NMR study of molecular motions in two disordered organic solids

The molecular motions are studies in two disordered materials that undergo glass transitions. Glycerol is a conventional glass former and cyclohexanol is an orientational glass former.;The technique used in the glycerol experiments was spectral hole burning. Chemical shift anisotropy produces inhomogeneous broadening of NMR lines in orientationally disordered and polycrystalline solids. By saturating or inverting a portion of the anisotropic line, \u27burning a hole\u27, molecules of certain orientations are tagged. Subsequent molecular reorientations result in spectral diffusion which is not related to spin-spin interactions. By measuring the broadening and recovery of the hole as a function of time, detailed knowledge of the reorientation is obtained. For example, the mean jump rate and the lower limit of angular reorientations are determined. The reorientation rate in supercooled glycerol is followed from 10(\u27-2)s(\u27-1) to 10(\u272) s(\u27-1). Our measurements agreed with previous results and extended them to lower frequencies. The mean jump size was determined to be greater than 45 degrees. The hole recovery curves were not exponential, but were fitted with the Williams-Watts function, exp = ((tau)/(tau)(,0))(\u27(beta)) with (beta) = 0.5.;The motions in the rotor phase of solid cyclohexanol are studied with proton NMR from the melt down to 5 K. Particular attention is paid to the variation of the linewidth with temperature and to the temperature and frequency dependences of T(,1). We find there are two distinct motions that cause minima in T(,1) as a function of temperature. These two motions were observed in dielectric experiments. From the proton line narrowing and C(\u2713) high-resolution solid state spectra, the high temperature \u27(alpha)\u27 motion is identified as overall molecular rotation. The low temperature \u27(beta)\u27 motion is identified as a uniaxial internal rotation of the cyclohexyl ring about the CO bond, with the COH group remaining stationary. This explains both the strong spin relaxation and the weak dielectric relaxation peak associated with the (beta) motion. Both motions have distributions of correlation times, as seen from the shallow T(,1) minima and the weak temperature and frequency dependences of T(,1). From 100 K to 5 K, the temperature dependence continues to be weak. The frequency dependence remains less than (omega)(,0)(\u272). These results indicate that some components of the motion remain faster than the NMR frequency (omega)(,0) even at 5K. The behavior of cyclohexanol is compared to that of other disordered solids

Carrier-Concentration Dependence of the Pseudogap Ground State of Superconducting Bi2Sr2-xLaxCuO6+delta Revealed by 63,65Cu-Nuclear Magnetic Resonance in Very High Magnetic Fields

We report the results of the Knight shift by 63,65Cu-nuclear-magnetic resonance (NMR) measurements on single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely and the pseudogap ground state is revealed. The 63Cu-NMR Knight shift shows that there remains a finite density of states (DOS) at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x > 0.8, which suggests that the DOS plunges to zero upon approaching the Mott insulating phase.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

Epigenome-wide association study of serum urate reveals insights into urate co-regulation and the SLC2A9 locus

Elevated serum urate levels, a complex trait and major risk factor for incident gout, are correlated with cardiometabolic traits via incompletely understood mechanisms. DNA methylation in whole blood captures genetic and environmental influences and is assessed in transethnic meta-analysis of epigenome-wide association studies (EWAS) of serum urate (discovery, n = 12,474, replication, n = 5522). The 100 replicated, epigenome-wide significant (p < 1.1E–7) CpGs explain 11.6% of the serum urate variance. At SLC2A9, the serum urate locus with the largest effect in genome-wide association studies (GWAS), five CpGs are associated with SLC2A9 gene expression. Four CpGs at SLC2A9 have significant causal effects on serum urate levels and/or gout, and two of these partly mediate the effects of urate-associated GWAS variants. In other genes, including SLC7A11 and PHGDH, 17 urate-associated CpGs are associated with conditions defining metabolic syndrome, suggesting that these CpGs may represent a blood DNA methylation signature of cardiometabolic risk factors. This study demonstrates that EWAS can provide new insights into GWAS loci and the correlation of serum urate with other complex traits

Epigenome-wide association study of serum urate reveals insights into urate co-regulation and the SLC2A9 locus

Serum urate concentration can be studied in large datasets to find genetic and epigenetic loci that may be related to cardiometabolic traits. Here the authors identify and replicate 100 urate-associated CpGs, which provide insights into urate GWAS loci and shared CpGs of urate and cardiometabolic traits.Elevated serum urate levels, a complex trait and major risk factor for incident gout, are correlated with cardiometabolic traits via incompletely understood mechanisms. DNA methylation in whole blood captures genetic and environmental influences and is assessed in transethnic meta-analysis of epigenome-wide association studies (EWAS) of serum urate (discovery, n = 12,474, replication, n = 5522). The 100 replicated, epigenome-wide significant (p < 1.1E-7) CpGs explain 11.6% of the serum urate variance. At SLC2A9, the serum urate locus with the largest effect in genome-wide association studies (GWAS), five CpGs are associated with SLC2A9 gene expression. Four CpGs at SLC2A9 have significant causal effects on serum urate levels and/or gout, and two of these partly mediate the effects of urate-associated GWAS variants. In other genes, including SLC7A11 and PHGDH, 17 urate-associated CpGs are associated with conditions defining metabolic syndrome, suggesting that these CpGs may represent a blood DNA methylation signature of cardiometabolic risk factors. This study demonstrates that EWAS can provide new insights into GWAS loci and the correlation of serum urate with other complex traits.</p

\u3csup\u3e27\u3c/sup\u3eAl field-swept and frequency-stepped NMR for sites with large quadrupole coupling constants

Spectra of nonspinning samples with large quadrupole coupling constants, 16-32 MHz, are acquired by frequency-stepping. A series of spin-echoes are acquired at arbitrary frequency increments, frequency-shifted in the time domain, and co-added as magnitude spectra. This procedure is derived from a method in use for field-swept NMR. The two methods are compared. © 2000 Elsevier Science B.V. All rights reserved

Two coupled chains are simpler than one: field-induced chirality in a frustrated spin ladder

Although the frustrated (zigzag) spin chain is the Drosophila of frustrated magnetism, our understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is still lacking. We address this problem through nuclear magnetic resonance (NMR) experiments on BiCu2PO6 in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjointly, we present advanced numerical calculations showing that even a moderate rung coupling dramatically simplifies the phase diagram below half-saturation magnetization by stabilizing a field-induced chiral phase. Surprisingly for a one-dimensional model, this phase and its response to Dzyaloshinskii-Moriya (DM) interactions adhere to classical expectations. While explaining the behavior at the highest accessible magnetic fields, our results imply a different origin for the solitonic phases occurring at lower fields in BiCu2PO6. An exciting possibility is that the known, DM-mediated coupling between chirality and crystal lattice may give rise to a new kind of spin-Peierls instability.ISSN:2045-232

The CD4 transmembrane GGXXG and juxtamembrane (C/F)CV+C motifs mediate pMHCII-specific signaling independently of CD4-LCK interactions

CD4+ T cell activation is driven by five-module receptor complexes. The T cell receptor (TCR) is the receptor module that binds composite surfaces of peptide antigens embedded within MHCII molecules (pMHCII). It associates with three signaling modules (CD3γε, CD3δε, and CD3ζζ) to form TCR-CD3 complexes. CD4 is the coreceptor module. It reciprocally associates with TCR-CD3-pMHCII assemblies on the outside of a CD4+ T cells and with the Src kinase, LCK, on the inside. Previously, we reported that the CD4 transmembrane GGXXG and cytoplasmic juxtamembrane (C/F)CV+C motifs found in eutherian (placental mammal) CD4 have constituent residues that evolved under purifying selection (Lee et al., 2022). Expressing mutants of these motifs together in T cell hybridomas increased CD4-LCK association but reduced CD3ζ, ZAP70, and PLCγ1 phosphorylation levels, as well as IL-2 production, in response to agonist pMHCII. Because these mutants preferentially localized CD4-LCK pairs to non-raft membrane fractions, one explanation for our results was that they impaired proximal signaling by sequestering LCK away from TCR-CD3. An alternative hypothesis is that the mutations directly impacted signaling because the motifs normally play an LCK-independent role in signaling. The goal of this study was to discriminate between these possibilities. Using T cell hybridomas, our results indicate that: intracellular CD4-LCK interactions are not necessary for pMHCII-specific signal initiation; the GGXXG and (C/F)CV+C motifs are key determinants of CD4-mediated pMHCII-specific signal amplification; the GGXXG and (C/F)CV+C motifs exert their functions independently of direct CD4-LCK association. These data provide a mechanistic explanation for why residues within these motifs are under purifying selection in jawed vertebrates. The results are also important to consider for biomimetic engineering of synthetic receptors

Synthesis, Structure, and Spectroscopic and Magnetic Characterization of [Mn₁₂O₁₂(O₂CCH₂Bu^t)₁₆(MeOH)₄]·MeOH, a Mn₁₂ Single-Molecule Magnet with True Axial Symmetry

The synthesis and properties are reported of a rare example of a Mn₁₂ single-molecule magnet (SMM) in truly axial symmetry (tetragonal, <i>I</i>4̅). [Mn₁₂O₁₂(O₂CCH₂Bu^t)₁₆(MeOH)₄]·MeOH (<b>3</b>·MeOH) was synthesized by carboxylate substitution on [Mn₁₂O₁₂(O₂CMe)₁₆(H₂O)₄]·2MeCO₂H·4H₂O (<b>1</b>). The complex was found to possess an <i>S</i> = 10 ground state, as is typical for the Mn₁₂ family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on <b>3·</b>MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for <b>1</b>. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for <b>1</b>, suggesting that the crystals of <b>3</b> are homogeneous and do not contain multiple Mn₁₂ environments. In the single-crystal ⁵⁵Mn NMR spectrum in zero applied field, three well-resolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn⁴⁺ peak was detectable, suggesting a possible decrease in the local symmetry of the Mn⁴⁺ site. Spin–lattice (<i>T</i>₁) relaxation studies were performed on single crystals of <b>3</b>·MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn₁₂ complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as <b>1</b>