Radio Frequency-driven Recoupling Research Articles

Reactivity of NH4H2PO4 toward LaCl3 in LiCl-KCl Melt Flux. Step by Step Formation of Monazite-Like LaPO4.

The synthesis of lanthanum phosphates in molten LiCl-KCL eutectic was chosen to address the preliminary treatment of chlorinated wastes containing fission products that are already present in a Li/Cl eutectic. The obtained monazite compound shows interesting properties to be considered as a good candidate to trap lanthanum for a long-time. The synthesis route based on LaCl(3) reaction with NH(4)H(2)PO(4) in a stoichiometric amount is a key point to obtain monazite as a pure phase. Hence, the salt composition is not modified during the synthesis reaction. The chemical reactivity of ammonium dihydrogenphosphate (NH(4)H(2)PO(4), hereafter abbreviated ADP) toward lanthanum chloride (LaCl(3)) in molten LiCl-KCl eutectic is probed by NMR spectroscopy to follow the formation of LaPO(4). Formally, a direct transformation of the two aforementioned precursors into LaPO(4), NH(4)Cl and HCl can be discarded on the basis of the low thermal stability of ADP. To shed some light on the formation of LaPO(4), in situ and ex situ NMR experiments were carried out on LiCl-KCl/LaCl(3)/ADP, as well as LiCl-KCl/ADP, KCl/ADP, and LiCl/ADP mixtures. First, the reactivity of the precursors in contact with the eutectic was studied from room temperature to 600 degrees C by means of (31)P, (35)Cl, and (139)La high temperature NMR. Second, ex situ room temperature magic angle spinning (MAS) and RadioFrequency driven recoupling (RFDR) (31)P solid-state NMR experiments were carried out on solid samples prepared in different conditions (i.e., temperature and atmosphere) and quenched at room temperature to identify frozen intermediate species in their metastable state. On the basis of this approach, we propose a model for the LaPO(4) formation based on a multistep mechanism which highlights the strong reactivity of ADP toward the alkaline salts but without final change in the composition of the solvent.

Radio frequency-driven recoupling at high magic-angle spinning frequencies: Homonuclear recoupling sans heteronuclear decoupling

We describe solid-state NMR homonuclear recoupling experiments at high magic-angle spinning (MAS) frequencies using the radio frequency-driven recoupling (RFDR) scheme. The effect of heteronuclear decoupling interference during RFDR recoupling at high spinning frequencies is investigated experimentally and via numerical simulations, resulting in the identification of optimal decoupling conditions. The effects of MAS frequency, RF field amplitude, bandwidth, and chemical shift offsets are examined. Most significantly, it is shown that broadband homonuclear correlation spectra can be efficiently obtained using RFDR without decoupling during the mixing period in fully protonated samples, thus considerably reducing the rf power requirements for acquisition of (13)C-(13)C correlation spectra. The utility of RFDR sans decoupling is demonstrated with broadband correlation spectra of a peptide and a model protein at high MAS frequencies and high magnetic field.

Investigation of finite-pulse radiofrequency-driven recoupling methods for measurement of intercarbonyl distances in polycrystalline and membrane-associated HIV fusion peptide samples

Two finite-pulse radiofrequency-driven recoupling (RFDR) methods were compared and applied to the measurement of 3-6 Å (13)CO-(13)CO distances in polycrystalline and membrane-associated HIV fusion peptide (HFP) samples. The RFDR methods were based on π pulses and were relatively straightforward to implement and insensitive to pulse imperfections. The two tested methods were: (i) constant-time double-quantum buildup with finite pulses (fpCTDQBU) for which the pulse sequence maintained a constant transverse relaxation period while allowing a variable period of dipolar dephasing; and (ii) constant-time finite-pulse rf-driven recoupling (fpRFDR-CT) for which the duration of transverse relaxation increased with increasing dephasing period. The fpRFDR-CT method yielded higher signal-to-noise and an accurate determination of a ~5 Å intercarbonyl distance was made in a crystalline peptide which had T(2) ≈ 55 ms. In some contrast, the HFP samples had T(2) ≈ 15 ms and the fpRFDR-CT data were dominated by transverse relaxation. Examination of the fpCTDQBU sequence showed: (i) the most rapid signal buildup was obtained with application of one (13) C π pulse per rotor period rather than one (13)C π pulse per multiple rotor periods and (ii) the data were insensitive to ~15 ppm transmitter offset and to ~5° variation of π pulse nutation angle. For HFP samples which were (13)CO labeled at a single residue, analyses of the fpCTDQBU data were interpreted with a model of mixed parallel and antiparallel β-strand arrangements in the N-terminal region of HFP and loss of parallel β-sheet structure in the C-terminal region of HFP.

Structural Analysis of Alanine Tripeptide with Antiparallel and Parallel β-Sheet Structures in Relation to the Analysis of Mixed β-Sheet Structures in Samia cynthia ricini Silk Protein Fiber Using Solid-State NMR Spectroscopy

The structural analysis of natural protein fibers with mixed parallel and antiparallel beta-sheet structures by solid-state NMR is reported. To obtain NMR parameters that can characterize these beta-sheet structures, (13)C solid-state NMR experiments were performed on two alanine tripeptide samples: one with 100% parallel beta-sheet structure and the other with 100% antiparallel beta-sheet structure. All (13)C resonances of the tripeptides could be assigned by a comparison of the methyl (13)C resonances of Ala(3) with different [3-(13)C]Ala labeling schemes and also by a series of RFDR (radio frequency driven recoupling) spectra observed by changing mixing times. Two (13)C resonances observed for each Ala residue could be assigned to two nonequivalent molecules per unit cell. Differences in the (13)C chemical shifts and (13)C spin-lattice relaxation times (T(1)) were observed between the two beta-sheet structures. Especially, about 3 times longer T(1) values were obtained for parallel beta-sheet structure as compared to those of antiparallel beta-sheet structure, which could be explicable by the difference in the hydrogen-bond networks of both structures. This very large difference in T(1) becomes a good measure to differentiate between parallel or antiparallel beta-sheet structures. These differences in the NMR parameters found for the tripeptides may be applied to assign the parallel and antiparallel beta-sheet (13)C resonances in the asymmetric and broad methyl spectra of [3-(13)C]Ala silk protein fiber of a wild silkworm, Samia cynthia ricini.

Structural investigation of cellulose Iα and Iβ by 2D RFDR NMR spectroscopy: determination of sequence of magnetically inequivalent d-glucose units along cellulose chain

In our previous studies of the crystal structure of native cellulose (cellulose I) by solid-state two-dimensional (2D) 13C–13C INADEQUATE, it was revealed that cellulose Iα contains two kinds of β-d-glucose residues (A and A′) in the unit cell and that cellulose Iβ contains another two kinds of residues (B and B′). In the present study, the sequence of residues A and A′ along the same chains in cellulose Iα and that of residues B and B′ in Iβ were investigated by 2D 13C–13C rotor-synchronized radiofrequency-driven recoupling (RFDR) experiments using, respectively, uniformly 13C6-labeled (U−13C6) bacterial cellulose and the same [U−13C6] cellulose sample after thermal treatment at 260 °C. The RFDR spectra recorded with a short mixing time (1.0 ms) showed dipolar-coupled 13C spin pairs of only the neighboring carbon of the both phases, while those recorded with a longer mixing time (3.0–15 ms) provided correlations between weakly coupled 13C spin pairs as well as strongly coupled 13C spin pairs such as neighboring carbon nuclei. In the RFDR spectrum of the [U−13C6] cellulose recorded with a mixing time of 15 ms, the inter-residue 13C–13C correlation between C4 of residue A and C2 of residue A′ and that between C3 of residue A and C4 of residue A′ were clearly observed. In the case of cellulose Iβ, however, inter-residue 13C–13C correlations between residues B and B′ could not be detected in the series of RFDR spectra recorded with different mixing times of annealed [U−13C6] cellulose. From these findings, that cellulose Iα was revealed to have the –A–A′– repeating units along the cellulose chain. For cellulose Iβ, it was revealed that the respective residues B and B′ are composed of independent chains (–B–B– and –B′–B′– repeating units) and that there are no –B–B′– repeating units in the chain.

Two-dimensional spin-exchange solid-state NMR study of the crystal structure of cellulose II

A two-dimensional (2D) 13C– 13C spin-exchange NMR experiment was applied to the uniformly 13C-enriched cellulose II in order to obtain interatomic distance information in the cellulose II crystal. A radio frequency-driven recoupling (RFDR) rotor-synchronized π pulse sequence was incorporated during the mixing time of the 2D experiment. The 2D spin-exchange NMR recorded with a short mixing time (0.80–2.58 ms) provided the correlations between a pair of strongly coupled 13C spins such as neighbor carbon nuclei. This spectrum enabled us to assign all 13C resonance lines of two kinds of anhydroglucose residues A and B in the structure of cellulose II. On the basis of the 13C resonance assignment of residues A and B, the interatomic distances from each C1 to the other carbon nuclei were compared by measuring the 2D spectra recorded with longer mixing times (5.12–20.48 ms). As a result, it was revealed that the respective residues A and B are composed of independent chains (– A– A– and – B– B– repeating units) and that there are no – A– B– repeating units in the chain. This experimental technique is expected to be applicable to conformation analysis of polysaccharides as well as the other cellulose polymorphs.

Increasing the Amphiphilicity of an Amyloidogenic Peptide Changes the β-Sheet Structure in the Fibrils from Antiparallel to Parallel

Solid-state NMR measurements have been reported for four peptides derived from β-amyloid peptide A β(1–42): A β(1–40), A β(10–35), A β(16–22), and A β(34–42). Of these, the first two are predicted to be amphiphilic and were reported to form parallel β-sheets, whereas the latter two peptides appear nonamphiphilic and adopt an antiparallel β-sheet organization. These results suggest that amphiphilicity may be significant in determining fibril structure. Here, we demonstrate that acylation of A β(16–22) with octanoic acid increases its amphiphilicity and changes the organization of fibrillar β-sheet from antiparallel to parallel. Electron microscopy, Congo Red binding, and one-dimensional 13C NMR measurements demonstrate that octanoyl-A β(16–22) forms typical amyloid fibrils. Based on the stability of monolayers at the air-water interface, octanoyl-A β(16–22) is more amphiphilic than A β(16–22). Measurements of 13C- 13C and 15N- 13C nuclear magnetic dipole-dipole couplings in isotopically labeled fibril samples, using the constant-time finite-pulse radiofrequency-driven recoupling (fpRFDR-CT) and rotational echo double resonance (REDOR) solid-state NMR techniques, demonstrate that octanoyl-A β(16–22) fibrils are composed of parallel β-sheets, whereas A β(16–22) fibrils are composed of antiparallel β-sheets. These data demonstrate that amphiphilicity is critical in determining the structural organization of β-sheets in the amyloid fibril. This work also shows that all amyloid fibrils do not share a common supramolecular structure, and suggests a method for controlling the structure of amyloid fibrils.

Fluorine-19 solid-state NMR investigation of regiodefective semicrystalline α-poly(vinylidenefluoride)

Solid-state 19F NMR has been applied to poly(vinylidenefluoride) to investigate, inter alia, the location of the reverse units. The application of relaxation filters in pulse sequences revealed fundamental differences relating to the domain structure of PVDF. A T 1 ρ (F) spin-lock experiment gave a spectrum of the crystalline phase along with some intensity from signals associated with reverse units. The proximity of reverse units to the amorphous and crystalline domains was further investigated by T 1 ρ (F)-filtered radio frequency driven recoupling (RFDR) and spin-diffusion experiments. The results showed the majority of reverse units to be relatively mobile (i.e. amorphous). However, weak RFDR cross-peaks were detected which suggest the presence of some reverse units in relatively rigid domains. A signal arising from a highly mobile site was detected at δ F =−115 ppm by a delayed acquisition experiment and is tentatively assigned to –CH 2CF 2H end-groups.

Supramolecular Structure in Full-Length Alzheimer's β-Amyloid Fibrils: Evidence for a Parallel β-Sheet Organization from Solid-State Nuclear Magnetic Resonance

We report constraints on the supramolecular structure of amyloid fibrils formed by the 40-residue β-amyloid peptide associated with Alzheimer's disease (A β 1–40) obtained from solid-state nuclear magnetic resonance (NMR) measurements of intermolecular dipole-dipole couplings between 13C labels at 11 carbon sites in residues 2 through 39. The measurements are carried out under magic-angle spinning conditions, using the constant-time finite-pulse radiofrequency-driven recoupling (fpRFDR-CT) technique. We also present one-dimensional 13C magic-angle spinning NMR spectra of the labeled A β 1–40 samples. The fpRFDR-CT data reveal nearest-neighbor intermolecular distances of 4.8 ± 0.5 Å for carbon sites from residues 12 through 39, indicating a parallel alignment of neighboring peptide chains in the predominantly β-sheet structure of the amyloid fibrils. The one-dimensional NMR spectra indicate structural order at these sites. The fpRFDR-CT data and NMR spectra also indicate structural disorder in the N-terminal segment of A β 1–40, including the first nine residues. These results place strong constraints on any molecular-level structural model for full-length β-amyloid fibrils.

Measurement of interfluorine distances in solids.

(19)F homonuclear dipolar recoupling methods were used to measure internuclear distances ranging from 5 to 12 A in fluorinated organic compounds in the solid state. Magic-angle-spinning-based high-resolution techniques were utilized. Trifluoromethyl and aromatic fluorine groups were separated by rigid aromatic spacers; these compounds were diluted into nonfluorinated host molecule matrices to give isolated homonuclear spin pairs with known internuclear distances. Radiofrequency-driven recoupling (RFDR) was used to elicit magnetization exchange between the spin pairs in 1D and 2D experiments. Simulation of the exchange was accomplished using a Monte Carlo-type algorithm to search the parameter space. These methods allow the determination of distances with an accuracy of 1 A at shorter distances and 2 A at longer distances, with the assumption of no prior knowledge of T(2)(ZQ).

Compound Radiofrequency-Driven Recoupling Pulse Sequences for Efficient Magnetization Transfer by Homonuclear Dipolar Interaction under Magic-Angle Spinning Conditions

The maximum of the transferred magnetization in rotating powdered solids under the radiofrequency-driven recoupling (RFDR) pulse sequence is enhanced by reducing the orientation dependence of the effective recoupled homonuclear dipolar interaction. The compound RFDR (CRFDR) pulse sequence for this enhancement consists of RFDR pulse units (tau(i)-pi-tau(R)-pi-1171;tau(i)) with different tau(i), where tau(R) is the sample rotation period, tau(i) and 1171;tau(i) (=tau(R) - tau(i)) are delays, and pi is a 180 degrees pulse. The delay tau(i) modifies the zero-quantum spin operators and the sample rotation-angle dependence of the recoupled dipolar Hamiltonian. The CRFDR pulse sequences were optimized for mixing by varying tau(i). Numerical simulation for the two-spin system only with a dipolar interaction and isotropic chemical shifts indicates that the transfer efficiency of CRFDR averaged over the powder is about 70%, which is 30% higher than the efficiency of the RFDR pulse over a broad range of about 1/tau(R) in resonance frequency difference. The CRFDR sequences need about 60% longer mixing times to maximize the transferred magnetizaion in comparison with the original RFDR sequence. Chemical shift anisotropy, the other dipolar interactions, and relaxation generally reduce the enhancement by CRFDR. Experiments for fully (13)C-labeled alanine, however, show that the maximum of the magnetization transferred with CRFDR from the carboxyl to alpha carbon is about 15% greater than that with RFDR. Copyright 2000 Academic Press.

Homonuclear radio frequency-driven recoupling in rotating solids

We discuss several aspects of homonuclear recoupling and longitudinal exchange using rotor-synchronized spin echo sequences in solid state magic-angle spinning (MAS) experiments. These include the accurate measurement of weak dipole–dipole couplings between rare spins, the behavior of dipolar trajectories in multiple spin environments, and chemical shift correlation spectroscopy via polarization exchange. To describe dipolar trajectories accurately, we adopt an approach to the simulation of these experiments which includes finite pulses and the influence of coherence decay. The latter effect becomes competitive with the strength of weak couplings in many experiments, and a simple empirical approach is outlined for the selection of decay parameters. Dipolar trajectories are shown to be dominated by the largest couplings in multiple spin systems via comparison of two and three interacting spins. Two-dimensional correlation spectroscopy based on dipolar exchange among proximate nuclei is illustrated with a uniformly N15,C13-labeled sample of the tetrapeptide achatin-II (Gly-L-Phe-L-Ala-L-Asp). In addition, a frequency-selective approach to recoupling dipolar interactions among homonuclear spins is introduced; selective approaches have possible utility in examining weak dipole–dipole couplings in the presence of strong interactions.