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The phospholipid-water partition coefficient is a commonly measured parameter that correlates with drug efficacy, small-molecule toxicity, and accumulation of molecules in biological systems in the environment. Despite the utility of this parameter, methods for measuring phospholipid- water partition coefficients are limited. This is due to the difficulty of making quantitative measurements in vesicle membranes or supported phospholipid bilayers, both of which are small-volume phases that challenge the sensitivity of many analytical techniques. In this work, we employ in situ confocal Raman microscopy to probe the partitioning of a model membrane-active compound, 2-(4-isobutylphenyl) propionic acid or ibuprofen, into both hybrid- and supportedphospholipid bilayers deposited on the pore walls of individual chromatographic particles. The large surface-area-to-volume ratio of chromatographic silica allows interrogation of a significant lipid bilayer area within a very small volume. The local phospholipid concentration within a confocal probe volume inside the particle can be as high as 0.5 M, which overcomes the sensitivity limitations of making measurements in the limited membrane areas of single vesicles or planar supported bilayers. Quantitative determination of ibuprofen partitioning is achieved by using the phospholipid acyl-chains of the within-particle bilayer as an internal standard. This approach is tested for measurements of pH-dependent partitioning of ibuprofen into both hybrid-lipid and supported-lipid bilayers within silica particles, and the results are compared with octanol-water partitioning and with partitioning into individual optically trapped phospholipid vesicle membranes. Additionally, the impact of ibuprofen partitioning on bilayer structure is evaluated for both within-particle model membranes and compared with the structural impacts of partitioning into vesicle lipid bilayers. [ABSTRACT FROM AUTHOR]

Permeabilization of the outer mitochondrial membrane is an integral step in apoptosis. The resulting release of pro-apoptotic signaling proteins leads to cell destruction through activation of the cysteine-aspartic protease (caspase) cascade. However, the mechanism of outer mitochondrial membrane (OMM) permeabilization remains unclear. It was recently shown that cytochrome c can induce pore formation in cardiolipin-containing phospholipid membranes, leading to large dextran and protein permeability. In this work, the interaction of cytochrome c with cardiolipin-containing phospholipid vesicles, serving as models of the OMM, is investigated to probe cytochrome c-induced permeability. Lipid vesicles having either a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or mixed-DPPC/cardiolipin membrane and containing a membrane-impermeable Raman tracer 3-nitrobenzenesulfonate (3-NBS) were optically trapped, translated into a solution containing cytochrome c, and monitored for 3-NBS leakage. Cytochrome-correlated leakage was observed only in cardiolipin-containing vesicles. Structural changes observed in the Raman spectra during permeabilization indicated acyl chain disordering along with decreased intensity of the cardiolipin cis-double-bond stretching modes. When the vesicle-associated cytochrome c Raman spectrum is compared with a spectrum in buffer, heme-resonance bands are absent, indicating loss of Met-80 coordination. To verify selective interactions of cytochrome c with cardiolipin, these experiments were repeated where the DPPC acyl chains were deuterated (D62-DPPC), allowing spectral resolution of the DPPC acyl chain response from that of cardiolipin. Interestingly, D62-DPPC acyl chains were unaffected by cytochrome c accumulation, while cardiolipin showed major changes in acyl chain structure. These results suggest that cytochrome-induced permeabilization proceeds through selective interaction of cytochrome c with cardiolipin, resulting in protein unfolding, where the unfolded form interacts with cardiolipin acyl chains within the bilayer to induce permeability. [ABSTRACT FROM AUTHOR]

Porous silica is used as a support in a variety of separation processes, including chromatographic separation and solid-phase extraction. The resolution and efficiency of these applications is significantly impacted by the kinetics of partitioning and molecular transport into the interior of the porous particles. Molecular transport in porous silica has been explored previously by measuring chromatographic elution profiles, but such measurements are limited to relatively low retention conditions, where within-particle molecular transport must be inferred from elution profiles of solutes emerging from a packed column. In this work, a measurement of within-particle molecular transport is carried out using confocal Raman microscopy to probe the time-dependent accumulation of pyrene from an aqueous mobile phase into the center of individual C18-chromatographic particles. The measured time constants for pyrene accumulation were much slower than diffusion-limited transport of solute in solution to the particle surface. Furthermore, the accumulation into the center of the particle did not show a time-lag characteristic of slow-transport into the particle interior. The exponential rise of pyrene concentration is, however, consistent with first-order Langmuir adsorption kinetics at low surface coverages. The linear dependence of the time-constant on particle radius indicates an adsorption barrier near the outer boundary of the particle, where the accumulation rate depends on flux across the boundary (proportional to the particle area) to satisfy the within-particle capacity at equilibrium (proportional to the particle volume). The pyrene accumulation kinetics into the porous particle, expressed as a heterogeneous rate constant, were nearly 50-times faster than the pyrene adsorption rate at a planar C18-functionalized silica surface, which demonstrates the impact of multiple surface encounters within the porous structure leading to much greater capture efficiency compared to a planar surface. Monte Carlo simulations of within-particle pyrene diffusion, with the adsorption efficiency estimated from the planar-surface adsorption rate, predict a diffusion-to-capture distance within the porous particle that is within 40% of that observed in the radial dependence of the pyrene within-particle accumulation results. [ABSTRACT FROM AUTHOR]

CO2 injected into depleted oil or gas reservoirs for long-term storage has the potential to mobilize organic compounds and distribute them between sediments and reservoir brines. Understanding this process is important when considering health and environmental risks, but little quantitative data currently exists on the partitioning of organics between supercritical CO2 and water. In this work, a high-pressure, in situ measurement capability was developed to assess the distribution of organics between CO2 and water at conditions relevant to deep underground storage of CO2. The apparatus consists of a titanium reactor with quartz windows, near-infrared and UV spectroscopic detectors, and switching valves that facilitate quantitative injection of organic reagents into the pressurized reactor. To demonstrate the utility of the system, partitioning coefficients were determined for benzene in water/supercritical CO2 over the range 35-65 °C and approximately 25-150 bar. Density changes in the CO2 phase with increasing pressure were shown to have dramatic impacts on benzene's partitioning behavior. Our partitioning coefficients were approximately 5-15 times lower than values previously determined by ex situ techniques that are prone to sampling losses. The in situ methodology reported here could be applied to quantify the distribution behavior of a wide range of organic compounds that may be present in geologic CO2 storage scenarios. [ABSTRACT FROM AUTHOR]

Chemically informative J couplings between pairs of quadrupolar nuclei in dimetallic and dimetalloid coordination motifs are measured using J-resolved solid-state NMR experiments. It is shown that the application of a double-quantum filter is necessary to observe the J splittings and that, under these conditions, only a simple doublet is expected. Interestingly, the splitting is amplified if the spins are magnetically equivalent, making it possible to measure highly precise J couplings and unambiguously probe the symmetry of the molecule. This is demonstrated experimentally by chemically breaking the symmetry about a pair of boron spins by reaction with an N-heterocyclic carbene to form a β-borylation reagent. The results show that the J coupling is a sensitive probe of bonding in diboron compounds and that the J values quantify the weakening of the B-B bond which occurs when forming an sp2-sp3 diboron compound, which is relevant to their reactivity. Due to the prevalence of quadrupolar nuclei among transition metals, this work also provides a new approach to probe metal-metal bonding; results for Mn2(CO)10 are provided as an example. [ABSTRACT FROM AUTHOR]

Natural-abundance 43Ca solid-state NMR spectroscopy at 21.1 T and gauge-including projector-augmented-wave (GIPAW) DFT calculations are developed as tools to provide insight into calcium binding environments, with special emphasis on the calcium chemical shift (CS) tensor. The first complete analysis of a 43Ca solid-state NMR spectrum, including the relative orientation of the CS and electric field gradient (EFG) tensors, is reported for calcite. GIPAW calculations of the 43Ca CS and EFG tensors for a series of small molecules are shown to reproduce experimental trends; for example, the trend in available solid-state chemical shifts is reproduced with a correlation coefficient of 0.983. The results strongly suggest the utility of the calcium CS tensor as a novel probe of calcium binding environments in a range of calcium-containing materials. For example, for three polymorphs of CaCO3 the CS tensor span ranges from 8 to 70 ppm and the symmetry around calcium is manifested differently in the CS tensor as compared with the EFG tensor. The advantages of characterizing the CS tensor are particularly evident in very high magnetic fields where the effect of calcium CS anisotropy is augmented in hertz while the effect of second-order quadrupolar broadening is often obscured for 43Ca because of its small quadrupole moment. Finally, as an application of the combined experimental-theoretical approach, the solid-state structure of the vaterite polymorph of calcium carbonate is probed and we conclude that the hexagonal P63/mmc space group provides a better representation of the structure than does the orthorhombic Pbnm space group, thereby demonstrating the utility of 43Ca solid-state NMR as a complementary tool to X-ray crystallographic methods. [ABSTRACT FROM AUTHOR]

Reaction of RuCl2(PPh3)3 with LiNN′ (NN′ = 2-[(2,6-diisopropylphenyl)imino]pyrrolide) affords a single product, with the empirical formula RuCl[(2,6-iPr2C6H3)N=CHC4H3N](PPh3)2. We identify this species as a σ-pyrrolato complex, [Ru(NN′)(PPh3)2]2(μ-Cl)2 (3b), rather than mononuclear RuCl(NN′)(PPh3)2 (3a), on the basis of detailed 1D and 2D NMR characterization in solution and in the solid state. Retention of the chelating, σ-bound iminopyrrolato unit within 3b, despite the presence of labile (dative) chloride and PPh3 donors, indicates that the chelate effect is sufficient to inhibit σ → π isomerization of 3b to a piano-stool, π-pyrrolato structure. 2D COSY, SECSY, and J-resolved solid-state 31p NMR experiments confirm that the PPh3 ligands on each metal center are magnetically and crystallographically inequivalent, and 31p CP/MAS NMR experiments reveal the largest 99Ru-31P spin-spin coupling constant (¹J(99Ru,31P) = 244 ± 20 Hz) yet measured. Finally, 31P dipolar-chemical shift spectroscopy is applied to determine benchmark phosphorus chemical shift tensors for phosphine ligands in hexacoordinate ruthenium complexes. [ABSTRACT FROM AUTHOR]

The results of a detailed systematic chlorine solid-state NMR study of several hydrochloride salts of amino acids implicated in chloride ion transport channel selectivity are reported. 35Cl and 37Cl NMR spectra have been obtained for stationary and/or magic-angle spinning powdered samples of the following compounds on 500 and/or 900 MHz spectrometers: DL-arginine HCI monohydrate, L-lysine HCI, L-serine HCI, L-glutamic acid HCI, L-proline HCI, L-isoleucine HCI, L-valine HCI, L-phenylalanine HCI, and glycine HCI. Spectral analyses provide information on the anisotropic properties and relative orientations of the chlorine electric field gradient and chemical shift (CS) tensors, which are intimately related to the local molecular and electronic structure. Data obtained at 900 MHz provide unique examples of the effects of CS anisotropy on the NMR spectrum of a quadrupolar nucleus. The range of chlorine quadrupolar coupling constants (CQ) measured, -6.42 to 2.03 MHz, demonstrates the sensitivity of this parameter to the chloride ion environment and suggests the applicability of chlorine solid-state NMR as a novel experimental tool for defining chloride binding environments in larger ion channel systems. Salts of hydrophobic amino acids are observed to tend to exhibit larger values of CQ than salts of hydrophilic amino acids. A simple model for rationalizing the observed trend in CQ is proposed. For salts for which neutron diffraction structures are available, we identify a quantum chemical method which reproduces experimental values of CQ with a root-mean-square deviation of 0.1 MHz and a correlation coefficient of 0.9998. On the basis of this, chlorine NMR tensors are predicted for the Cl- binding site in CIC channels. [ABSTRACT FROM AUTHOR]

Incomplete motional averaging of chemical shift anisotropy upon weak alignment of nucleic acids and proteins in a magnetic field results in small changes in chemical shift. Knowledge of nucleus-specific chemical shift (CS) tensor magnitudes and orientations is necessary to take full advantage of these measurements in biomolecular structure determination. We report the determination by liquid crystal NMR of the CS tensors for all ribose carbons in A-form helical RNA, using a series of novel 3D NMR pulse sequences for accurate and resolved measurement of the ribose 13C chemical shifts. The orientation of the riboses relative to the rhombic alignment tensor of the molecule studied, a stem-loop sequence corresponding to helix-35 of 238 rRNA, is known from an extensive set of residual dipolar couplings (RDC), previously used to refine its structure. Singular-value-decomposition fits of the chemical shift changes to this structure, or alternatively to a database of helical RNA X-ray structures, provide the CS tensor for each type of carbon. Quantum chemical calculations complement the experimental results and confirm that the most shielded tensor component lies approximately along the local carbon-oxygen bond axis in all cases and that shielding anisotropy for C3' and C4' is much larger than for C1' and C2', with C5' being intermediate. [ABSTRACT FROM AUTHOR]

Solid-state [sup 17]O NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of [sup 17]O-enriched triphenylphosphine oxide, Ph[sub 3]PO. Precise data on the orientationdependent [sup 17]O electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the [sup 17]O nuclear quadrupolar coupling constants (C[sub Q]) are essentially identical for the two polymorphs (C[sub Q] = -4.59 ± 0.01 MHz (orthorhombic); C[sub Q] = -4.57 ± 0.01 MHz (monoclinic)), the spans (Ω) of the CS tensors are distinctly different (Ω = 135 ± 3 ppm (orthorhombic); Ω = 155 ± 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramsey's theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar σ-bonded R[sub 3]P[sup +]-O[sup -] end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between [sup 31]P and [sup 17]O is observed directly in [sup 17]O magicangle-spinning (MAS) NMR spectra as well as in [sup 31]P MAS NMR spectra. Ab initio and density-functional theory calculations of the [sup 17]O EFG, CS, and ¹J([sup 31]P,[sup 17]O) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed [sup 17]O NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials. [ABSTRACT FROM AUTHOR]

Examines the structure of silver cyanide by solid-state multinuclear magnetic resonance spectroscopy. Linearity of polymeric chains; Domination of two-site model by indirect nuclear spin-spin coupling between [sup 109/107]Ag and [sup 12]C nuclei; Application of the zeroth-order regular approximation density functional theory calculation.

Examines the nuclear spin-spin coupling tensors for polyatomic xenon fluorides and group 17 fluorides. Reduction of coupling constants for halogen-noble gas spin pairs; Use of zeroth-order regular approximation density-functional theory method in interpreting spin-spin coupling of polyatomic systems; Mechanisms of chlorine-fluorine isotropic coupling constants.

Examines the advantages on the calculations of an indirect nuclear pin-spin coupling tensors. Usability of nuclear magnetic resonance spectroscopy; Characterization of an inorganic semiconducting materials; Complexity of the polyatomic molecules.

Presents a computational quantum chemistry experiment that illustrates the connection between nuclear magnetic resonance parameters and state-of-the-art ab initio calculations. Chemical shifts and nuclear magnetic shielding; Connection of nuclear magnetic shielding tensors to nuclear spin-rotation tensors; Quadropolar coupling constants.

Compares results obtained by experiment and first principle calculations on indirect nuclear spin to spin coupling tensors in diatomic molecules. Equivalence of hyperfine parameter in isotropic spin to spin coupling constant; Calculation of complete tensor determination reliability; Observation of isolated molecules in the experimental data.

The anisotropic magnetic susceptibility exhibited by nucleic acids in solution causes such molecules to adopt a small degree of alignment in a magnetic field, resulting in residual dipolar couplings (RDC) between nuclear spins." RDCs, induced either by magnetic field alignment or by anisotropic media such as dilute liquid crystals, have been successfully exploited to provide precise information on biomolecular structure. Critical prerequisites to the interpretation of dynamics of nucleic acids from magnetic field-induced RDCs are accurate reference values of the magnetic susceptibility anisotropies (Ax).

Access to orientational information through the measurement of short-range dipolar couplings in weakly oriented molecules has created new opportunities for the structure determination of proteins′ and nucleic acids by NMR. The ability to determine a local order matrix, and thereby define dynamics along a protein backbone or in different regions of multisubunit nucleic acids, offers the possibility of gaining unique insights not only into the time-averaged structure, but also into the degree of internal dynamics, averaged over the entire time regime from milli- to femtoseconds.

Knowledge of 13C chemical shift anisotropy (CSA) tensors in nucleotide bases is important for interpretation of NMR relaxation data in terms of local dynamic properties of nucleic acids and for analysis of residual chemical shift anisotropy (RCSA) resulting from weak alignment. CSA tensors for protonated nucleic acid base carbons have been derived from measurements on a uniformly 13C-enriched helical A-form RNA segment and a helical B-form DNA dodecamer at natural 13C abundance. The magnitudes of the derived CSA principal values are tightly restricted by the magnetic field dependencies of the 13C transverse relaxation rates, whereas the tensor orientation and asymmetry follow from quantitative measurements of interference between 13C-{1H} dipolar and 13C CSA relaxation mechanisms. Changes in the chemical shift between the isotropic and aligned states, Δδ, complement these measurements and permit cross-validation. The CSA tensors are determined from the experimental Δδ values and relaxation rates, under the assumption that the CSA tensor of any specific carbon in a given type of base is independent of the base position in either the RNA or DNA helix. However, the experimental data indicate that for pyrimidine C6 carbons in A-form RNA the CSA magnitude is considerably larger than in B-form DNA. This result is supported by quantum chemical calculations and is attributed in part to the close proximity between intranucleotide C6H and O5′ atoms in RNA. The magnitudes of the measured CSA tensors, on average, agree better with previous solid-state NMR results obtained on powdered nucleosides than with prior results from quantum chemical calculations on isolated bases, which depend rather strongly on the level of theory at which the calculations are carried out. In contrast, previously computed orientations of the chemical shift tensors agree well with the present experimental results and exhibit less dependence on the level of theory at which the computations are performed. [ABSTRACT FROM AUTHOR]

The J([11]B,[11]B) coupling constants of various salts of the electron-precise hexacyanodiborane(6) dianion, [B[sub 2](CN)[sub 6]][2-], were obtained using [11]B double-quantum-filtered (DQF) J-resolved solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Our results show that the magnitude of the DQF J splitting is influenced by both the crystallographic symmetry of the system and the presence of dynamics. The splittings are amplified by a factor of 3 as compared to the corresponding theoretical J coupling constants for cases where (1) there is an absence of dynamics but the boron pairs are crystallographically equivalent or (2) the boron pairs are crystallographically inequivalent but are rendered magnetically equivalent on the time scale of the experiment due to dynamic disorder, which was identified by [11]B and [13]C SSNMR experiments. Consequently, molecular motions need to be taken into consideration when interpreting the results of DQF J-resolved experiments, and conversely, these experiments may be used to identify dynamic disorder. Variable-temperature NMR data support the notion of three different motional processes with correlation times ranging from 10[2] to 10[6] s[-1] over the temperature range of 248-306 K. When molecular motion and crystallographic symmetry are both accounted for, the J([11]B,[11]B) coupling constants for various [B[sub 2](CN)[sub 6]][2-] salts were measured to range from 29.4 to 35.8 Hz, and their electronic origins were determined using natural localized molecular orbital and natural bond orbital analyses. The coupling constants were found to strongly correlate to the hybridization states of the boron orbitals that form the B -- B bonds and to the strength of the B -- B bonds. This study provides a novel tool to study dynamics in ordered and disordered solids and provides new perspectives on electron-precise dianionic diboranes featuring two-center -- two-electron bonds in the context of related compounds featuring multiply and singly bonded boron spin pairs. [ABSTRACT FROM AUTHOR]

The dramatic reactivity difference between the Grubbs metathesis catalysts and their resting-state methylidene derivatives was probed in an integrated crystallographic, solid-state NMR and localized molecular orbital analysis study. A principal focus was the second-generation Grubbs system RuCl2(H2IMes)(PCy3)(=CHR) (GII, R = Ph; GIIm, R = H); supporting studies were carried out with the first-generation species RuCl2(PCy3)2(=CHR) (GI, GIm). The compiled rate constants for PCy3 dissociation demonstrate the limited lability of the methylidene complexes (e.g., ca. 275-fold lower for GIIm than GII and nearly 2000 times lower for the IMes analogue GIIm'). This is important because it impedes catalyst re-entry from the resting state into the active cycle. The 31P chemical shift (CS) tensors for the PCy3 ligand exhibited the expected changes (i.e., those characteristic of an increased Ru-P orbital interaction) in GIIm relative to GII, as did GIm vs GI. Greater insight was offered by the 13C CS tensors. Whereas calculations on truncated models predict significant differences in 13C CS tensor values for GII compared with GIIm, the experimental values are equivalent, implying a compensating effect that weakens the Ru=C interaction in the benzylidene complex. Published X-ray crystallographic parameters for GII and GI reveal that one chloride ligand is displaced below the basal plane by steric interactions with the benzylidene phenyl group, an effect absent in GIIm and GIm. During PCy3 loss from the [Ru]=CHPh systems, established processes of alkylidene rotation transform Ph-Cl repulsion into Ph-PCy3 repulsion. Displacing the PCy3 ligand below the plane does not relieve this conflict, instead incurring steric interactions with the H2IMes ligand. Enhanced PCy3 lability in the benzylidene complexes, relative to their methylidene analogues, is hence proposed to originate in the steric pressure exerted by the Ph substituent. [ABSTRACT FROM AUTHOR]

Borinium cations (R2B+) are of particular fundamental and applied interest in part due to their pronounced Lewis acidity which enables unique chemical transformations. Solid-state NMR spectroscopy of magic-angle spinning and stationary powdered samples of the dicoordinate boron cation in the recently reported dimesitylborinium tetrakis(pentafluorophenyl)borate compound (Shoji et al. Nature Chem. 2014, 6, 498) is applied to characterize the 11B electric field gradient (EFG) and chemical shift (CS) tensors. The experimental data are consistent with linear C--B+--C geometry. The 11B quadrupolar coupling constant, 5.44 ± 0.08 MHz, and the span of the CS tensor, 130 ± 1 ppm, are both particularly large relative to literature data for a variety of boron functional groups, and represent the first such data for the linear C--B+--C borinium moiety. The NMR data are similar to those for the neutral tricoordinate analogue, trimesitylborane, but contrast with those of the Cp*2B+ cation. Quantum chemical calculations are applied to provide additional insights into the relationship between the NMR observables and the molecular and electronic structure of the dimesitylborinium cation. [ABSTRACT FROM AUTHOR]

Noncovalent interactions play a ubiquitous role in the structure, stability, and reactivity of a wade range of molecular and ionic cocrystals, pharmaceuticals, materials, and biomolecules. The halogen bond continues to be the focus of much attention, due in part to its strength and unique directionality. Here, we report a multifaceted experimental and computational study of halogen bonds in the solid state. A series of cocrystals of three different diiodobenzene molecules and various onium halide (Cl- or Br-) salts, designed to exhibit moderately strong halogen bonds (C-I···X -) in the absence of competing hydrogen bonds, has been prepared and characterized by single-crystal X-ray diffraction. Interestingly, a wide range of geometries about the halide anion are observed. 35/37Cl and 79/81Br solid state NMR spectroscopy is applied to characterize the nuclear quadrupolar coupling constants (CQ) and asymmetry parameters (ηQ) for the halogen-bonded anions at the center of bonding environments ranging from approximately linear to distorted square planar to octahedral. The relationship between the halogen bond environment and the quadrupolar parameters is elucidated through a natural localized molecular orbital (NLMO) analysis in the framework of density functional theory (DFT). These calculations reveal that the lone pair type orbitals on the halogen-bonded anion govern the magnitude and orientation of the quadrupolar tensor as the geometry about the anion is systematically altered. In - C - I··· X-··· I - C - environments, the value of ηQ is well-correlated to the I···X-···I angle. 13C NMR and DFT calculations show a correlation between chemical shifts and halogen bond strength (through the C-I distance) in o-diiodotetrafluorobenzene cocrystals. Overall, this work provides a chemically intuitive understanding of the connection between the geometry and electronic structure of halogen bonds and various NMR parameters with the aid of NLMO analysis. [ABSTRACT FROM AUTHOR]

A series of calcium and strontium complexes featuring aryl carboxylate ligands has been prepared and characterized by alkaline-earth (43Ca and 87Sr) solid-state NMR experiments in a magnetic field of 21.1 T. In the 11 compounds studied as part of this work, a range of coordination motifs are observed including nitrogen atom binding to Ca2+ and Sr2+, a binding mode which has not been investigated previously by 43Ca or 87Sr solid-state NMR. 43Ca isotopic enrichment has enabled the full characterization of the 43Ca electric field gradient (EFG) and chemical shift tensors of the two calcium sites in calcium p-aminosalicylate (Ca(pams)), where both NMR interactions are affected by the presence of a nitrogen atom in the first coordination sphere of one of the metal sites. The 43Ca isotropic chemical shift is sensitive to the Ca-N distance as exemplified by the NMR parameters of a second form of Ca(pams) and density functional theory (DFT) calculations. Studies of the strontium analogue, Sr(pams), confirm a similar sensitivity of the 87Sr EFG tensor to the presence or absence of nitrogen in the first coordination sphere. To our knowledge, this is the first systematic 87Sr NMR study of strontium complexes featuring organic ligands. The |CQ(87Sr)| values are found to be sensitive to the coordination number about Sr2+. In general, this work has also established a larger data set of reliable experimental |CQ(43Ca)| values which correlate well with those obtained using gauge-including projector-augmented-wave (GIPAW) DFT calculations. It is found that the use of a recently recommended quadrupole moment for 43Ca, -44.4 mbarn, improves the agreement with experimental values. This contribution lays the groundwork for the interpretation of 43Ca and 87Sr NMR spectra of more challenging systems, particularly where nitrogen-alkaline earth metal bonding is occurring. [ABSTRACT FROM AUTHOR]

A series of monohaloanilinium halides exhibiting weakhalogen bonding(XB) has been prepared and characterized by 35Cl, 81Br, and 127I solid-state nuclear magnetic resonance(SSNMR) spectroscopy in magnetic fields of up to 21.1 T. The quadrupolarand chemical shift (CS) tensor parameters for halide ions (Cl–, Br–, I–) whichact as electron density donors in the halogen bonds of these compoundsare measured to provide insight into the possible relationship betweenhalogen bonding and NMR observables. The NMR data for certain seriesof related compounds are strongly indicative of when such compoundspack in the same space group, thus providing practical structuralinformation. Careful interpretation of the NMR data in the contextof novel and previously available X-ray crystallographic data, andnew gauge-including projector-augmented-wave density functional theory(GIPAW DFT) calculations has revealed several notable trends. Whena series of related compounds pack in the same space group, it hasbeen possible to interpret trends in the NMR data in terms of thestrength of the halogen bond. For example, in isostructural series,the halide quadrupolar coupling constant was found to increase asthe halogen bond weakens. In the case of a series of haloaniliniumbromides, the 81Br isotropic chemical shift and CS tensorspan both decrease as the bromide–halogen XB is weakened. Thesetrends were reproduced using both GIPAW DFT and cluster-model calculationsof the bromide ion magnetic shielding tensor. Such trends are particularlyexciting given the well-known role that NMR has played historicallyin the characterization of hydrogen bonding. [ABSTRACT FROM AUTHOR]

A family of molecular heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temperature solution and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT) calculations. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host-guest and supramolecular phenomena. Specifically, dimerization of these metallocavitands yields capsules with vacant coordination and hydrogen-bonding sites accessible to encapsulated guests. Strong host-guest interactions explain the exceptionally high packing coefficient (0.80) observed for encapsulated N, N-dimethylformamide (DMF). The guest-accessible hydrogen-bonding sites arise from an unusual μ3-OH ligand bridging three cadmium ions. Thermodynamic and kinetic studies show that dimerization is an entropy-driven process with a highly associative mechanism. In DMF the exchange rate of peripheral cluster supporting carboxylate ligands is intrinsically linked to the rate of dimerization and these two seemingly different events have a common rate-determining step. Investigation of guest dynamics with solid-state ²H NMR spectroscopy revealed 3-fold rotation of an encapsulated DMF molecule. These studies provide a solid understanding of the host-guest and dynamic properties of a new family of metallocavitands and may help in designing new supramolecular catalysts and materials. [ABSTRACT FROM AUTHOR]

A large fraction of hydrogens in proteins and nucleic acids is of the methylene type. Their detailed study, however, in terms of structure and dynamics by NMR spectroscopy is hampered by their fast relaxation properties, which give rise to low sensitivity and resolution. It is demonstrated that six different relaxation interference processes, involving 1H-13H and 1H-1H dipolar interactions and 1H and 13C chemical shift anisotropy, can be used simultaneously to mitigate these problems effectively. The approach is applicable to the majority of NMR experiments commonly used to study side chain and backbone conformation. For proteins, its efficiency is evaluated quantitatively for two samples: the third IgG-binding domain from Streptococcal Protein G and the protein calmodulin complexed with a 26-residue target peptide. Gains in both resolution and sensitivity by up to factors of 3.2 and 2.0, respectively, are observed for Gly residues at high magnetic field strengths, but even at much lower fields gains remain substantial. The resolution enhancement obtained for methylene groups makes possible a detailed analysis of spectral regions commonly considered inaccessible due to spectral crowding. For DNA, the high resolution now obtainable for C5' sites permits an H5'/H5"-based sequential NOE assignment procedure, complementary to the conventional base-H1'/H2'/H2" pathway. [ABSTRACT FROM AUTHOR]