Radio Frequency Driven Dipolar Recoupling Research Articles

Triclinic La7Zn2P11 with P3-, P24-, and P35- units: a combined study by 31P solid-state NMR spectroscopy and single crystal X-ray diffraction.

The ternary polyphosphide La7Zn2P11 was synthesized from the elements by using a salt flux or via a ceramic method in sealed quartz ampoules. The obtained samples were investigated by X-ray powder and single crystal diffraction: own type, P1̄, a = 775.33(13), b = 827.45(13), c = 1502.8(3) pm, α = 82.111(3), β = 77.034(3), γ = 89.996(3)°, wR2 = 0.1553, 5852 F2 values and 183 variables. This peculiar structure is characterized by the simultaneous presence of three distinct anionic phosphide species, namely P3-, P24-, and P35- units. La7Zn2P11 is an electron precise Zintl phase: (7La3+)21+(2Zn2+)4+(4P3-)12-(2P24-)8-(P35-). The P-P single bond distances range from 219.2 to 223.0 pm. The zinc sites show tetrahedral phosphorus coordination by three P3- and one P24- species. The tetrahedra are condensed to chains via common corners. The P35- units with P-P-P angles of 113.7° have exclusively lanthanum coordination. 31P solid-state NMR was used to probe the phosphorus local environments, connectivities and spatial proximities. The eleven crystallographically distinct phosphorus atoms were assigned with the help of two-dimensional homonuclear dipolar correlation experiments. Even though the application of 2D measurements on such phosphorus-based polyanionic compounds is exceedingly challenging because of the wide dispersion of chemical shifts, the fast irreversible decay of the transverse magnetization, and slow spin-lattice relaxation, a complete assignment is possible using radiofrequency-driven dipolar recoupling (RFDR), J-RESOLVED and total-through-bond correlation with R-sequence (R-TOBSY) techniques.

Determination of Long-Range Distances by Fast Magic-Angle-Spinning Radiofrequency-Driven 19F-19F Dipolar Recoupling NMR.

Nanometer-range distances are important for restraining the three-dimensional structure and oligomeric assembly of proteins and other biological molecules. Solid-state NMR determination of protein structures typically utilizes 13C-13C and 13C-15N distance restraints, which can only be measured up to ∼7 Å because of the low gyromagnetic ratios of these nuclear spins. To extend the distance reach of NMR, one can harvest the power of 19F, whose large gyromagnetic ratio in principle allows distances up to 2 nm to be measured. However, 19F possesses large chemical shift anisotropies (CSAs) as well as large isotropic chemical shift dispersions, which pose challenges to dipolar coupling measurements. Here, we demonstrate 19F-19F distance measurements at high magnetic fields under fast magic-angle spinning (MAS) using radiofrequency-driven dipolar recoupling (RFDR). We show that 19F-19F cross-peaks for distances up to 1 nm can be readily observed in two-dimensional 19F-19F correlation spectra using less than 5 ms of RFDR mixing. This efficient 19F-19F dipolar recoupling is achieved using practically accessible MAS frequencies of 15-55 kHz, moderate 19F radio frequency field strengths, and no 1H decoupling. Experiments and simulations show that the fastest polarization transfer for aromatic fluorines with the highest distance accuracy is achieved using either fast MAS (e.g., 60 kHz) with large pulse duty cycles (>50%) or slow MAS with strong 19F pulses. Fast MAS considerably reduces relaxation losses during the RFDR π-pulse train, making finite-pulse RFDR under fast-MAS the method of choice. Under intermediate MAS frequencies (25-40 kHz) and intermediate pulse duty cycles (15-30%), the 19F CSA tensor orientation has a quantifiable effect on the polarization transfer rate; thus, the RFDR buildup curves encode both distance and orientation information. At fast MAS, the impact of CSA orientation is minimized, allowing pure distance restraints to be extracted. We further investigate how relayed transfer and dipolar truncation in multifluorine environments affect polarization transfer. This fast-MAS 19F RFDR approach is complementary to 19F spin diffusion for distance measurements and will be the method of choice under high-field fast-MAS conditions that are increasingly important for protein structure determination by solid-state NMR.

2D 31P solid state NMR spectroscopy, electronic structure and thermochemistry of PbP7

Phase pure polycrystalline PbP7 was prepared from the elements via a lead flux. Crystalline pieces with edge-lengths up to 1mm were obtained. The assignment of the previously published 31P solid state NMR spectrum to the seven distinct crystallographic sites was accomplished by radio-frequency driven dipolar recoupling (RFDR) experiments. As commonly found in other solid polyphosphides there is no obvious correlation between the 31P chemical shift and structural parameters. PbP7 decomposes incongruently under release of phosphorus forming liquid lead as remainder. The thermal decomposition starts at T>550K with a vapor pressure almost similar to that of red phosphorus. Electronic structure calculations reveal PbP7 as a semiconductor according to the Zintl description and clearly shows the stereo-active Pb-6s2 lone pairs in the electron localization function ELF.

3D 15N/15N/1H chemical shift correlation experiment utilizing an RFDR-based 1H/1H mixing period at 100 kHz MAS

Homonuclear correlation NMR experiments are commonly used in the high-resolution structural studies of proteins. While 13C/13C chemical shift correlation experiments utilizing dipolar recoupling techniques are fully utilized under MAS, correlation of the chemical shifts of 15N nuclei in proteins has been a challenge. Previous studies have shown that the negligible 15N–15N dipolar coupling in peptides or proteins necessitates the use of a very long mixing time (typically several seconds) for effective spin diffusion to occur and considerably slows down a 15N/15N correlation experiment. In this study, we show that the use of mixing proton magnetization, instead of 15N, via the recoupled 1H–1H dipolar couplings enable faster 15N/15N correlation. In addition, the use of proton-detection under ultrafast MAS overcomes the sensitivity loss due to multiple magnetization transfer (between 1H and 15N nuclei) steps. In fact, less than 300nL (∼1.1micromole quantity) sample is sufficient to acquire the 3D spectrum within 5h. Our results also demonstrate that a 3D 15N/15N/1H experiment can render higher resolution spectra that will be useful in the structural studies of proteins at ultrafast MAS frequencies. 3D 15N/15N/1H and 2D radio frequency-driven dipolar recoupling (RFDR)-based 1H/1H experimental results obtained from a powder sample of N-acetyla-l-15N-valyl-l-15N-leucine at 70 and 100kHz MAS frequencies are presented.

Finite-pulse radio frequency driven recoupling with phase cycling for 2D 1H/1H correlation at ultrafast MAS frequencies

The first-order recoupling sequence radio frequency driven dipolar recoupling (RFDR) is commonly used in single-quantum/single-quantum homonuclear correlation 2D experiments under magic angle spinning (MAS) to determine homonuclear proximities. From previously reported analysis of the use of XY-based super-cycling schemes to enhance the efficiency of the finite-pulse-RFDR (fp-RFDR) pulse sequence, XY814 phase cycling was found to provide the optimum performance for 2D correlation experiments on low-γ nuclei. In this study, we analyze the efficiency of different phase cycling schemes for proton-based fp-RFDR experiments. We demonstrate the advantages of using a short phase cycle, XY4, and its super-cycle XY414 that only recouples the zero-quantum homonuclear dipolar coupling, for the fp-RFDR sequence in 2D 1H/1H correlation experiments at ultrafast MAS frequencies. The dipolar recoupling efficiencies of XY4, XY414 and XY814 phase cycling schemes are compared based on results obtained from 2D 1H/1H correlation experiments, utilizing the fp-RFDR pulse sequence, on powder samples of U–13C,15N-l-alanine, N-acetyl–15N-l-valyl–15N-l-leucine, and glycine. Experimental results and spin dynamics simulations show that XY414 performs the best when a high RF power is used for the 180° pulse, whereas XY4 renders the best performance when a low RF power is used. The effects of RF field inhomogeneity and chemical shift offsets are also examined. Overall, our results suggest that a combination of fp-RFDR-XY414 employed in the recycle delay with a large RF-field to decrease the recycle delay, and fp-RFDR-XY4 in the mixing period with a moderate RF-field, is a robust and efficient method for 2D single-quantum/single-quantum 1H/1H correlation experiments at ultrafast MAS frequencies.

Structural Characterization of Phosphorus‐Based Networks and Clusters: 31P MAS NMR Spectroscopy and Magnetic Shielding Calculations on Hittorf’s Phosphorus

The (31)P MAS NMR spectrum of Hittorf's phosphorus has been measured and assigned to the 21 crystallographically distinct phosphorus atoms based on two-dimensional dipolar correlation spectroscopies. Application of such 2D techniques to phosphorus-based networks is particularly challenging owing to the wide chemical shift dispersions, rapid irreversible decay of transverse magnetization, and extremely slow spin-lattice relaxation in these systems. Nevertheless, a complete assignment was possible by using the combination of correlated spectroscopy (COSY) and radiofrequency-driven dipolar recoupling (RFDR). The assignment is supported further by DFT-based ab initio chemical shift calculations using a cluster-model approach, which gives good agreement between experimental and calculated chemical shift values. The (31)P chemical shifts appear to be strongly correlated with the average P-P bond lengths within the P(P(1/3))(3) coordination environments, whereas no clear dependence on average P-P-P bond angles can be detected. Calculations of localized Kohn-Sham orbitals reveal that this bond-length dependence is reflected in energy variations in the corresponding localized p-p-σ orbitals influencing the paramagnetic deshielding contribution in Ramsey's equation. In contrast, the contributions of the lone pairs to shielding differences are small and/or do not vary in a systematic manner for the different crystallographically distinct phosphorus sites. The combined spectroscopic and quantum chemical approach applied here and the increased theoretical understanding of (31)P chemical shifts will facilitate the structural elucidation of other phosphorus-based clusters and networks.

Solid-State NMR Reveals the Hydrophobic-Core Location of Poly(amidoamine) Dendrimers in Biomembranes

Poly(amidoamine) (PAMAM) dendrimer nanobiotechnology shows great promise in targeted drug delivery and gene therapy. Because of the involvement of cell membrane lipids with the pharmacological activity of dendrimer nanomedicines, the interactions between dendrimers and lipids are of particular relevance to the pharmaceutical applications of dendrimers. In this study, solid-state NMR was used to obtain a molecular image of the complex of generation-5 (G5) PAMAM dendrimer with the lipid bilayer. Using (1)H radio frequency driven dipolar recoupling (RFDR) and (1)H magic angle spinning (MAS) nuclear Overhauser effect spectroscopy (NOESY) techniques, we show that dendrimers are thermodynamically stable when inserted into zwitterionic lipid bilayers. (14)N and (31)P NMR experiments on static samples and measurements of the mobility of C-H bonds using a 2D proton detected local field protocol under MAS corroborate these results. The localization of dendrimers in the hydrophobic core of lipid bilayers restricts the motion of bilayer lipid tails, with the smaller G5 dendrimer having more of an effect than the larger G7 dendrimer. Fragmentation of the membrane does not occur at low dendrimer concentrations in zwitterionic membranes. Because these results show that the amphipathic dendrimer molecule can be stably incorporated in the interior of the bilayer (as opposed to electrostatic binding at the surface), they are expected to be useful in the design of dendrimer-based nanobiotechnologies.

Penetration of Aromatic Residues into Membrane Bilayers: A New Approach

Penetration of Aromatic Residues into Membrane Bilayers: A New ApproachLipid bilayers are characterized by a unique molecular motional regime that makes it possible to apply both solid-state NMR and solution-state NMR methods together for structural studies. The combination of magic angle spinning (MAS) with the high-resolution 1H NOESY NMR experiment is an established method for measuring through-space 1H¼1H dipolar couplings in biological membranes, and has been applied extensively in the past to biological membranes to determine the location of bound drugs and peptides. The segmental motion of the lipid acyl chains along with the overall rotational diffusion of the lipids provides sufficient motion to average the 1H dipolar interaction to within the range where MAS can be effective. One drawback of the approach is the relatively long NOESY mixing times needed for relaxation processes to generate significant crosspeak intensity. In order to drive magnetization transfer more rapidly, we introduce the use of solid-state radiofrequency driven dipolar recoupling (RFDR) pulses during the mixing time. We compare the established 1H MAS NOESY experiment with a new 1H MAS RFDR experiment on dimyristoylphosphocholine, a bilayer forming lipid, and show that the 1H MAS RFDR experiment provides considerably faster magnetization exchange than the 1H MAS NOESY experiment. We apply the method to model compounds containing basic and aromatic amino acids bound to membrane bilayers to illustrate the ability to locate the position of aromatic groups that have penetrated to below the level of the lipid headgroups.

High-resolution 1H MAS RFDR NMR of biological membranes

The combination of magic angle spinning (MAS) with the high-resolution 1H NOESY NMR experiment is an established method for measuring through-space 1H… 1H dipolar couplings in biological membranes. The segmental motion of the lipid acyl chains along with the overall rotational diffusion of the lipids provides sufficient motion to average the 1H dipolar interaction to within the range where MAS can be effective. One drawback of the approach is the relatively long NOESY mixing times needed for relaxation processes to generate significant crosspeak intensity. In order to drive magnetization transfer more rapidly, we use solid-state radiofrequency driven dipolar recoupling (RFDR) pulses during the mixing time. We compare the 1H MAS NOESY experiment with a 1H MAS RFDR experiment on dimyristoylphosphocholine, a bilayer-forming lipid and show that the 1H MAS RFDR experiment provides considerably faster magnetization exchange than the standard 1H MAS NOESY experiment. We apply the method to model compounds containing basic and aromatic amino acids bound to membrane bilayers to illustrate the ability to locate the position of aromatic groups that have penetrated to below the level of the lipid headgroups.

The Kagomé Topology of the Gallium and Indium Metal-Organic Framework Types with a MIL-68 Structure: Synthesis, XRD, Solid-State NMR Characterizations, and Hydrogen Adsorption

The vanadium-based terephthalate analogs of MIL-68 have been obtained with gallium and indium (network composition: M(OH)(O(2)C-C(6)H(4)-CO(2)), M = Ga or In) by using a solvothermal synthesis technique using N,N-dimethylformamide as a solvent (10 and 48 h, for Ga and In, respectively, at 100 degrees C). They have been characterized by X-ray diffraction analysis; vibrational spectroscopy; and solid-state (1)H and (1)H-(1)H radio-frequency-driven dipolar recoupling (RFDR), (1)H-(1)H double quantum correlation (DQ), and (13)C{(1)H} cross polarization magic angle spinning (CPMAS) NMR spectroscopy. The three-dimensional network with a Kagomé-like lattice is built up from the connection of infinite trans-connected chains of octahedral units MO(4)(OH)(2) (M = Ga or In), linked to each other through the terephthalate ligands in order to generate triangular and hexagonal one-dimensional channels. The presence of DMF molecules with strong interactions within the channels as well as their departure upon calcination (150 degrees C under a primary vacuum) of the materials has been confirmed by subjecting MIL-68 (Ga) to solid-state (1)H MAS NMR. The (1)H-(1)H RFDR and (1)H-(1)H DQ spectra revealed important information on the spatial arrangement of the guest species with respect to the hybrid organic-inorganic network. (13)C{(1)H} CPMAS NMR of activated samples provided crystallographically independent sites in agreement with X-ray diffraction structure determination. Brunauer-Emmett-Teller surface areas are 1117(24) and 746(31) m(2) g(-1) for MIL-98 (Ga) and MIL-68 (In), respectively. Hydrogen adsorption isotherms have been measured at 77 K, and the storage capacities are found to be 2.46 and 1.98 wt % under a saturated pressure of 4 MPa for MIL-68 (Ga) and MIL-68 (In), respectively. For comparison, the hydrogen uptake for the aluminum trimesate MIL-110, which has an open framework with 16 A channels, is 3 wt % under 4 MPa.

Solid State NMR Connectivity Studies in Dipolarly Coupled Inorganic Networks: Site Assignments in the Solid Electrolyte Material (CuI)8P12

Abstract The local structure of the fast solid ion conductor Cu2P3I2 = (CuI)8P12 is investigated using one- and two-dimensional 31P and 65Cu magic-angle spinning NMR techniques. Peak assignments are proposed on the basis of anisotropic and isotropic spin-spin interactions recovered via radio frequency driven dipolar recoupling (RFDR) and total through bond correlation spectroscopy (TOBSY) methods. Especially the previously proposed R306 14 pulse symmetry (R-TOBSY), which exploits scalar spin-spin interactions for coherence transfer, offers outstanding spectral selectivity with respect to 1 J coupling. Using sufficiently short mixing times it is thus possible to correlate selectively those 31P spins that are directly bonded, thereby providing detailed insights into phosphorus-phosphorus connectivities in the network. The final assignment of the nine resolved 31P resonances to the twelve crystallographically distinct phosphorus sites suggests a significant relationship between 31P chemical shifts and the detailed pyramidal P(P1/3)3 bonding geometry.

Proton Dipolar Recoupling in Resin-Bound Peptides under High-Resolution Magic Angle Spinning

Rotational resonance and radiofrequency-driven dipolar recoupling (RFDR) experiments have been used to recover the weak proton dipolar interaction present in peptides bound to swollen resins spun at the magic angle. The intensity of the correlation peaks obtained using these sequences is shown to be significantly stronger than the one obtained using the classical NOESY experiment. In addition, it is found that during the relatively long mixing times required to transfer magnetization in such soft materials, the RFDR sequence also achieves magnetization transfer via the scalar J-coupling.

(1)H and (13)C MAS NMR evidence for pronounced ligand-protein interactions involving the ionone ring of the retinylidene chromophore in rhodopsin.

Rhodopsin is a member of the superfamily of G-protein-coupled receptors. This seven alpha-helix transmembrane protein is the visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In the active binding site of this protein the ligand or chromophore, 11-cis-retinal, is covalently bound via a protonated Schiff base to lysine residue 296. Here we present the complete (1)H and (13)C assignments of the 11-cis-retinylidene chromophore in its ligand-binding site determined with ultra high field magic angle spinning NMR. Native bovine opsin was regenerated with 99% enriched uniformly (13)C-labeled 11-cis-retinal. From the labeled pigment, (13)C carbon chemical shifts could be obtained by using two-dimensional radio frequency-driven dipolar recoupling in a solid-state magic angle spinning homonuclear correlation experiment. The (1)H chemical shifts were assigned by two-dimensional heteronuclear ((1)H-(13)C) dipolar correlation spectroscopy with phase-modulated Lee-Goldburg homonuclear (1)H decoupling applied during the t(1) period. The data indicate nonbonding interactions between the protons of the methyl groups of the retinylidene ionone ring and the protein. These nonbonding interactions are attributed to nearby aromatic acid residues Phe-208, Phe-212, and Trp-265 that are in close contact with, respectively, H-16/H-17 and H-18. Furthermore, binding of the chromophore involves a chiral selection of the ring conformation, resulting in equatorial and axial positions for CH(3)-16 and CH(3)-17.

Quantized field description of rotor frequency-driven dipolar recoupling

A formalized many-particle nonrelativistic classical quantized field interpretation of magic angle spinning (MAS) nuclear magnetic resonance (NMR) radio frequency-driven dipolar recoupling (RFDR) is presented. A distinction is made between the MAS spin Hamiltonian and the associated quantized field Hamiltonian. The interactions for a multispin system under MAS conditions are described in the rotor angle frame using quantum rotor dynamics. In this quasiclassical theoretical framework, the chemical shift, the dipolar interaction, and radio frequency terms of the Hamiltonian are derived. The effect of a generalized RFDR train of π pulses on a coupled spin system is evaluated by creating a quantized field average dipolar-Hamiltonian formalism in the interaction frame of the chemical shift and the sample spinning. This derivation shows the analogy between the Hamiltonian in the quantized field and the normal rotating frame representation. The magnitude of this Hamiltonian peaks around the rotational resonance conditions and has a width depending on the number of rotor periods between the π pulses. Its interaction strength can be very significant at the n=0 condition, when the chemical shift anisotropies of the interacting spins are of the order of their isotropic chemical shift differences.

Fluorine-19 NMR from retrosynthesis to NMR crystallography

Recently, the need for more accurate structure description that will allow to analyze possible retrosynthesis of microporous compounds has led us to use and develop new structural strategies to access structure determination by solid state NMR techniques. At first, an example, AlPO4-CJ2, between crystallogenesis and crystal structure is explored. AlPO4-CJ2 has been studied by hydrothermal in-situ NMR. A soluble species exists in supersaturated liquid as well as the product of redissolution of AlPO4-CJ2. This species, the prenucleation building units (PNBU) is different from structural buildings units (SBU) the sub-ensembles of the crystal. How this PNBU gets into the structure and becomes therefore a structural building unit (SBU) is our main question. To answer it, an HETCOR experiment between fluorine and aluminum has been performed. This experiment shows that when the PNBU enters the solid, it forms at the same time than growing the solid, a fluorinated bridge. On ULM-18 after structure elucidation positioning hydrogen, a dynamic charge compensation mechanism is proposed for the solid formation. On TMPGaPO, a set of 2D experiments using radio frequency driven dipolar recoupling (RFDR) on 19F, Double Quanta on 31P and HETCOR between 19F and 31P allowed analyzing the topology of the network.

Cross-polarization/magic-angle-spinning nuclear magnetic resonance in selectively 13C-labeled synthetic eumelanins

We report a solid-state NMR study of synthetic eumelanins prepared by oxidation of 5,6-dihydroxyindole (DHI) selectively 13C-labeled at positions 2 or 3 of the indole ring. The 13C– 1H couplings have been used to quantify the carbons by selecting the non-protonated and protonated carbon resonances. By comparing the data of non-labeled melanin to that obtained using [2- 13C]- and [3- 13C]-enriched DHI, it was possible to clearly demonstrate the high chemical reactivity of position 2 and, to a lesser extent, position 3 of the DHI unit. These two sites together are responsible for three-quarters of the proton loss during polymerization. The cross-polarization/magic-angle-spinning spectra likewise point to a partial oxidation of positions 2 and 3 to the carboxyl and carbonyl oxidation states during the formation of melanin. Furthermore, it is shown that 13C– 13C dipolar interactions in [2- 13C]-enriched DHI melanins can be observed using radiofrequency-driven dipolar recoupling (RFDR) 2D experiments. An upper limit of about 4 Å for the distance between the C-2 carbons is deduced from the RFDR experiments. This result is in agreement with the basic arrangement of the different atoms expected in the DHI melanins.