Coherent backscattering of Bose-Einstein condensates in two-dimensional disorder potentials

We study quantum transport of an interacting Bose-Einstein condensate in a two-dimensional disorder potential. In the limit of vanishing atom-atom interaction, a sharp cone in the angle-resolved density of the scattered matter wave is observed, arising from constructive interference between amplitudes propagating along reversed scattering paths. Weak interaction transforms this coherent backscattering peak into a pronounced dip, indicating destructive instead of constructive interference. We reproduce this result, obtained from the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic theory of weak localization in presence of a nonlinearity.Comment: 4 pages, 4 figure

Local Structure in Disordered Melilite Revealed by Ultrahigh Field 71Ga and 139La Solid‐State Nuclear Magnetic Resonance

Multinuclear Nuclear Magnetic Resonance (NMR) spectroscopy of quadrupolar nuclei at ultrahigh magnetic field provides compelling insight into the short‐range structure in a family of fast oxide ion electrolytes with La1+xSr1–xGa3O7+0.5x melilite structure. The striking resolution enhancement in the solid‐state 71Ga NMR spectra measured with the world’s unique series connected hybrid magnet operating at 35.2 T distinctly resolves Ga sites in four‐ and five‐fold coordination environments. Detection of five‐coordinate Ga centers in the site‐disordered La1.54Sr0.46Ga3O7.27 melilite is critical given that the GaO5 unit accommodates interstitial oxide ions and provides excellent transport properties. This work highlights the importance of ultrahigh magnetic fields for the detection of otherwise broad spectral features in systems containing quadrupolar nuclei and the potential of ensemble‐based computational approaches for the interpretation of NMR data acquired for site‐disordered materials.</jats:p

Dipolar Heteronuclear Correlation Solid-State NMR Experiments Between Half-Integer Quadrupolar Nuclei: The Case of 11B-17O

With 73 % of all NMR-active nuclei being quadrupolar, there is great interest in the development of NMR experiments that can probe the proximity between quadrupolar spins. Here, several pulse sequences for magic angle spinning (MAS) 11B-17O Resonance-Echo Saturation-Pulse DOuble-Resonance (RESPDOR) and dipolar Heteronuclear Multiple Quantum Correlation (D-HMQC) solid-state NMR experiments were investigated. In these pulse sequences, Rotational-Echo Double-Resonance (REDOR) recoupling with central transition (CT) selective 180 pulses were applied to either the 11B or 17O spins to recouple the 11B-17O dipolar interactions. 11B{17O} RESPDOR experiments on model 17O-enriched boronic acids showed that application of dipolar recoupling on the 11B channel yielded more signal dephasing than when recoupling is applied on the 17O channel; however, short effective 11B transverse relaxation time constants (T2’) hinders the acquisition of dephasing curves out to long recoupling durations. Application of REDOR recoupling to 17O spins was found to produce significant dephasing without compromising the 11B T2’. Comparison of experimental 11B{17O} RESPDOR curves to that of numerical simulations enabled the 17O isotopic enrichment to be estimated. 2D 11B{17O} D-HMQC spectra were recorded with either 11B or 17O REDOR recoupling at a variety of radio frequency field conditions. Lastly, 2D 11B{17O} and 23Na{17O} D-HMQC spectra of a 17O-enriched sodium borate glass were acquired to demonstrate the practical application of these heteronuclear correlation experiments to probe structural connectivity between two quadrupolar spins. Importantly, the high-field 2D 11B-17O D-HMQC NMR spectrum revealed two unique 17O sites correlating to 4-coordinate BO4 (B[4]), which were attributed to the expected B[3]-O-B[4] and unexpected B[4]-O-B[4] bridging O atoms. The heteronuclear correlation experiments outlined here should be applicable to a variety of quadrupolar spin pairs.This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication as Dorn, Rick W., Alexander L. Paterson, Ivan Hung, Peter L. Gor’kov, Austin J. Thompson, Aaron D. Sadow, Zhehong Gan, and Aaron J. Rossini. "Dipolar Heteronuclear Correlation Solid-State NMR Experiments between Half-Integer Quadrupolar Nuclei: The Case of 11B–17O." The Journal of Physical Chemistry C 126, no. 28 (2022): 11652-11666. Copyright 2022 American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acs.jpcc.2c02737. Posted with permission. DOE Contract Number(s): AC02-07CH11358; CHE-1900393; NSF/DMR-1644779; DMR-1039938; DMR-0603042

Magic Angle Spinning and Oriented Sample Solid-State NMR Structural Restraints Combine for Influenza A M2 Protein Functional Insights

As a small tetrameric helical membrane protein, the M2 proton channel structure is highly sensitive to its environment. As a result, structural data from a lipid bilayer environment have proven to be essential for describing the conductance mechanism. While oriented sample solid-state NMR has provided a high-resolution backbone structure in lipid bilayers, quaternary packing of the helices and many of the side-chain conformations have been poorly restrained. Furthermore, the quaternary structural stability has remained a mystery. Here, the isotropic chemical shift data and interhelical cross peaks from magic angle spinning solid-state NMR of a liposomal preparation strongly support the quaternary structure of the transmembrane helical bundle as a dimer-of-dimers structure. The data also explain how the tetrameric stability is enhanced once two charges are absorbed by the His37 tetrad prior to activation of this proton channel. The combination of these two solid-state NMR techniques appears to be a powerful approach for characterizing helical membrane protein structure

Evidence from 900 MHz ¹H MAS NMR of Displacive Behavior of the Model Order–Disorder Antiferroelectric NH₄H₂AsO₄

NH₄H₂AsO₄ (ADA) is a model compound for understanding the mechanism of phase transitions in the KH₂PO₄ (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at <i>T</i>_N ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (¹H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, <i>T</i>₁, of the O–H···O and NH₄⁺ protons through the <i>T</i>_N. As <i>T</i> → <i>T</i>_N, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its <i>T</i>_N, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at <i>T</i>_N, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH₄⁺ protons. Both sets of protons exhibited order–disorder characteristics in their <i>T</i>₁ data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into <i>order–disorder</i> and <i>displacive</i> ones, should rather be “<i>order–disorder cum displacive</i>” type

¹⁷O MAS NMR Correlation Spectroscopy at High Magnetic Fields

The structure of two protected amino acids, FMOC-l-leucine and FMOC-l-valine, and a dipeptide, <i>N</i>-acetyl-l-valyl-l-leucine (N-Ac-VL), were studied via one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy. Utilizing ¹⁷O magic-angle spinning (MAS) NMR at multiple magnetic fields (17.6–35.2 T/750–1500 MHz for ¹H) the ¹⁷O quadrupolar and chemical shift parameters were determined for the two oxygen sites of each FMOC-protected amino acids and the three distinct oxygen environments of the dipeptide. The one- and two-dimensional, ¹⁷O, ¹⁵N–¹⁷O, ¹³C–¹⁷O, and ¹H–¹⁷O double-resonance correlation experiments performed on the uniformly ¹³C,¹⁵N and 70% ¹⁷O-labeled dipeptide prove the attainability of ¹⁷O as a probe for structure studies of biological systems. ¹⁵N–¹⁷O and ¹³C–¹⁷O distances were measured via one-dimensional REAPDOR and ZF-TEDOR experimental buildup curves and determined to be within 15% of previously reported distances, thus demonstrating the use of ¹⁷O NMR to quantitate interatomic distances in a fully labeled dipeptide. Through-space hydrogen bonding of N-Ac-VL was investigated by a two-dimensional ¹H-detected ¹⁷O R³-R-INEPT experiment, furthering the importance of ¹⁷O for studies of structure in biomolecular solids