with the formation of a hydrogen bond network (RCOO−↔HOOCR) at the surface of the sodium oleate/oleic acid system in the course of the transformation of micelles into the closed bilayers (vesicles). Vesicles formation below pH 10 was determined independently with a spin labeled glucose derivative.

EPR Investigations of Spin‐Probe Dynamics in Aqueous Dispersions of a Nonionic Amphiphilic Compound

AbstractDynamics of various spin probes in aqueous dispersions of nonionic amphiphilic compound, [poly(oxyethylene) hydrogenated castor oil, HCO], were investigated by EPR (electron paramagnetic resonance) and saturation recovery (SR) spectroscopies. Partitioning, rotational correlation time (τR), rotational diffusion coefficient, and electron spin‐lattice relaxation time (T1e) in dispersions of the HCO membrane were obtained. The partitioning of water soluble spin probes, DTBN and TEMPO, in the aqueous and vesicle phases was determined by an EPR linewidth simulation as a function of temperature. The results suggest that DTBN and TEMPO have a similar partitioning in the vesicle phase throughout the temperatures studied. The longer τR and shorter T1e (~0.33 μs) values of DTBN in the vesicle phase were obtained, and could be attributed to the probe environment in the membrane. The simulation results for fast tumbling probes were quite different from those of conventional intensity analysis (spectral parameter, f). Thus, the simulation and T1e analyses have provided a quantitative understanding of the probe dynamics in both phases. Aliphatic spin probes, doxylstearic acids (DSAs) and 3β‐doxyl‐5α‐cholestane (CHL), were used for monitor of various membrane motions. The EPR spectra were quantitatively analyzed by a slow tumbling simulation. The rotational diffusion coefficients and order parameter were obtained by the simulation. In addition, the direct observations of the behavior of the probes were measured by SR method. The results were consistent with T1e obtained for spin probes. Thus, the quantitative results regarding EPR, SR method, various simulation analyses have provided detailed information regarding physicochemical properties of the various moieties of the probe region in the amphiphilic compound.

Anyonic loops in three-dimensional spin liquid and chiral spin liquid

We established a large class of exactly soluble spin liquids and chiral spin liquids on three-dimensional helix lattices by introducing Kitaev-type's spin coupling. In the chiral spin liquids, exact stable ground states with spontaneous breaking of the time reversal symmetry are found. The fractionalized loop excitations in both the spin and chiral spin liquids obey non-Abelian statistics. We characterize this kind of statistics by non-Abelian Berry phase and quantum algebra relation. The topological correlation of loops is independent of local order parameter and it measures the intrinsic global quantum entanglement of degenerate ground states.

Gauge symmetry in Kitaev-type spin models and index theorems on odd manifolds

We construct an exactly soluble spin- 1 2 model on a honeycomb lattice, which is a generalization of Kitaev model. The topological phases of the system are analyzed by study of the ground state sector of this model, the vortex-free states. Basically, there are two phases, A phase and B phase. The behaviors of both A and B phases may be studied by mapping the ground state sector into a general p-wave paired states of spinless fermions with tunable pairing parameters on a square lattice. In this p-wave paired state theory, the A phase is shown to be the strong paired phase, an insulating phase. The B phase may be either gapped or gapless determined by the generalized inversion symmetry is broken or not. The gapped B is the weak pairing phase described by either the Moore–Read Pfaffian state of the spinless fermions or anti-Pfaffian state of holes depending on the sign of the next nearest neighbor hopping amplitude. A phase transition between Pfaffian and anti-Pfaffian states are found in the gapped B phase. Furthermore, we show that there is a hidden SU(2) gauge symmetry in our model. In the gapped B phase, the ground state has a non-trivial topological number, the spectral first Chern number or the chiral central charge, which reflects the chiral anomaly of the edge state. We proved that the topological number is identified to the reduced eta-invariant and this anomaly may be cancelled by a bulk Wess–Zumino term of SO(3) group through an index theorem in 2 + 1 dimensions.

Exactly soluble quantum spin models on a double layer: The net spin model

We study a set of exactly soluble spin models in one and two dimensions for any spin S. Its ground state, the excitation spectrum, and quantum phase transition points, as well as dimensional crossover are determined.

Mobility of solutes in frozen pork studied by electron spin resonance spectroscopy: evidence for two phase transition temperatures

The mobility of solutes in frozen pork ( longissimus dorsi) was studied by X band electron spin resonance spectroscopy (ESR) using the three water soluble spin probes TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), CAT 1 (4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl) and TEMPO choline (4-(N,N-dimethyl-N-(2-hydroxyethyl))ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl). The mobility was quantified as the observed hyperfine coupling constant A′ zz. The three spin probes, of which only the neutral TEMPOL may penetrate membranes, gave very similar values of A′ zz as a function of the temperature. A′ zz was constant at temperatures below −55 °C, whereas A′ zz decreased as the temperature was increased above −55 °C. This behavior has been interpreted as an increase in the amplitude of the rotational oscillations of the spin probes (and other solutes) with temperature. Above −13 °C the spin probes gave isotropic ESR spectra indicating free rotational mobility in a molten liquid phase. These results are discussed in relation to glass transitions in frozen pork.

Spin-labeling study of membranes in wheat embryo axes. 1. Partitioning of doxyl stearates into the lipid domains

The interaction of lipid soluble spin labels with wheat embryo axes has been investigated to obtain insight into the structural organization of lipid domains in embryo cell membranes, using conventional electron paramagnetic resonance (EPR) and saturation transfer EPR (ST-EPR) spectroscopy. Stearic acid spin labels ( n-SASL) and their methylated derivatives ( n-MeSASL), labelled at different positions of their doxyl group ( n=5, 12 and 16), were used to probe the ordering and molecular mobility in different regions of the lipid moiety of axis cell membranes. The ordering and local polarity in relation to the position of the doxyl group along the hydrocarbon chain of SASL, determined over the temperature range from −50 to +20°C, are typical for biological and model lipid membranes, but essentially differ from those in seed oil droplets. Positional profiles for ST-EPR spectra show that the flexibility profile along the lipid hydrocarbon chain does exist even at low temperatures, when most of the membrane lipids are in solid state (gel phase). The ordering of the SASL nitroxide radical in the membrane surface region is essentially higher than that in the depth of the membrane. The doxyl groups of MeSASLs are less ordered (even at low temperatures) than those of the corresponding SASLs, indicating that the MeSASLs are located in the bulk of membrane lipids rather than in the protein boundary lipids. The analysis of the profiles of EPR and ST-EPR spectral parameters allows us to conclude that the vast majority of SASL and MeSASL molecules accumulated in embryo axes is located in the cell membranes rather than in the interior of the oil bodies. The preferential partitioning of the doxyl stearates into membranes demonstrates the potential of the EPR spin-labelling technique for the in situ study of membrane behavior in seeds of different hydration levels.

Exact Ground States and Excited States of Net Spin Models

We study a set of exactly soluble net spin models. There exist two kinds of ground state, one is a complete dimerized state, and the other one is the ground state of corresponding spin-1 model. For the excitation gap, various phases were discovered and determined.

An EPR investigation of surfactant action on bacterial membranes.

The effects of the surfactants, alcohol ethoxylate, amine ethoxylate, amine oxide and SDS on cell membranes were investigated using the lipid soluble spin label 5-doxyl stearic acid (5-DS). Electron paramagnetic resonance (EPR) spectroscopy revealed that the action of the surfactants was to significantly increase membrane fluidity of Proteus mirabilis, Staphylococcus aureus and Saccharomyces cerevisiae. The action of these surfactants as biocides was investigated and found to be dependent on the type of organism tested. There was, however, no direct correlation between enhanced membrane fluidity observed due to the action of the surfactants and biocidal activity. Data presented suggest that perturbing the fluidity of the cytoplasmic membrane is not immediately responsible for cell death.

An EPR investigation of surfactant action on bacterial membranes

The effects of the surfactants, alcohol ethoxylate, amine ethoxylate, amine oxide and SDS on cell membranes were investigated using the lipid soluble spin label 5-doxyl stearic acid (5-DS). Electron paramagnetic resonance (EPR) spectroscopy revealed that the action of the surfactants was to significantly increase membrane fluidity of Proteus mirabilis, Staphylococcus aureus and Saccharomyces cerevisiae. The action of these surfactants as biocides was investigated and found to be dependent on the type of organism tested. There was, however, no direct correlation between enhanced membrane fluidity observed due to the action of the surfactants and biocidal activity. Data presented suggest that perturbing the fluidity of the cytoplasmic membrane is not immediately responsible for cell death.

EPR study of mucoadhesive ointments for delivery of liposomes into the oral mucosa

Local drug delivery to the oral cavity tissues has been used for treatment of periodontal disease, aphthosus stomatitis, lichen planus, bacterial and fungal infections. The liposome stability in different mucoadhesive ointments, their transport into dog's oral mucosa and gingiva and washing out of ointments from tissue surface was investigated by electron paramagnetic resonance (EPR). Liposomes were composed of soya lecithin, cholesterol and lipoaminosalt (55:35:10 w/w) and encapsulated with spin labeled water soluble spin probe ASL ( N-1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl- N-dimethyl- N-hydroxyethylammonium iodide). Polymethyl methacrylate, Carbopol 934P and Orabase as the bioadhesive ointments were used. The stability of liposomes did not change significantly after mixing with polymethyl methacrylate, but decreases in Carbopol and even more in Orabase. Washing out experiment shows that all three ointments adhere well to oral mucosa and gingiva. After 10 min of washing, approximately 30% of hydrogels were washed out. The transport experiment showed that liposomes limit the transport of hydrophilic substance to the superficial layer of epithelium. In oral mucosa, as opposed to gingiva, hydrogels enhance the transport as compared to solution of ASL or liposomes. Among the examined ointments, polymethyl methacrylate proved to be the most appropriate for local application of liposome entrapped drug to oral mucosa or gingiva.

Investigation of some soluble lattice spin models

Investigation of some soluble lattice spin models

An NMR and spin label study of the effects of binding calcium and troponin I inhibitory peptide to cardiac troponin C

The paramagnetic relaxation reagent, 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HyTEMPO), was used to probe the surface exposure of methionine residues of recombinant cardiac troponin C (cTnC) in the absence and presence of Ca2+ at the regulatory site (site II), as well as in the presence of the troponin I inhibitory peptide (cTnIp). Methyl resonances of the 10 Met residues of cTnC were chosen as spectral probes because they are thought to play a role in both formation of the N-terminal hydrophobic pocket and in the binding of cTnIp. Proton longitudinal relaxation rates (R1's) of the [13C-methyl] groups in [13C-methyl]Met-labeled cTnC(C35S) were determined using a T1 two-dimensional heteronuclear single- and multiple-quantum coherence pulse sequence. Solvent-exposed Met residues exhibit increased relaxation rates from the paramagnetic effect of HyTEMPO. Relaxation rates in 2Ca(2+)-loaded and Ca(2+)-saturated cTnC, both in the presence and absence of HyTEMPO, permitted the topological mapping of the conformational changes induced by the binding of Ca2+ to site II, the site responsible for triggering muscle contraction. Calcium binding at site II resulted in an increased exposure of Met residues 45 and 81 to the soluble spin label HyTEMPO. This result is consistent with an opening of the hydrophobic pocket in the N-terminal domain of cTnC upon binding Ca2+ at site II. The binding of the inhibitory peptide cTnIp, corresponding to Asn 129 through Ile 149 of cTnI, to both 2Ca(2+)-loaded and Ca(2+)-saturated cTnC was shown to protect Met residues 120 and 157 from HyTEMPO as determined by a decrease in their measured R1 values. These results suggest that in both the 2Ca(2+)-loaded and Ca(2+)-saturated forms of cTnC, cTnIp binds primarily to the C-terminal domain of cTnC.

A 1H NMR study on the interaction of aminoxyl paramagnetic probes with unfolded peptides

The use of soluble spin labels to filter out the cross peaks of outer proton nuclei in 2D NMR spectra has been proposed as a general method to obtain structural information for complex molecules. Here the paramagnetic effects observed on backbone protons of an unfolded 27 amino acid peptide are discussed. The lack of any differential intensity change of the NH–Hα cross-peaks in TOCSY spectra is suggested as an additional general criterion for the identification of unfolded structures.

Probing protein structure by solvent perturbation of nuclear magnetic resonance spectra: Nuclear magnetic resonance spectral editing and topological mapping in proteins by paramagnetic relaxation filtering

Soluble spin labels, which “bleach” the surface proton resonances of a protein to n.m.r. measurements, can provide useful information about protein conformation and dynamics. The use of the soluble nitroxide, TEMPOL, has been explored to show the correlation of the paramagnetic perturbations of protein two-dimensional n.m.r. data with proton exposure to the free radical in hen egg-white lysozyme. The results demonstrate that the nitroxide approaches the protein randomly, and that the extent of the observed paramagnetic effects reflects the native folding pattern of the protein. A correlation of spectral simplification with the known tertiary structure establishes the feasibility of new strategies for topological mapping of surface and buried protons of the protein. Application to the elucidation of protein structure and to the study of dynamical processes is discussed.

Evidence for L· against LOO· being spin-trapped by 4-POBN during the reaction of Fe2+-induced lipid peroxidation

The character of a water soluble spin trap-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN) in the peroxidation of linoleic acid induced by Fe2+ has been studied. An ESR spectrum with six lines (a N=1.58 mT,a H=0.26 mT) was formed after Fe2+ and 4-POBN were added to linoleic acid solution. Its intensity depends on the concentration of Fe2+ and the incubation time of the system. It increased when the reaction solution was flushed with nitrogen, comparing with the reaction solution exposed to air at the same concentration of Fe2+, 4-POBN and linoleic acid. And it decreased greatly when the system was bubbled with oxygen. At the same time, the concentration of Fe2+ did not decrease distinctly with the existence of oxygen detected colorimetrically using phenanthroline, by ultraviolet spectruphotometer. When N-t-butyl-phenylnitrone (PBN) was used instead of 4-POBN, a similar spectrum (a N=1.61 mT,a H=0.32 mT) was obtained. The hyperfine coupling constants (hfcs) of the signal are very similar to that of the adduct formed from alkyl radical with PBN. According to above results it is reasonable to conclude that the trapped radical should be assigned as a carbon-centered species (i.e. L·), not the oxygen-derived peroxyl one (i.e. LOO·). The adduct of 4-POBN and L· can survive as long as two hours.