Studies of heterogeneous catalysis using high-resolution solid state NMR

Recent results obtained at the institute of Catalysis from studies of heterogeneous catalysis using high-resolution¹H,¹³C,¹⁵N,²⁷Al,²⁹Si,³¹P and⁵¹V solid state NMR have been summarized. Emphasis is made on: (1) structural studies of active in catalysis compounds and sites in supported oxides, hydrides and inorganic acids; (2) studies of structures and properties of surface OH groups active in Bronsted acid catalysis; (3) studies of Lewis acidity of heterogeneous catalysts using¹⁵N NMR of adsorbed N₂O and (4) studies of adsorption mechanisms for molecules of reactants over various catalysts.     

FTIR, XRD, SEM and solid state NMR investigations of carbonate-containing hydroxyapatite nano-particles synthesized by hydroxide-gel technique

Nano-crystalline hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂ has been synthesized by a precipitate conversion technique using hydroxide gel at lower temperatures, e.g. 80 °C. HA crystallizes in hexagonal structure (space group: P6₃/m) having lattice parameters: and and around 17 nm in crystallite-size for the 80 °C-heated sample. SEM micrographs show hexagonal crystallites of average particle dimensions approximately 50×20 nm for 80 °C heated sample. The structure analysis by XRD, FTIR, ¹H and ³¹P MAS NMR show the existence of structural disorder at the particle surface that either does not form hydrogen bonding due to lack of adequate bonding parameters or forms a very weak dipolar bonding. The structural disorder has been explained as a result of chemical interactions between the phosphate groups either with the surface adsorbed water or the hydroxyl groups at the surface of the nano-particles.     

Methodology for solid state NMR off-resonance study of molecular dynamics in heteronuclear systems

Methodology for the study of dynamics in heteronuclear systems in the laboratory frame was described in the previous paper [1]. Now the methodology for the study of molecular dynamics in the solid state heteronuclear systems in the rotating frame is presented. The solid state NMR off-resonance experiments were carried out on a homemade pulse spectrometer operating at the frequency of 30.2 MHz for protons. This spectrometer includes a specially designed probe which contains two independently tuned and electrically isolated coils installed in the coaxial position on the dewar. A unique probe design allows working at three slightly differing frequencies off and on resonance for protons and at the frequency of 28.411 MHz for fluorine nuclei with complete absence of their electrical interference. The probe allows simultaneously creating rf magnetic fields at off-resonance frequencies within the range of 30.2–30.6 MHz and at the frequency of 28.411 MHz. Presented heteronuclear cross-relaxation off-resonance experiments in the rotating frame provide information about molecular dynamics.     

The role of solid state 13C NMR spectroscopy in studies of the nature of native celluloses

Published spectroscopic observations pertaining to the crystal structure of native celluloses are reviewed for the purpose of defining our current level of understanding about crystalline polymorphism in these materials. Emphasis is placed on observations from solid state 13C nuclear magnetic resonance (NMR), which first led to the postulate that most native, semicrystalline celluloses are composites of two crystalline allomorphs, labeled Ialpha and Ibeta. Historical background is presented, highlighting the structural controversies which mainly arose because different native celluloses were used, each one representing a different mixture of allomorphs. Input from Raman, infrared (IR) and electron diffraction data is included in the discussion of our current understanding of polymorphism in native celluloses. Also noted is the input from more recently studied celluloses (e.g., Halocynthia) as well as from newer processes that convert the Ialpha to the Ibeta form. On the basis of Raman and IR observations, it is argued that the Ialpha and Ibeta allomorphs differ in hydrogen bonding patterns only and that backbone conformations are nearly identical. Also, the point is made that the absence of correlation field splittings in the Raman spectra calls into question (although it does not disprove) whether the normal two-chain-per-unit-cell, monoclinic Ibeta allomorph really possesses two equivalent chains. Considerable discussion is devoted to the allomorphic composition of cellulose crystallites in higher plants. Published methods of NMR lineshape analysis for the higher plant celluloses are reviewed and critiqued, both from the point of view of lineshape theory and from the point of view of self-consistency of inferences that are based on lineshape analyses for different carbons (particularly C1 and C4). It is concluded that higher plant celluloses most likely possess a minor amount of the Ialpha allomorph where the Ialpha/Ibeta ratio is probably less than 0.25.     

Improvement of resolution in solid state NMR spectra with J-decoupling: an analysis of lineshape contributions in uniformly 13C-enriched amino acids and proteins

In magic angle spinning (MAS) NMR spectra of highly and uniformly 13C,15N-enriched amino acids and proteins, homo-nuclear coupling interactions contribute significantly to the 13C linewidths, particularly for moderate applied magnetic field strengths and sample spinning frequencies. In this work, we attempted to dissect, analyze, and control the contributions of J-coupling and residual homo-nuclear dipolar coupling interactions to the linewidths of uniformly 13C,15N-enriched crystalline alanine; these studies were carried out at 9.4 T using a range of spinning frequencies from 5 to 15 kHz. The anisotropic second-order dipolar shifts and the J-splittings are comparable in their contribution to the linewidths, but behave very differently in terms of experimental protocols for line narrowing. In contrast to the J-coupling interactions, the second-order dipolar broadening cannot be refocused using selective pulses on the passively coupled spin. We carried out experiments to remove or refocus the 13C J-coupling interactions (omega1 J-decoupling) using a selective DANTE pulse in the center of the indirect evolution period. Inversion profiles and bandwidths of selective DANTE pulses acting on transverse magnetization, in the regime of moderate spinning frequencies, were characterized computationally and experimentally. A dramatic improvement in the resolution of the 2D spectrum was achieved when this decoupling protocol was employed.     

Improvement of resolution in solid state NMR spectra with J-decoupling: an analysis of lineshape contributions in uniformly C-enriched amino acids and proteins

In magic angle spinning (MAS) NMR spectra of highly and uniformly 13C,15N-enriched amino acids and proteins, homo-nuclear coupling interactions contribute significantly to the 13C linewidths, particularly for moderate applied magnetic field strengths and sample spinning frequencies. In this work, we attempted to dissect, analyze, and control the contributions of J-coupling and residual homo-nuclear dipolar coupling interactions to the linewidths of uniformly 13C,15N-enriched crystalline alanine; these studies were carried out at 9.4 T using a range of spinning frequencies from 5 to 15 kHz. The anisotropic second-order dipolar shifts and the J-splittings are comparable in their contribution to the linewidths, but behave very differently in terms of experimental protocols for line narrowing. In contrast to the J-coupling interactions, the second-order dipolar broadening cannot be refocused using selective pulses on the passively coupled spin. We carried out experiments to remove or refocus the 13C J-coupling interactions (omega1 J-decoupling) using a selective DANTE pulse in the center of the indirect evolution period. Inversion profiles and bandwidths of selective DANTE pulses acting on transverse magnetization, in the regime of moderate spinning frequencies, were characterized computationally and experimentally. A dramatic improvement in the resolution of the 2D spectrum was achieved when this decoupling protocol was employed.     

Structural characterization of the thermal transformation of halloysite by solid state NMR

Structural changes on thermal decomposition of the clay mineral halloysite (Al₂Si₂O₇·4H₂O) with progressive heating up to 1400°C have been followed by a combination of²⁹Si and²⁷Al solid state NMR, X-ray diffraction and transmission electron microscopy. Thermal transformations closely follow those previously outlined for the related mineral kaolinite. Clear evidence is presented here that the major exotherm at 1000°C is principally associated with the formation of γ-Al₂O₃ and results in phase separation of alumina-rich and silicarich regions on a scale of <5 nm. On further heating initially an extremely alumina-rich mullite forms. This progressively reacts with further silica, increasing the silica-content of the mullite, finally producing 3: 2 mullite with the residual silica crystallising as cristobalite.     

Dipolar recoupling in solid state NMR by phase alternating pulse sequences

We describe some new developments in the methodology of making heteronuclear and homonuclear recoupling experiments in solid state NMR insensitive to rf-inhomogeneity by phase alternating the irradiation on the spin system every rotor period. By incorporating delays of half rotor periods in the pulse sequences, these phase alternating experiments can be made γ encoded. The proposed methodology is conceptually different from the standard methods of making recoupling experiments robust by the use of ramps and adiabatic pulses in the recoupling periods. We show how the concept of phase alternation can be incorporated in the design of homonuclear recoupling experiments that are both insensitive to chemical shift dispersion and rf-inhomogeneity.     

Formation of lactose-mannitol molecular alloys by solid state vitrification

In this paper, we report the possibility to form glassy molecular alloys (α-lactose)_1−x(mannitol)_x for x<0.5 by co-milling two crystalline powders of pure α-lactose and pure mannitol β. The results have been established by differential scanning calorimetry and by powder X-ray diffraction. The concentration dependence of the glass transition temperature is found to obey the Gordon Taylor rule expected for regular solutions. It is also shown that the milling of pure mannitol β (x=1) leads to a polymorphic transformation towards the metastable form α of mannitol.     

Preparation of protein nanocrystals and their characterization by solid state NMR

Preparation of proteins in their crystalline state has been found to be important in producing stable therapeutic protein formulations, cross-linked enzyme crystals for application in industrial processes, generating novel porous media for separations, and of course in structure elucidation. Of these applications only X-ray crystallography requires large crystals, defined here as being crystals 100s of microns or greater in size. Smaller crystals have attractive attributes in many instances, and are just as useful in structure determination by solid state NMR (ssNMR) as are large crystals. In this paper we outline a simple set of procedures for preparing nanocrystalline protein samples for ssNMR or other applications and describe the characterization of their crystallinity by ssNMR and X-ray powder diffraction. The approach is demonstrated in application to five different proteins: ubiquitin, lysozyme, ribonuclease A, streptavidin, and cytochrome c. In all instances the nanocrystals produced are found to be highly crystalline as judged by natural abundance 13C ssNMR and optical and electron microscopy. We show for ubiquitin that nanocrystals prepared by rapid batch crystallization yield equivalent 13C ssNMR spectra to those of larger X-ray diffraction quality crystals. Single crystal and powder X-ray diffraction measurements are made to compare the degree of order present in polycrystalline, nanocrystalline, and lyophilized ubiquitin. Solid state 13C NMR is also used to show that ubiquitin nanocrystals are thermally robust, giving no indication of loss of local order after repeated temperature cycling between liquid nitrogen and room temperature. The methods developed are rapid and should scale well from the tenths of milligram to multi-gram scales, and as such should find wide utility in the preparation of protein nanocrystals for applications in catalysis, separations, and especially in sample preparation for structural studies using ssNMR.     

Fluorine-19 solid state NMR study of vinylidenefluoride polymers using selective relaxation filters

Two fluoropolymers, poly(vinylidenefluoride) (PVDF) and a vinylidenefluoride telomer (VDFT), with molecular weights of 1 x 10(6) and 2 x 10(3) Da by GPC, respectively, have been analysed by 19F solid-state nuclear magnetic resonance (NMR) spectroscopy. Relaxation-filtered proton-decoupled magic-angle spinning (MAS) experiments, namely T1rho filter, dipolar filter (DF), direct-polarisation delayed acquisition (DPDA) and discrimination induced by variable-amplitude minipulses (DIVAM), allowed signals in the direct polarisation (DP) spectra of PVDF and the VDFT to be discussed in terms of rigid and mobile domains. Both samples showed signals, which were multi-componential, but they differ in the nature of the crystalline form present. Thus, the Vinylidenefluoride (VDF) telomer exhibited a crystalline component corresponding to beta PVDF, whereas the PVDF contained crystallites of the alpha form. Signals relating to end groups and reverse units, plus an anomalous signal displaying long-time transverse relaxation in the DPDA spectrum, were found for both polymers, though they showed diversity in chemical shift and content. Signals related to reverse units and/or end groups were seen between approximately -115 and approximately -117 ppm for both samples. High-speed MAS at higher magnetic field resulted in an increase in resolution so that signals previously attributed to single-phase characteristics are shown to indicate the possibility of several different mobilities. The results are debated with respect to molecular weight and relaxation parameters.     

Solid state 19F NMR parameters of fluorine-labeled amino acids. Part II: aliphatic substituents

A representative set of amino acids with aliphatic 19F-labels has been characterized here, following up our previous compilation of NMR parameters for single 19F-substituents on aromatic side chains. Their isotropic chemical shifts, chemical shift tensor parameters, intra-molecular 19F dipole-dipole couplings and temperature-dependent T1 and T2 relaxation times were determined by solid state NMR on twelve polycrystalline amino acid samples, and the corresponding isotropic 19F chemical shifts and scalar couplings were obtained in solution. Of particular interest are amino acids carrying a trifluoromethyl-group, because not only the 19F chemical shift but also the intra-CF3 homonuclear dipolar coupling can be used for structural studies of 19F-labeled peptides and proteins. The CF3-groups are further compared with CH2F-, CD2F-, and CD3-groups, using both 19F and 2H NMR to describe their motional behavior and to examine the respective linebroadening effects of the protonated and deuterated neighbors. We have also characterized two unnatural amino acids in which a CF3-label is rigidly connected to the backbone by a phenyl or bicyclopentyl moiety, and which are particularly well suited for structure analysis of membrane-bound polypeptides. The 19F NMR parameters of the polycrystalline amino acids are compared with data from the correspondingly labeled side chains in synthetic peptides.     

NMR study of solid state reaction in Co-Sn multilayers

Amorphization by solid state reaction at room temperature in Co-Sn multilayers with periodicities of 65, 130 and 195 Å and relative Co to Sn ratio varying as 4, 3, 2 has been studied by zero field NMR method. The distribution of⁵⁹Co hyperfine fields and that of NMR enhancement factors were used as a probe of the ferromagnetic part of a sample and its evolution with the annealing time. The critical role of the interface during the first hours of annealing is pointed out. On the long time scale the diffusion process is slowed down by the creation of Kirkendall voids and after 3 months of annealing some crystalline Co is still present.     

Solid state 19F NMR parameters of fluorine-labeled amino acids. Part I: aromatic substituents

Structural parameters of peptides and proteins in biomembranes can be directly measured by solid state NMR of selectively labeled amino acids. The 19F nucleus is a promising label to overcome the low sensitivity of 2H, 13C or 15N, and to serve as a background-free reporter group in biological compounds. To make the advantages of solid state 19F NMR fully available for structural studies of polypeptides, we have systematically measured the chemical shift anisotropies and relaxation properties of the most relevant aromatic and aliphatic 19F-labeled amino acids. In this first part of two consecutive contributions, six different 19F-substituents on representative aromatic side chains were characterized as polycrystalline powders by static and MAS experiments. The data are also compared with results on the same amino acids incorporated in synthetic peptides. The spectra show a wide variety of lineshapes, from which the principal values of the CSA tensors were extracted. In addition, temperature-dependent T(1) and T(2) relaxation times were determined by 19F NMR in the solid state, and isotropic chemical shifts and scalar couplings were obtained in solution.     

The equation of state of solid molecular hydrogen and deuterium

Isochores of solid H₂ and D₂ have been measured to 2 kbar. Our results lead to reevaluation of the 4.2 K isotherm of Anderson and Swenson to 25 kbar. The equation of state has been determined, including ortho-para dependence. The pre-melting phase transition cited in recent russian literature has not been detected.     

Two-dimensional J-resolved and SUPERCOSY NMR experiments in the solid state.

Two 2D experiments, novel to solid-state NMR, are demonstrated using the trimethylsilyl ester of cubic octameric silicate (Q8M8). J-Resolved 13C NMR with BLEW-12 proton homonuclear decoupling reveals the scalar 13C-1H couplings and 29Si SUPERCOSY the connectivities of silicon atoms in the distorted Q8 cube.     

Sensitivity enhancement in structural measurements by solid state NMR through pulsed spin locking

Free induction decay (FID) signals in solid state NMR measurements performed with magic angle spinning can often be extended in time by factors on the order of 10 by a simple pulsed spin locking technique. The sensitivity of a structural measurement in which the structural information is contained in the dependence of the integrated FID amplitude on a preceding evolution period can therefore be enhanced substantially by pulsed spin locking in the signal detection period. We demonstrate sensitivity enhancements in a variety of solid state NMR techniques that are applicable to selectively isotopically labeled samples, including 13C-15N rotational echo double resonance (REDOR), 13C-13C dipolar recoupling measurements using the constant-time finite-pulse radio-frequency-driven recoupling (fpRFDR-CT) and constant-time double-quantum-filtered dipolar recoupling (CTDQFD) techniques, and torsion angle measurements using the double quantum chemical shift anisotropy (DQCSA) technique. Further, we demonstrate that the structural information in the solid state NMR data is not distorted by pulsed spin locking in the detection period.