Conformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1H --> 19F CP/MAS NMR spectroscopy

The changes in the conformation and molecular mobility accompanied by a phase transition in the crystalline domain were analyzed for ethylene (E) and tetrafluoroethylene (TFE) copolymer, ETFE, using variable-temperature (VT) solid-state 19F magic angle spinning (MAS) and 1H --> 19F cross-polarization (CP)/MAS NMR spectroscopy. The shifts of the signals for fluorines in TFE units to higher frequency and the continuing decrease and increase in the T1rho(F) values suggest that conformational exchange motions exist in the crystalline domain between 42 and 145 degrees C. Quantum chemical calculations of magnetic shielding constants showed that the high-frequency shift of TFE units should be induced by trans to gauche conformational changes at the CH2-CF2 linkage in the E-TFE unit. Although the 19F signals of the crystalline domain are substantially overlapped with those of the amorphous domain at ambient probe temperature (68 degrees C), they were successfully distinguished by using the dipolar filter and spin-lock pulse sequences at 145 degrees C. The dipolar coupling constants for the crystalline domain, which can be estimated by fitting the dipolar oscillation behaviors in the 1H --> 19F CP curve, showed a significant decrease with increasing temperature from 42 to 145 degrees C. This is due to the averaging of 1H-19F dipolar interactions originating from the molecular motion in the crystalline domain. The increase in molecular mobility in the crystalline domain was clearly shown by VT T1rho(F) and 1H --> 19F CP measurements in the phase transition temperature range.     

Structure and molecular dynamics of solid-state inclusion complexes of cyclodextrin and permethylated cyclodextrin with benzaldehyde studied by high-resolution CP/MAS13C NMR

The structure and molecular dynamics of the benzaldehyde inclusion-complexes with -and-cyclodextrins and permethylated -cyclodextrin in the solid state have been studied by high-resolution cross-polarization/magic angle sample-spinning¹³C NMR spectroscopy. It is shown that the guest benzaldehyde molecule undergoes motion in the host cyclodextrin cavity and the rate of motion depend on the cavity size. In the -cyclodextrin complex, compared to -and permethylated -cyclodextrin complexes, the benzaldehyde motion is severely restricted, but under high-vacuum benzaldehyde is released more easily from the cavity.     

Structure of ultrahigh-molecular-weight polyethylene in the solid state as studied by variable-temperature 13C CP/MAS NMR spectroscopy

In order to obtain some insight into the structure of an ultrahigh-molecular-weight polyethylene sample, ¹³C CP/MAS NMR experiments have been carried out at temperatures from 23 to ?108°C. The peak for the crystalline component moves upfield with a decrease in temperature, which is contrary to what was reported previously for a melt-crystallized polyethylene sample. On the basis of x-ray diffraction results and quantum-chemical calculations, it is suggested that the methylene carbons are in the distorted orthorhombic form at low temperatures.     

Solid‐state 1H → 19F/19F → 1H CP/MAS NMR study of poly(vinylidene fluoride)

Solid‐state ¹H → ¹⁹F and ¹⁹F → ¹H cross‐polarization magic angle spinning (CP/MAS) NMR spectra have been investigated for a semicrystalline fluoropolymer, namely poly(vinylidene fluoride) (PVDF). The ¹H → ¹⁹F CP/MAS spectra can be fitted by five Lorentzian functions, and the amorphous peaks were selectively observed by the DIVAM CP pulse sequences. Solid‐state spin‐lock experiments showed significant differences in T_1ρ^F and T_1ρ^H between the crystalline and amorphous domains, and the effective time constants, T_HF* and T_1ρ*, which were estimated from the ¹H → ¹⁹F CP curves, also clarify the difference in the strengths of dipolar interactions. Heteronuclear dipolar oscillation behaviour is observed in both standard CP and ¹H → ¹⁹F inversion recovery CP (IRCP) experiments. The inverse ¹⁹F → ¹H CP‐MAS and ¹H → ¹⁹F CP‐drain MAS experiments gave complementary information to the standard ¹H → ¹⁹F CP/MAS spectra in a manner reported in our previous papers for other fluoropolymers. The value of N_F/N_H (where N is a spin density) estimated from the CP‐drain curve is within experimental error equal to unity, which is consistent with the chemical structure. Copyright © 2001 John Wiley & Sons, Ltd.     

1H NMR study of inclusion compounds of phenylurea derivatives in beta-cyclodextrin

Proton nuclear magnetic resonance spectroscopy ((1)H NMR), which has become an important tool for the study "in situ" of beta-cyclodextrin (beta-CD) complexes, was used to study and structurally characterize the inclusion complexes formed between beta-CD and isoproturon, fenuron, monuron and diuron. The high variation of the chemical shifts from the proton located inside the cavity (H-3, H-5 and H-6) coupled with the non variation of the one located outer sphere of the beta-CD (H-1, H-2 and H-4) provided clear evidence of the inclusion phenomena. Two-dimensional rotating frame Overhauser effect spectroscopy (ROESY) experiments were carried out to further support the proposed inclusion mode.     

Chemical bonding and molecular dynamics in inclusion compounds of fluorinated graphite according to 19F NMR spectroscopy data

In Inclusion compounds of fluorinated graphite with chlorine trifluoride C₂. xClF₃ and hexafluoro-benzene C₂F. xC₆F₆ , the guest molecules are characterized by rotational mobility and weak bonds with the host matrix. ¹⁹F NMR chemical shift tensors are determined for the fluorine nuclei of the matrix and the guest molecules, including the structurally nonequivalent fluorine atoms ofClFj molecules [δ (Fl) = −700, δ(F1) = −280; δ|| (F2) = −440, δ±(F₂) = −220ppm relative to F2]. It is shown that C-F bonds in the host matrix are close to those in aromatic fluorocarbons. Translated from Zhumal Stmktumoi Khimii, Vol. 41, No. 1, pp. 80-85, January–February, 2000.     

Supramolecular assembly of dendritic polymers elucidated by 1H and 13C solid-state MAS NMR spectroscopy

Advanced solid-state NMR methods under fast magic-angle spinning (MAS) are used to study the structure and dynamics of large supramolecular systems, which consist of a polymer backbone with dendritic side groups and self-assemble into a columnar structure. The NMR experiments are performed on as-synthesized samples, i.e., no isotopic enrichment is required. The analysis of (1)H NMR chemical-shift effects as well as dipolar (1)H-(1)H or (1)H-(13)C couplings provide site-specific insight into the local structure and the segmental dynamics, in particular, of phenyl rings and -CH(2)O- linking units within the dendrons. Relative changes of (1)H chemical shifts (of up to -3 ppm) serve as distance constraints and allow protons to be positioned relative to aromatic rings. Together with dipolar spinning sideband patterns, pi-pi packing phenomena and local order parameters (showing variations between 30% and 100%) are selectively and precisely determined, enabling the identification of the dendron cores as the structure-directing moieties within the supramolecular architecture. The study is carried out over a representative selection of systems which reflect characteristic differences, such as different polymer backbones, sizes of dendritic side groups, or length and flexibility of linking units. While the polymer backbone is found to have virtually no effect on the overall structure and properties, the systems are sensitively affected by changing the generation or the linkage of the dendrons. The results help to understand the self-assembly process of dendritic moieties and aid the chemical design of self-organizing molecular structures.     

Characterization of monofluorinated polycyclic aromatic compounds by 1H, 13C and 19F NMR spectroscopy

Monofluorinated polycyclic aromatic hydrocarbons (F-PAHs) have attracted much attention in analytical, environmental, toxicological and mechanistic studies because of their physico-chemical properties, which are closely similar to those of the parent PAHs. Because of this, full NMR characterization has become of interest. Complete 1H, 13C and 19F NMR chemical shifts, and also 1J(H,C), (n)J(C,F), (n)J(H,F) and (n)J(H,H) coupling constants, have been assigned for the F-PAHs 1-fluoronaphthalene, 2-fluorofluorene, 5-fluoroacenaphthylene, 2-fluorophenanthrene, 3-fluorophenanthrene, 3-fluorofluoranthene, 1-fluoropyrene, 1-fluorochrysene, 2-fluorochrysene, 3-fluorochrysene and 9-fluorobenzo[k]fluoranthene. To allow comparison with the corresponding parent PAHs, the 1H and 13C chemical shifts of acenaphthylene, phenanthrene, fluoranthene, pyrene and benzo[k]fluoranthene were determined. Chemical shift increments and the effects on the coupling constants from the fluorine substitution are discussed.     

Structure,dynamics and interactions in polymer systems as studied by NMR spectroscopy

The possibilities of NMR spectroscopy in studies of interactions in polymer systems are demonstrated on the example of two types of macromolecular complexes: (i) By measuring ¹H NMR high resolution line intensities, the formation of ordered associated structures of syndiotactic (s) poly(methyl methacrylate)(PMMA) in mixed solvents was quantitatively characterized. The obtained results permit us to assume that the mechanism by which the solvent affects self-association of s-PMMA involves specific interactions of the solvent molecules with PMMA units. Solid state high resolution ¹³C NMR spectra of associated s-PMMA gels were also measured and compared with the spectra of a solid s-PMMA sample. (ii) By using ¹³C solid state NMR spectroscopy, the differences in the structure of the amorphous and crystalline phases in pure poly(ethylene oxide) and its complexes with p-dichlorobenzene or p-nitrophenol were characterized. Prounounced differences also in the dynamic structure of the crystalline phase in these systems are indicated by the relaxation times T₁(C), T_1ρ(C) and T_1ρ(H).     

Interaction and morphology of polyethylenimine/DNA complexes as studied by solid-state NMR spectroscopy

Solid-state ¹H wide-line and ³¹P magic angle spinning NMR were applied to a series of PEI(polyethylenimine)/DNA complexes. The experimental results revealed that the higher the nitrogen/phosphorus (N/P) molar ratio is, the more phosphorus atoms of DNA are engaged in the electrostatic interaction with PEI. ¹H spin–diffusion experiments manifested that the aggregation degree of DNA in the complexes decreases greatly when N/P ratio increases from 0.5 to 3 and changes only slightly with further increase of N/P ratio, indicating that DNA disperses in the matrix of PEI on the molecular level at higher N/P ratio.     

Concentration of hydroxyl groups in dental apatites: a solid-state 1H MAS NMR study using inverse 31P -->1H cross-polarization

The concentration of structural hydroxyl groups in the apatite mineral of enamel, dentin and cementum of healthy human teeth was estimated by reference to stoichiometric hydroxyapatite to be 73 +/- 3, 18 +/- 2 and 18 +/- 1%, respectively.     

Characterization of dynamics of perdeuterated proteins by MAS solid-state NMR

We show in this communication that dynamic information for uniformly 2H,13C,15N isotopically enriched, crystalline proteins can be obtained by MAS solid-state NMR spectroscopy. The experiments make use of the deuterium quadrupolar tensor, which is the dominant interaction mechanism. Dynamic properties are accessed by measurement of the size of the quadrupolar coupling constant, Cq, and the value of the asymmetry parameter, eta, via evolution of the deuterium chemical shift, as well as by measurement of deuterium T1 relaxation times. Three-dimensional experiments are performed in order to obtain site-specific resolution. Due to proton dilution, no proton decoupling is required in the carbon evolution periods at MAS rotation frequencies of 10 kHz.     

Crystalline order in polyethylene: A 13C NMR CP/MAS solid-state T1 study

The carbon-13 spin-lattice relaxation times T₁ of the crystalline components of four solid ethylene-octene copolymers have been studied as a function of thermal history, branching number, and branching distribution. Slowly cooled samples (1 deg/min from melt to room temperature) exhibited similar or longer T₁s with respect to the same sample quench cooled (from the melt into 20°C water). The greater the degree of branching and the more homogeneous the branching distribution, the shorter were the observed crystal lattice T₁s. Differences of up to a factor of 3 in T₁ were observed for the same sample undergoing the two thermal treatments. Different degrees of branching homogeneity (for the same total number of branches) resulted in differences approaching a factor of 7 for samples with the same thermal history. These variations were attributed to the differing effects of side-chain disruption of the crystal lattice.     

13C CP/MAS NMR studies of vitamin E model compounds

13C cross-polarization magic angle spinning (CP/MAS) NMR data for 2,2,5,7,8-pentamethylchroman-6-ol (2), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox c) (3) and its acetate (4), 2-methoxy-2,2,5,7,8-pentamethylchroman-6-ol (5), 2-hydroxy-2,2,5,7,8-pentamethylchroman-6-ol (6) and 2,2,5,7,8-pentamethylchroman (7) are reported. A deshielding of 7.7 ppm for the carboxylic carbon was observed in solid Trolox due to formation of intermolecular hydrogen bonds within cyclic dimers. Such crystal packing permits effective cross-polarization and fast relaxation (short T1rho(H)). The impact of the proton concentration on the CP dynamics is reflected by the longer T(CP) and T1rhoH for Trolox-d2 (deuterated at mobile proton sites). The calculated GIAO RHF shielding constants are sensitive to intramolecular effects: rotation around the C-6-O bond (changes of sigma up to 8 ppm) and conformation at C-2.     

Structure calculation from unambiguous long-range amide and methyl 1H-1H distance restraints for a microcrystalline protein with MAS solid-state NMR spectroscopy

Magic-angle spinning (MAS) solid-state NMR becomes an increasingly important tool for the determination of structures of membrane proteins and amyloid fibrils. Extensive deuteration of the protein allows multidimensional experiments with exceptionally high sensitivity and resolution to be obtained. Here we present an experimental strategy to measure highly unambiguous spatial correlations for distances up to 13 ?. Two complementary three-dimensional experiments, or alternatively a four-dimensional experiment, yield highly unambiguous cross-peak assignments, which rely on four encoded chemical shift dimensions. Correlations to residual aliphatic protons are accessible via synchronous evolution of the (15)N and (13)C chemical shifts, which encode valuable amide-methyl distance restraints. On average, we obtain six restraints per residue. Importantly, 50% of all restraints correspond to long-range distances between residues i and j with |i - j| > 5, which are of particular importance in structure calculations. Using ARIA, we calculate a high-resolution structure for the microcrystalline 7.2 kDa α-spectrin SH3 domain with a backbone precision of ～1.1 ?.     

Characterization of amorphous carbon films by 13C CP MAS NMR spectroscopy

¹³C CP MAS NMR spectroscopy was used to characterize α-C:H materials generated from methane and hydrogen mixtures using a microwave plasma. Dipolar dephasing experiments indicate a range of T_dd making quantification of quaternary “diamond”-like carbons difficult. Unconstrained lineshape analysis is not suitable for the deconvolution of the NMR spectra, but linewidth constrained analyses gave reasonable results. © 1993 John Wiley & Sons, Inc.     

Nature of acidic protons in metallosilicate molecular sieves as studied by variable temperature 1H MAS NMR

The dynamic properties of protons in H-[Ga]-ZSM-5, H-[B]-ZSM-5 and H-[Al-B]-ZSM-5 were compared with that of protons in H-[Al]-ZSM-5 by temperature dependence of ¹H MAS NMR in the range of 298 k and 473 K. The temperature dependence of the line width of ¹H MAS NMR reveals that protons in H-[Ga]-ZSM-5 were more mobile than those in H-[Al]-ZSM-5 at temperature as low as 373 K. The protons in H-[B]-ZSM-5 were not mobile at 473 K and fixed in the zeolite frame work as the bridging hydroxyl groups, ≡B-OH-Si≡. The thermal motion of protons in ≡Al-OH-Si≡ was suppressed by introducing B³⁺ cations into the framework of H-[Al]-ZSM-5.     

The organization of aromatic side groups in an amyloid fibril probed by solid-state 2H and 19F NMR spectroscopy

Some 25 diseases are associated with proteins and peptides that assemble into amyloid fibrils composed of beta-strands connected by hydrogen bonds oriented parallel to the fiber long axis. There is mounting evidence that amyloid formation involves specific interactions between amino acid side groups, which bring together beta-sheets to form layers with buried and exposed faces. This work demonstrates how a combination of solid-state 2H and 19F NMR experiments can provide constraints on fibril architecture by probing the environment and spatial organisation of aromatic side groups. It is shown that phenylalanine rings within fibrils formed by a decapeptide fragment of the islet amyloid polypeptide, amylin, are highly motionally restrained and are situated within 6.5 A of one another. Taken together with existing structural constraints for this peptide, these results are consistent with a fibril architecture that comprises layers of two or more beta-sheets, with the aromatic residues facing into the inter-sheet space and possibly engaged in pi-pi interactions. The methods presented will be of general utility in exploring the architecture of fibrils of larger, full-length peptides and proteins, including amylin itself.