Selective Binding of Monovalent Cations to the Stacking G-Quartet Structure Formed by Guanosine 5‘-Monophosphate: A Solid-State NMR Study

We report a solid-state multinuclear (23Na, 15N, 13C, and 31P) NMR study on the relative affinity of monovalent cations for a stacking G-quartet structure formed by guanosine 5‘-monophosphate (5‘-GMP) self-association at pH 8. Two major types of cations are bound to the 5‘-GMP structure:  one at the surface and the other within the channel cavity between two G-quartets. The channel cation is coordinated to eight carbonyl oxygen atoms from the guanine bases, whereas the surface cation is close to the phosphate group and likely to be only partially hydrated. On the basis of solid-state 23Na NMR results from a series of ion titration experiments, we have obtained quantitative thermodynamic parameters concerning the relative cation binding affinity for each of the two major binding sites. For the channel cavity site, the values of the free energy difference (ΔG° at 25 °C) for ion competition between M+ and Na+ ions are K+ (−1.9 kcal mol-1), NH4+ (−1.8 kcal mol-1), Rb+ (−0.3 kcal mol-1), and Cs+ (1.8 kcal mol-1). For the surface site, the values ΔG° are K+ (2.5 kcal mol-1), NH4+ (−1.3 kcal mol-1), Rb+ (1.1 kcal mol-1), and Cs+ (0.9 kcal mol-1). Solid-state NMR data suggest that the affinity of monovalent cations for the 5‘-GMP structure follows the order NH4+ > Na+ > Cs+ > Rb+ > K+ at the surface site and K+ > NH4+ > Rb+ > Na+ > Cs+ > Li+ at the channel cavity site. We have found that the cation-induced stability of a 5‘-GMP structure is determined only by the affinity of monovalent cations for the channel site and that the binding of monovalent cations to phosphate groups plays no role in 5‘-GMP self-ordered structure. We have demonstrated that solid-state 23Na and 15N NMR can be used simultaneously to provide mutually complementary information about competitive binding between Na+ and NH4+ ions

A High-Resolution ⁴³Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

High resolution 43Ca solid-state NMR studies of hydroxyapatite (Ca10(PO4)6(OH)2) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using 1H-43Ca rotational echo double resonance and 1H-43Ca cross polarization magic angle spinning experiments

In Situ NMR Insights into the Electrochemical Reaction of Cu₃P Electrodes in Lithium Batteries

This study reports a multinuclei in situ (real-time) NMR spectroscopic characterization of the electrochemical reactions of a negative Cu₃P electrode toward lithium. Taking advantage of the different nuclear spin characteristics, we have obtained real-time ³¹P and ⁷Li NMR data for a comprehensive understanding of the electrochemical mechanism during the discharge and charge processes of a lithium battery. The large NMR chemical shift span of ³¹P facilitates the observation of the chemical evolutions of different lithiated and delithiated Li_<i>x</i>Cu_3–<i>x</i>P phases, whereas the quadrupolar line features in ⁷Li enable identification of asymmetric Li sites. These combined NMR data offer an unambiguous identification of four distinct Li_<i>x</i>Cu_3–<i>x</i>P phases, Cu₃P, Li_0.2Cu_2.8P, Li₂CuP, and Li₃P, and the characterization of their involvement in the electrochemical reactions. The NMR data led us to propose a delithiation process involving the intercalation of metallic Cu⁰ atomic aggregates into the Li₂CuP structure to form a Cu⁰-Li_2–<i>x</i>Cu_1+<i>x</i>P phase. This process might be responsible for the poor capacity retention in Cu₃P lithium batteries when cycled to a low voltage

Solid-State ²⁵Mg NMR Spectroscopic and Computational Studies of Organic Compounds. Square-Pyramidal Magnesium(II) Ions in Aqua(magnesium) Phthalocyanine and Chlorophyll <i>a</i>

We report a solid-state 25Mg NMR spectroscopic study of two magnesium-containing organic compounds:  monopyridinated aqua(magnesium) phthalocyanine (MgPc·H2O·Py) and chlorophyll a (Chla). Each of these compounds contains a Mg(II) ion coordinating to four nitrogen atoms and a water molecule in a square-pyramidal geometry. Solid-state 25Mg NMR spectra for MgPc·H2O·Py were obtained at 11.7 T (500 MHz for 1H) for a 25Mg-enriched sample (99.1% 25Mg atom) using both Hahn-echo and quadrupole Carr−Purcell Meiboom−Gill (QCPMG) pulse sequences. Solid-state 25Mg NMR spectra for Chla were recorded at 25Mg natural abundance (10.1%) at 19.6 T (830 MHz for 1H). The 25Mg quadrupole parameters were determined from spectral analyses:  MgPc·H2O·Py, CQ = 13.0 ± 0.1 MHz and ηQ = 0.00 ± 0.05; Chla, CQ = 12.9 ± 0.1 MHz and ηQ = 1.00 ± 0.05. This work represents the first time that Mg(II) ions in a square-pyramidal geometry have been characterized by solid-state 25Mg NMR spectroscopy. Extensive quantum mechanical calculations for electric-field-gradient (EFG) and chemical shielding tensors were performed at restricted Hartee−Fock (RHF), density functional theory (DFT), and second-order Møller−Plesset perturbation theory (MP2) levels for both compounds. Computed 25Mg nuclear quadrupole coupling constants at the RHF and MP2 levels show a reasonable basis-set convergence at the cc-pV5Z basis set (within 7% of the experimental value); however, B3LYP results display a drastic divergence beyond the cc-pVTZ basis set. A new crystal structure for MgPc·H2O·Py is also reported

Solid-State ²⁵Mg NMR Spectroscopic and Computational Studies of Organic Compounds. Square-Pyramidal Magnesium(II) Ions in Aqua(magnesium) Phthalocyanine and Chlorophyll <i>a</i>

We report a solid-state 25Mg NMR spectroscopic study of two magnesium-containing organic compounds:  monopyridinated aqua(magnesium) phthalocyanine (MgPc·H2O·Py) and chlorophyll a (Chla). Each of these compounds contains a Mg(II) ion coordinating to four nitrogen atoms and a water molecule in a square-pyramidal geometry. Solid-state 25Mg NMR spectra for MgPc·H2O·Py were obtained at 11.7 T (500 MHz for 1H) for a 25Mg-enriched sample (99.1% 25Mg atom) using both Hahn-echo and quadrupole Carr−Purcell Meiboom−Gill (QCPMG) pulse sequences. Solid-state 25Mg NMR spectra for Chla were recorded at 25Mg natural abundance (10.1%) at 19.6 T (830 MHz for 1H). The 25Mg quadrupole parameters were determined from spectral analyses:  MgPc·H2O·Py, CQ = 13.0 ± 0.1 MHz and ηQ = 0.00 ± 0.05; Chla, CQ = 12.9 ± 0.1 MHz and ηQ = 1.00 ± 0.05. This work represents the first time that Mg(II) ions in a square-pyramidal geometry have been characterized by solid-state 25Mg NMR spectroscopy. Extensive quantum mechanical calculations for electric-field-gradient (EFG) and chemical shielding tensors were performed at restricted Hartee−Fock (RHF), density functional theory (DFT), and second-order Møller−Plesset perturbation theory (MP2) levels for both compounds. Computed 25Mg nuclear quadrupole coupling constants at the RHF and MP2 levels show a reasonable basis-set convergence at the cc-pV5Z basis set (within 7% of the experimental value); however, B3LYP results display a drastic divergence beyond the cc-pVTZ basis set. A new crystal structure for MgPc·H2O·Py is also reported

Operando Magnetic Resonance Imaging Reveals Phase Transitions Driven by Nonuniform Cathode Lithiation in Li-Ion Pouch Cells

Li-ion cells based on layered transition metal oxides (LTMO) demonstrate the best overall performance to date. A detailed understanding of ion transport and charge storage mechanisms in these cathode materials is key to improved design, performance, and safety of cells. The magnetism of LTMO-based materials depends on the concentration and the type of the intercalant. This phenomenon provides a source of sensitive magnetic resonance imaging (MRI) contrast for studies of Li-ion cell function and failure mechanisms. Surface-scan MRI is a nondestructive operando technique designed for artifact-free mapping of strongly inhomogeneous magnetic fields near various portable devices and battery cells. Recent experiments revealed nonuniform distributions of current density and magnetic susceptibility in common Li-ion pouch cells. Further analysis of the surface-scan MRI data suggests the coexistence of several magnetic phases and the presence of transient Li concentration gradients in the cathode. These hypotheses are validated herein through the observation of propagating magnetic susceptibility fronts in LixCoO2 cathodes of resting state pouch cells. We show evidence for the cathode lithium distribution to follow the areas of high current densities, which is a surprising result, given that the cathode generally has very high conductivity. Furthermore, equalization of the lithiation levels is a slow process happening over several days. Such observations of structural varieties and solid-state ion transport are possible in any material with pronounced intercalation-dependent magnetic properties. The methodology described in this work is a powerful tool for the analysis of kinetic phenomena in a wide range of pouch cells

The Sodium Ions Inside a Lipophilic G-Quadruplex Channel as Probed by Solid-State ²³Na NMR

We report solid-state 23Na NMR and X-ray crystallographic results for a self-assembled G-quadruplex channel formed by a guanine nucleoside, 5‘-tert-butyl-dimethylsilyl-2‘,3‘-O-isopropylidene guanosine (G 1). The study provides an unambiguous 23Na NMR identification for the Na+ ions inside a lipophilic G-quadruplex channel. The crystalline nature of the sample yields a remarkably high resolution in the 23Na multiple-quantum magic-angle spinning (MQMAS) spectrum, making it possible to extract very accurate 23Na NMR parameters for each of the three crystallographically distinct Na sites. The observation of a single Na+ ion from a 9-kDa system demonstrates the potential of solid-state 23Na NMR as a complementary technique to X-ray for detecting Na+ ions in biological structures

The Sodium Ions Inside a Lipophilic G-Quadruplex Channel as Probed by Solid-State ²³Na NMR

We report solid-state 23Na NMR and X-ray crystallographic results for a self-assembled G-quadruplex channel formed by a guanine nucleoside, 5‘-tert-butyl-dimethylsilyl-2‘,3‘-O-isopropylidene guanosine (G 1). The study provides an unambiguous 23Na NMR identification for the Na+ ions inside a lipophilic G-quadruplex channel. The crystalline nature of the sample yields a remarkably high resolution in the 23Na multiple-quantum magic-angle spinning (MQMAS) spectrum, making it possible to extract very accurate 23Na NMR parameters for each of the three crystallographically distinct Na sites. The observation of a single Na+ ion from a 9-kDa system demonstrates the potential of solid-state 23Na NMR as a complementary technique to X-ray for detecting Na+ ions in biological structures

μHigh Resolution-Magic-Angle Spinning NMR Spectroscopy for Metabolic Phenotyping of <i>Caenorhabditis elegans</i>

Analysis of model organisms, such as the submillimeter-size Caenorhabditis elegans, plays a central role in understanding biological functions across species and in characterizing phenotypes associated with genetic mutations. In recent years, metabolic phenotyping studies of C. elegans based on 1H high-resolution magic-angle spinning (HR-MAS) nuclear magnetic resonance (NMR) spectroscopy have relied on the observation of large populations of nematodes, requiring labor-intensive sample preparation that considerably limits high-throughput characterization of C. elegans. In this work, we open new platforms for metabolic phenotyping of C. elegans mutants. We determine rich metabolic profiles (31 metabolites identified) from samples of 12 individuals using a 1H NMR microprobe featuring high-resolution magic-angle coil spinning (HR-MACS), a simple conversion of a standard HR-MAS probe to μHR-MAS. In addition, we characterize the metabolic variations between two different strains of C. elegans (wild-type vs slcf-1 mutant). We also acquire a NMR spectrum of a single C. elegans worm at 23.5 T. This study represents the first example of a metabolomic investigation carried out on a small number of submillimeter-size organisms, demonstrating the potential of NMR microtechnologies for metabolomics screening of small model organisms