Kinetics of 1H →31P NMR cross-polarization and dynamics in a layered crystalline α-Sn(IV) phosphate.

The proton-phosphorus (H-P) cross-polarization (CP) is effective in Sn(HPO4)2·H2O despite of the presence of paramagnetic ion impurities. Polarization constants TH-P and 1H T1ρ times are measured in static Sn(HPO4)2·H2O by the kinetic variable-temperature H-P CP experiments. The temperature dependence of the 1H T1ρ times is interpreted in terms of proton movements in the interlayer space occurring between the phosphate groups without participation of the water molecules. The process requires an activation energy of 8.7 ± 0.7 kcal/mol. The MAS effect on the 1H T1ρ times is shown and discussed.

Spin diffusion in the Phosphorus-31 NMR relaxation in a layered crystalline α-Sn(IV) phosphate contaminated by paramagnetic impurities.

The study of a layered crystalline Sn(IV) phosphate by solid-state NMR has demonstrated that the 31P T1 relaxation of phosphate groups, dependent on spinning rate is completely controlled by the limited spin diffusion to paramagnetic ions found by EPR. The spin-diffusion constant, D(SD), was estimated as 2.04 10-14 cm2s-1. The conclusion was supported by the 31P T1 time measurements in zirconium phosphate 1-1, also showing paramagnetic ions and in diamagnetic compound (NH4)2HPO4.

Acidic Centers on the Surface of a Crystalline α-Sn(IV) Phosphate Characterized by the Solid-State 1H, 2H, 31P, and 119Sn MAS NMR Techniques.

A layered crystalline phosphate α-Sn(HPO4)2·H2O (1), prepared and characterized in the present study by the multinuclear solid-state nuclear magnetic resonance (NMR), powder X-ray diffraction, and thermogravimetric analysis techniques, was treated with D2O and HOD imitating the reaction conditions in a water medium. The 2H solid-echo magic angle spinning NMR spectra of the products have revealed on their surface low mobile water molecules and hydronium ions, forming a structure close to the Zundel cation, [D2O···D-OD2]+. All the deuterons in the hydronium ions are tangled by hydrogen bonds with the water and the surface phosphate groups and stabilized by ionic interactions.

On Librational and Rotational Motions of Aromatic Rings in Layered Sn(IV) and Zr(IV) Phosphonate Materials: A Variable-Temperature 13C, 31P Solid-State NMR Study.

According to the solid-state 13C, 31P NMR study and 13C chemical shift anisotropy (CSA) measurements, aromatic rings in the layered metal(IV) phosphonate materials behave as low-energy rotors at rotation activation energy, Eact, of 1.4-3.0 kcal/mol. The rotational mechanism consists of 180° flips and librations around C(1)-C(4) axis. The amplitude of the librations, added to the flips, grows with temperature, shifting the reorientations toward rotational diffusion at high temperatures.

Pyridine-d5 as a 2H NMR probe for investigation of macrostructure and pore shapes in a layered Sn(iv) phosphonate-phosphate material.

Isotropic and anisotropic motions and molecular states of pyridine-d5, adsorbed on the surface within the pores of a layered Sn(iv) phosphonate-phosphate material (1) have been characterized thermodynamically and kinetically by solid-state NMR. The data obtained provide formulation of macrostructure and shapes of pores in 1.

Guest Molecules in a Layered Microporous Tin(IV) Phosphonate-Phosphate Material: Solid State NMR Studies.

There is little systematic understanding of pore surfaces in layered microporous metal(IV) phosphate-phosphonate materials and their interactions with guest molecules. In this paper, we show how to probe the mobility of guest molecules in such poorly crystalline systems using multinuclear solid-state NMR and relaxation time measurements. Anisotropic motions of benzene- d6 molecules absorbed on the pore walls of material Sn(O3PC6H4PO3)0.85(O3POH)0.33 (1) have been recognized as the fast in-plane C6 rotation due to metal-π interactions with pore walls. The benzene- d6 absorption enthalpy due to Sn···π interactions has been determined as -Δ H = 5.9 kcal/mol. Specific interactions between pyridine and the pore walls of 1 have been observed as immobile pyridine, the population of which grows strongly at low temperatures to show thermodynamic parameters -Δ H of 5.0 kcal/mol and Δ S of -11.0 e.u. It has been suggested that these parameters characterize N···H-OP hydrogen bonding as a driving force for accumulation of immobile pyridine molecules in pores of compound 1.

Benzene-d6 and toluene-d8 as guest molecules in micropores of a layered zirconium phosphonate: 2 H, 13 C{1 H}, and 31 P{1 H} solid-state NMR, deuterium NMR relaxation, and molecular motions.

For the first time, pore spaces in the Zr (IV) phosphonate (1) as a representative of layered metal (IV) phosphonate materials have been investigated by studying mobility of guest molecules, benzene-d6 , and toluene-d8 . Guest molecules located in micropores of 1 have been characterized by solid-state 13 C{1 H} and 2 H NMR spectra in static samples with varying temperatures. At moderately low temperatures, the benzene and toluene molecules experience fast isotropic reorientations and show the motionally averaged liquid-like carbon and deuterium line shapes in the NMR spectra. At lower temperatures, two anisotropic motional modes have been found for benzene molecules by analyzing the 2 H NMR line shapes: the well-known in-plane C6 rotation and composite motions. Interpretation of the variable-temperature 2 H T1 relaxation times identifies the composite motions as 120° flips around the C6 axis perpendicular to the molecular plane and the rotations around the molecular para-C-C axis. The data obtained resulted in the idealized (cylinder-shaped) model of micropores in compound 1 with the diameter of 20-30 Å. Furthermore, the activation energy of 20.1 kJ/mol determined for the benzene motions classifies the molecule-surface interactions as weak but enough for absorption.

Backbone motion in elastin's hydrophobic domains as detected by (2) H NMR spectroscopy.

The elasticity of vertebrate tissue originates from the insoluble, cross-linked protein elastin. Here, the results of variable-temperature (2) H NMR spectra are reported for hydrated elastin that has been enriched at the Hα position in its abundant glycines. Typical powder patterns reflecting averaged quadrupolar parameters are observed for the frozen protein, as opposed to the two, inequivalent deuterons that are detected in a powder sample of enriched glycine. The spectra of the hydrated elastin at warmer temperatures are dominated by a strong central peak with features close to the baseline, reflective of both isotropic and very weakly anisotropic motions.

X-ray and NMR crystallography in an enzyme active site: the indoline quinonoid intermediate in tryptophan synthase.

Chemical-level details such as protonation and hybridization state are critical for understanding enzyme mechanism and function. Even at high resolution, these details are difficult to determine by X-ray crystallography alone. The chemical shift in NMR spectroscopy, however, is an extremely sensitive probe of the chemical environment, making solid-state NMR spectroscopy and X-ray crystallography a powerful combination for defining chemically detailed three-dimensional structures. Here we adopted this combined approach to determine the chemically rich crystal structure of the indoline quinonoid intermediate in the pyridoxal-5'-phosphate-dependent enzyme tryptophan synthase under conditions of active catalysis. Models of the active site were developed using a synergistic approach in which the structure of this reactive substrate analogue was optimized using ab initio computational chemistry in the presence of side-chain residues fixed at their crystallographically determined coordinates. Various models of charge and protonation state for the substrate and nearby catalytic residues could be uniquely distinguished by their calculated effects on the chemical shifts measured at specifically (13)C- and (15)N-labeled positions on the substrate. Our model suggests the importance of an equilibrium between tautomeric forms of the substrate, with the protonation state of the major isomer directing the next catalytic step.

De-Pake-ing transform analysis of fully deuterated malonic acid.

The analysis of deuterium wideline NMR spectra has been an essential step in characterizing the dynamics of molecules in the solid-state. Although clearly important, the identification of quadrupolar coupling constants (QCCs) from the powder patterns is often complicated by poor sensitivity and/or spectral overlap. Previously, others have demonstrated the utility of "de-Pake-ing", a mathematical transform that yields the QCCs in a straightforward manner for symmetric (eta=0) sites. In this short paper, we describe our analysis of a powder sample of perdeutero-malonic acid, a molecule with two distinct deuteron environments and asymmetries. The methylene sites are immediately amenable to the standard de-Pake-ing transform analysis due to their low asymmetry. However, the de-Pake-ing methodology for the acid deuterons, for which the asymmetry deviates significantly from zero, requires more analysis to extract their QCCs. The impact of this work on the future application of de-Pake-ing to a wider range of samples is also discussed.

Sensitive absorptive refocused scalar correlation NMR spectroscopy in solids.

A new two-dimensional NMR experiment is described which is suitable for obtaining magic angle spinning (MAS) scalar correlation spectra in solids. The new experiment has several advantages, including increased cross peak intensities, coupled with good suppression of the diagonal. Its utility is demonstrated via assignments of the carbon-13 MAS spectra of progesterone at natural abundance and of the polymer phase of 50%-U-13C-CsC60.

Penetration depth of surfactant peptide KL4 into membranes is determined by fatty acid saturation.

KL(4) is a 21-residue functional peptide mimic of lung surfactant protein B, an essential protein for lowering surface tension in the alveoli. Its ability to modify lipid properties and restore lung compliance was investigated with circular dichroism, differential scanning calorimetry, and solid-state NMR spectroscopy. KL(4) binds fluid lamellar phase PC/PG lipid membranes and forms an amphipathic helix that alters lipid organization and acyl chain dynamics. The binding and helicity of KL(4) is dependent on the level of monounsaturation in the fatty acid chains. At physiologic temperatures, KL(4) is more peripheral and dynamic in fluid phase POPC/POPG MLVs but is deeply inserted into fluid phase DPPC/POPG vesicles, resulting in immobilization of the peptide. Substantial increases in the acyl chain order are observed in DPPC/POPG lipid vesicles with increasing levels of KL(4), and POPC/POPG lipid vesicles show small decreases in the acyl chain order parameters on addition of KL(4). Additionally, a clear effect of KL(4) on the orientation of the fluid phase PG headgroups is observed, with similar changes in both lipid environments. Near the phase transition temperature of the DPPC/POPG lipid mixtures, which is just below the physiologic temperature of lung surfactant, KL(4) causes phase separation with the DPPC remaining in a gel phase and the POPG partitioned between gel and fluid phases. The ability of KL(4) to differentially partition into lipid lamellae containing varying levels of monounsaturation and subsequent changes in curvature strain suggest a mechanism for peptide-mediated lipid organization and trafficking within the dynamic lung environment.

Interactions of the C-terminus of lung surfactant protein B with lipid bilayers are modulated by acyl chain saturation.

Lung surfactant protein B (SP-B) is critical to minimizing surface tension in the alveoli. The C-terminus of SP-B, residues 59-80, has much of the surface activity of the full protein and serves as a template for the development of synthetic surfactant replacements. The molecular mechanisms responsible for its ability to restore lung compliance were investigated with circular dichroism, differential scanning calorimetry, and (31)P and (2)H solid-state NMR spectroscopy. SP-B(59-80) forms an amphipathic helix which alters lipid organization and acyl chain dynamics in fluid lamellar phase 4:1 DPPC:POPG and 3:1 POPC:POPG MLVs. At higher levels of SP-B(59-80) in the POPC:POPG lipid system a transition to a nonlamellar phase is observed while DPPC:POPG mixtures remain in a lamellar phase. Deuterium NMR shows an increase in acyl chain order in DPPC:POPG MLVs on addition of SP-B(59-80); in POPC:POPG MLVs, acyl chain order parameters decrease. Our results indicate SP-B(59-80) penetrates deeply into DPPC:POPG bilayers and binds more peripherally to POPC:POPG bilayers. Similar behavior has been observed for KL(4), a peptide mimetic of SP-B which was originally designed using SP-B(59-80) as a template and has been clinically demonstrated to be successful in treating respiratory distress syndrome. The ability of these helical peptides to differentially partition into lipid lamellae based on their degree of monounsaturation and subsequent changes in lipid dynamics suggest a mechanism for lipid organization and trafficking within the dynamic lung environment.

Constant-time through-bond 13C correlation spectroscopy for assigning protein resonances with solid-state NMR spectroscopy.

Even as available magnetic fields for NMR continue to increase, resolution remains one of the most critical limitations in assigning and solving structures of larger biomolecules. Here we present a novel constant-time through-bond correlation spectroscopy for solids that offers superior resolution for 13C chemical shift assignments in proteins. In this experiment, the indirect evolution and transfer periods are combined into a single constant time interval, offering increased resolution while not sacrificing sensitivity. In GB1, this allows us to resolve peaks that are otherwise unresolved and to make assignments in the absence of multibond transfers.

Uniform-sign cross-peak double-quantum-filtered correlation spectroscopy.

We detail the uniform-sign cross-peak double-quantum-filtered correlation spectroscopy (UC2QF COSY) experiment, a new through-bond correlation method for disordered solids. This experiment is a refocused version of the popular double-quantum-filtered correlation spectroscopy experiment in liquids. Its key feature is that it provides in-phase and doubly absorptive line shapes, which renders it robust for chemical shift correlation in solids. Both theory and experiment point to distinct advantages of this protocol, which are illustrated by several experiments under challenging conditions, including fast magic-angle spinning (30kHz), anisotropic molecular motion, and (13)C correlation spectroscopy at the natural abundance isotope level.

Through-bond 13C-13C correlation at the natural abundance level: refining dynamic regions in the crystal structure of vitamin-D3 with solid-state NMR.

Recently, we presented a novel nuclear magnetic resonance experiment for establishing through-bond connectivity in disordered solids using scalar coupling-driven correlation. This method, a variant of the popular double-quantum-filtered correlation spectroscopy experiment in liquids, is robust under fast magic-angle-spinning conditions and in the presence of dynamics. Here, we show that this new experiment, the UC2QF COSY, can be extended to 13C natural abundance correlation in moderately sized molecules, allowing the assignment of the 54 peaks of the solid-state NMR spectrum of microcrystalline vitamin-D3. In this case, comparison between the assigned peaks and ab initio calculations of the chemical shifts based on the crystal coordinates permits a refinement of the average structure in dynamic regions reported as disordered in the crystal structure.

Crystallographic evidence for a free silylium ion.

Evidence for a three-coordinate silyl cation is provided by the crystal structure of [(Mes)3Si][H-CB11Me5Br6].C6H6 (where Mes is 2,4,6-trimethylphenyl). Free (Mes)3Si+ cations are well separated from the carborane anions and benzene solvate molecules. Ortho-methyl groups of the mesityl substituents shield the silicon atom from the close approach of nucleophiles, while remaining innocent as significant ligands themselves. The silicon center is three-coordinate and planar. The downfield 29Si nuclear magnetic resonance chemical shift in the solid state (226.7 parts per million) is almost identical to that in benzene solution and in "gas phase" calculations, indicating that three-coordination can be preserved in all phases.

Establishing through-bond connectivity in solids with NMR: structure and dynamics in HC(60)(+).

We present a novel nuclear magnetic resonance experiment for establishing through-bond connectivity in solids using scalar coupling-driven correlation. This method, a variant of the popular double-quantum-filtered correlation spectroscopy experiment in liquids, is robust under fast magic-angle-spinning conditions and in the presence of dynamics. In HC(60)(+), where anisotropic molecular motion renders through-space dipolar-driven correlation ineffective, this through-bond correlation method answers a significant structural question by accurately identifying the direct bond between the protonated sp(3) hybridized carbon site and the sp(2) hybridized cationic site.