Conformation,ordering and mobility of macromolecules in aromatic polyesters as studied by IR and solid-state NMR spectroscopy

Thermotropic main-chain LC polyesters have been studied with the aid of IR and NMR spectroscopy. Rigid mesogens and mesogens with a possibility of conformational isomerism were examined. The dependence of the order parameters of the mesogens and spacers on the chemical structure of the polymer has been determined. The IR spectroscopy data and the data of the calculation of the possible conformations of the mesogen and the spacer confirm the theoretical predictions about the straightening of macromolecules in the anisotropic melt. Solid-state ²H NMR spectra show that p-oxybenzoic rings adjacent to a flexible spacer are more mobile than inner terphthalic rings. In decamethylene spacer the contrary holds true: the mobility of inner methylene groups is more marked than that of the α-methylene groups bound to the mesogen.     

Weak molecular complexes as studied by NMR spectroscopy of oriented molecules

Weak molecular interactions such as those in pyridine—iodine, benzene—iodine and benzene—chloroform systems oriented in thermotropic liquid crystals have been studied from the changes of the order parameters as a result of complex formation. The results indicate the formation of at least two types of charge transfer complexes in pyridine—iodine solutions. The pi-complexes in benzene—chloroform and benzene—iodine mixtures have also been detected. No detectable changes in the inter-proton distances in these systems were observed.     

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.     

Effects of antidepressants on the conformation of phospholipid headgroups studied by solid-state NMR

The effect of tricyclic antidepressants (TCA) on phospholipid bilayer structure and dynamics was studied to provide insight into the mechanism of TCA-induced intracellular accumulation of lipids (known as lipidosis). Specifically we asked if the lipid-TCA interaction was TCA or lipid specific and if such physical interactions could contribute to lipidosis. These interactions were probed in multilamellar vesicles and mechanically oriented bilayers of mixed phosphatidylcholine-phosphatidylglycerol (PC-PG) phospholipids using (31)P and (14)N solid-state NMR techniques. Changes in bilayer architecture in the presence of TCAs were observed to be dependent on the TCA's effective charge and steric constraints. The results further show that desipramine and imipramine evoke distinguishable changes on the membrane surface, particularly on the headgroup order, conformation and dynamics of phospholipids. Desipramine increases the disorder of the choline site at the phosphatidylcholine headgroup while leaving the conformation and dynamics of the phosphate region largely unchanged. Incorporation of imipramine changes both lipid headgroup conformation and dynamics. Our results suggest that a correlation between TCA-induced changes in bilayer architecture and the ability of these compounds to induce lipidosis is, however, not straightforward as imipramine was shown to induce more dramatic changes in bilayer conformation and dynamics than desipramine. The use of (14)N as a probe was instrumental in arriving at the presented conclusions.     

Selective oxidation of methanol over supported vanadium oxide catalysts as studied by solid-state NMR spectroscopy

Methanol catalytic oxidation over VO_x/Al₂O₃, VO_x/ZrO₂ and VO_x/MgO catalysts has been studied by solid-state nuclear magnetic resonance (NMR) spectroscopy. It was found that stronger acid sites in VO_x/Al₂O₃ result in almost the same selectivities for dimethoxymethane, paraformaldehyde and formic acid, and weaker acid sites in VO_x/ZrO₂ favor the formation of paraformaldehyde, while the VO_x/MgO catalyst with the base support shows high selectivity for formate. Supporting VO_x species on γ-Al₂O₃ and ZrO₂ leads to the formation of Brønsted acid sites as revealed by the adsorption of probe molecules. The acid strength of Brønsted acid sites on the VO_x/Al₂O₃ catalyst is found to be stronger than that of the VO_x/ZrO₂ catalyst which has the acid strength similar to zeolite HZSM-5's. The proposed bridging hydroxyl models accounting for the Brønsted acid sites formation were also confirmed by quantum chemical calculation.     

Microphase separation in RIM polyureas as studied by solid-state NMR

The microphase separation (MPS) in polyureas based on methylene diphenyl diisocyanate (MDI) hard segment, diethyltoluenediamine chain extender, and amino-terminated polypropylene glycol soft segment prepared by reaction injection molding (RIM) was studied by advanced solid-state NMR spectroscopy. Incomplete microphase separation leads to the presence of mobilized hard segments dispersed in the soft segment domains as well as immobilized soft segments residing in the hard domains. This is detected by ¹H-NMR spectra recorded under spinning at the magic angle (MAS) as well as two-dimensional wide-line separation (WISE) NMR spectra. The sizes of the various domains as well as the interfaces between them are quantified by spin diffusion measurements. In this way the impact of annealing, method of polymerization, and hard segment content on MPS is studied. Whereas annealing at temperatures up to 170°C results in improving the MPS, major changes are observed after annealing at higher temperatures (190°C), where the system changes from “soft-in-hard” to “hard-in-soft” behavior. The MPS decreases with increasing hard segment content. The highest MPS is observed for solution polymerized samples. The various NMR experiments clearly reveal the nonequilibrium nature of RIM systems. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 693–703, 1998     

Arrangement and mobility of water in vermiculite hydrates followed by 1H NMR spectroscopy

The arrangement of water molecules in one- and two-layer hydrates of high-charged vermiculites, saturated with alkaline (Li(+), Na(+)) and alkali-earth (Mg(2+), Ca(2+), Ba(2+)) cations, has been analyzed with (1)H NMR spectroscopy. Two different orientations for water molecules have been found, depending on the hydration state and the sites occupied by interlayer cations. As the amount of water increases, hydrogen bond interactions between water molecules increase at expenses of water-silicate interactions. This interaction favors water mobility in vermiculites. A comparison of the temperature dependence of relaxation times T(1) and T(2) for one and two-layer hydrates of Na-vermiculite shows that the rotations of water molecules around C(2)-axes and that of cation hydration shells around the c-axis is favored in the two-layer hydrate. In both hydrates, the anisotropic diffusion of water takes place at room temperature, preserving the orientation of water molecules relative to the silicate layers. Information obtained by NMR spectroscopy is compatible with that deduced by infrared spectroscopy and with structural studies carried out with X-ray and neutron diffraction techniques on single-crystals of vermiculite.     

The crystalline structures of ethylene-dimethylaminoethyl methacrylate copolymers as studied by solid-state high-resolution 13C NMR spectroscopy

The crystalline structures of ethylene-dimethylaminoethyl methacrylate (EDAM) copolymers, which were either melt-quenched (mq) or isothermally crystallized (iso), were studied by solid-state high-resolution ¹³C NMR spectroscopy. It revealed that the crystalline structures of EDAM copolymers are greatly dependent on the comonomer content, crystallization condition and the storage time after treatment. The ratio of monoclinic to orthorhombic crystal (M/O) increases with the increase in the dimethylaminoethyl methacrylate content. Higher crystallinity and lower monoclinic content were observed for iso samples compared to the mq ones. The monoclinic crystal was found to melt at lower temperatures compared to the orthorhombic one during the heating process. The degree of crystallinity as well as the contents of monoclinic and orthorhombic crystals and the M/O value are found to increase after storage at room temperature for a month.     

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).     

Dynamics of p-nitroaniline molecules in micoporous aluminophosphate AlPO4-5 studied by solid-state NMR

Dynamics of deuterated p-nitroaniline (pNA-d) molecules in the micropores of AlPO4-5 has been investigated by means of solid-state NMR. The adsorbed amounts of pNA-d were 5.0 and 10.1 mass % of the total mass. We have measured 13C magic-angle-spinning (MAS) and 2H NMR spectra of the guest molecules and 31P and 27Al MAS NMR spectra of the host framework. The pNA-d molecules distribute rather inhomogeneously in the channel, and do not coordinate to Al strongly like H2O. The intermolecular hydrogen bonds are formed between a part of the guest molecules only when the loading level is high. The 2H NMR spectra are successfully analyzed, elucidating the orientation and the motion of the guest molecules. The molecular axis of pNA-d is inclined to the channel axis, and the molecular plane is perpendicular to the inner wall. The guest molecule jumps among 12 sites or 12 orientations. This motion is faster in the sample of 5.0 mass % than in the sample of 10.1 mass %, suggesting that the guest-guest interaction hinders the motion. The mean residence times of the molecules are estimated from the analysis of the 2H NMR spectra, which are affected by the size of the nanospace as well as the property of the adsorbed site.     

Membrane insertion of a lipidated ras peptide studied by FTIR,solid-state NMR,and neutron diffraction spectroscopy

Membrane binding of a doubly lipid modified heptapeptide from the C-terminus of the human N-ras protein was studied by Fourier transform infrared, solid-state NMR, and neutron diffraction spectroscopy. The 16:0 peptide chains insert well into the 1,2-dimyristoyl-sn-glycero-3-phosphocholine phospholipid matrix. This is indicated by a common main phase transition temperature of 21.5 degrees C for both the lipid and peptide chains as revealed by FTIR measurements. Further, (2)H NMR reveals that peptide and lipid chains have approximately the same chain length in the liquid crystalline state. This is achieved by a much lower order parameter of the 16:0 peptide chains compared to the 14:0 phospholipid chains. Finally, proton/deuterium contrast variation of neutron diffraction experiments indicates that peptide chains are localized in the membrane interior analogous to the phospholipid chains. In agreement with this model of peptide chain insertion, the peptide part is localized at the lipid-water interface of the membrane. This is revealed by (1)H nuclear Overhauser enhancement spectra recorded under magic angle spinning conditions. Quantitative cross-peak analysis allows the examination of the average location of the peptide backbone and side chains with respect to the membrane. While the backbone shows the strongest cross-relaxation rates with the phospholipid glycerol, the hydrophobic side chains of the peptide insert deeper into the membrane interior. This is supported by neutron diffraction experiments that reveal a peptide distribution in the lipid-water interface of the membrane. Concurring with these experimental findings, the amide protons of the peptide show strong water exchange as seen in NMR and FTIR measurements. No indications for a hydrogen-bonded secondary structure of the peptide backbone are found. Therefore, membrane binding of the C-terminus of the N-ras protein is mainly due to lipid chain insertion but also supported by interactions between hydrophobic side chains and the lipid membrane. The peptide assumes a mobile and disordered conformation in the membrane. Since the C-terminus of the soluble part of the ras protein is also disordered, we hypothesize that our model for membrane binding of the ras peptide realistically describes the membrane binding of the lipidated C-terminus of the active ras protein.     

Interdiffusion of PMMA and PVF2, studied by solid-state NMR

A previously published new solid-state nuclear magnetic resonance (NMR) method is applied to the interdiffusion of poly(methacrylate) (PMMA) and poly(vinylidene fluoride) (PVF₂) above their T_g. Via a variation of the cross-polarization technique magnetization is transferred from protons to fluorines. When this magnetization is made to disappear at the fluorine sites, only those protons that are distant from fluorines greater than the distance over which cross-polarization functions will retain their magnetization. In this way we detect the fraction of PMMA near (ca. 20 Å) PVF₂. Starting from sheets of PMMA and PVF₂, which are then heated at 190°C for a variable time, and applying the above technique, we can determine the fractions of PMMA and PVF₂ that have diffused within a distance of a few Å of each other. The intrinsic diffusion coefficients of PMMA and PVF₂ determined from such experiments compare well with literature data. Initial attempts to fit the experimental data suggest that the concentration dependence of the diffusion coefficients cannot be neglected. © 1994 John Wiley & Sons, Inc.     

Acid sites and oxidation center in molybdena supported on tin oxide as studied by solid-state NMR spectroscopy and theoretical calculation

Solid-state NMR spectroscopy and density functional theory (DFT) calculations were employed to study the structure and properties, especially the solid acidity, of molybdenum oxide supported on tin oxide. As demonstrated by solid-state NMR experiments, Mo species are mainly dispersed on the surface of SnO(2) support rather than significantly dissolved into the SnO(2) structure and Br?nsted as well as Lewis acid sites are present on the MoO(3)/SnO(2) catalyst. Acid strength of the supported metal oxide is stronger than those of zeolites, e.g., HY and HZSM-5, though the concentration of acid sites is relatively lower. The DFT calculated (13)C chemical shift for acetone adsorbed on MoO(3)/SnO(2) is in good agreement with the experimental value, which confirms our proposed structure of -Mo-(OH)-Sn- for the Br?nsted acid site. Reducibility of the supported metal oxide is also demonstrated by solid-state NMR experiments and an active oxidation center of this catalyst is proposed as well.     

Conformations of banana-shaped molecules studied by 2H NMR spectroscopy in liquid crystalline solvents

ClPbis11BB and Pbis11BB, two banana-shaped mesogens differing by a chlorine substituent on the central phenyl ring, show a nematic and a B2 phase, respectively. To obtain information on the structural features responsible for their different mesomorphic behavior, a study of the preferred conformations of these mesogens has been performed by NMR spectroscopy in two nematic media (Phase IV and ZLI1167), which should mimic the environment of the molecules in their own mesophases, avoiding problems of sample alignment by a magnetic field. To this aim, 2H NMR experiments have been performed on selectively deuterated isotopomers of ClPbis11BB and Pbis11BB and of two parent molecules, ClPbisB and PbisB, assumed as models in previous theoretical and experimental conformational studies. We found that only a limited number of conformations is compatible with experimental data, often very different from those inferred from theoretical calculations in vacuo, indicating a strong influence of the liquid crystalline environment on molecular conformation. No significant differences between chlorinated and non-chlorinated molecules were found, this suggesting that chlorine does not change the molecular conformational equilibrium, as previously proposed.     

The magnetic properties of myoglobin as studied by NMR spectroscopy

Deoxymyoglobin has been investigated by NMR spectroscopy to determine the magnetic anisotropy through pseudocontact shifts and the total magnetic susceptibility through Evans measurements. The magnetic anisotropy values were found to be Deltachi(ax)=-2.03+/-0.08 x 10(-32) m(3) and Deltachi(rh)=-1.02+/-0.09 x 10(-32) m(3). The negative value of the axial susceptibility anisotropy originates from the z tensor axis lying in the heme plane, unlike all other heme systems investigated so far. This magnetic axis is almost exactly orthogonal to the axial histidine plane. The other two axes lie essentially in the histidine plane, the closest to the heme normal being tilted by about 36 degrees from it, towards pyrrole A on the side of the proximal histidine. From the comparison with cytochrome c' it clearly appears that the position of the one axis lying in the heme plane is related to the axial histidine orientation. Irrespective of the directions, the magnetic anisotropy is smaller than that of the analogous reduced cytochrome c' and of the order of that of low-spin iron(III). The magnetic anisotropy of the system permits the measurement of residual dipolar couplings, which, together with pseudocontact shifts, prove that the solution structure is very similar to that in the crystalline state. Magnetic measurements, at variance with previous data, demonstrate that there is an orbital contribution to the magnetic moment, micro(eff)=5.5 micro(B). Finally, from the magnetic anisotropy data, the hyperfine shifts of iron ligands could be separated in pseudocontact and contact components, and hints are provided to understand the spin-delocalisation mechanism in S=2 systems by keeping in mind the delocalisation patterns in low-spin S=1/2 and high-spin S= 5/2 iron(III) systems.     

Structure of neutral molecules and monoanions of selected oxopurines in aqueous solutions as studied by NMR spectroscopy and theoretical calculations

A methodology enabling investigation of a multicomponent tautomeric and acid-base equilibria by (13)C NMR spectroscopy supported by theoretical calculations has been proposed. The effectiveness of this method has been illustrated in a study of 2-oxopurine, 6-oxopurine (hypoxanthine), 8-oxopurine, and 2,6-dioxopurine (xanthine) in neutral and alkaline aqueous solutions. For each compound a series of (13)C NMR spectra were recorded at pH ranges in which neutral molecules, monoanions and/or dianions occurred in dynamic equilibrium. The carbon chemical shifts for these three forms of the investigated compounds were retrieved from the analysis of pH-dependence of the measured, dynamically averaged values of these parameters. The structures of several stable tautomers of the neutral and monoanionic oxopurine forms were predicted from theoretical calculations and nuclear magnetic shielding constants for (13)C nuclei in these tautomers were calculated. At both calculation steps (molecular geometry optimization and calculation of NMR parameters) the PBE1PBE/6-311++G(2d,p) level of theory was used. The populations of the most stable tautomers were determined from the experimental data analysis exploiting the fact that they were population-weighted averages of the chemical shifts of particular tautomers. It has been shown that only the oxo forms of the investigated oxopurines are present in aqueous solutions and that the determined populations in most cases remain in a qualitative agreement with the calculated free energies of the appropriate tautomers. The obtained results are in general agreement with other literature reports on oxopurine tautomerism and confirm importance of the hydration phenomena for the investigated systems. The data analysis has shown that the best compliance between theory and experiment is obtained when the hydration phenomenon is modeled by discrete hydration augmented by PCM (polarizable continuum solvation model).     

Native conformation at specific residues in recombinant inclusion body protein in whole cells determined with solid-state NMR spectroscopy

Inclusion bodies are insoluble aggregates that are formed by bacteria to store excess recombinant protein produced during expression. The structure of the protein in inclusion bodies is poorly understood but it has been hypothesized that the protein may form misfolded beta sheet aggregates. This paper presents an isotopic labeling and solid-state nuclear magnetic resonance approach to determine the secondary structure of individual residues within a recombinant influenza virus "FHA2" protein in inclusion bodies. The inclusion bodies were studied either in the context of the unlysed hydrated E. coli cells or in the hydrated pellet formed from centrifugation of the material insoluble in the cell lysate. The native structure of FHA2 is predominantly helical and native helical structure was also observed for several specific residues in the inclusion body FHA2. This approach will be applicable to structural analysis of many inclusion body proteins and should provide useful information for optimizing solubilization and purification protocols of these proteins.     

Cation distribution in co-doped ZnAl2O4 nanoparticles studied by X-ray photoelectron spectroscopy and 27Al solid-state NMR spectroscopy

Co(x)Zn(1-x)Al(2)O(4) (x = 0.01-0.6) nanoparticles were synthesized by the citrate sol-gel method and were characterized by X-ray powder diffraction and transmission electron microscopy to identify the crystalline phase and determine the particle size. X-ray photoelectron spectroscopy and (27)Al solid-state NMR spectroscopy were used to study the distribution of the cations in the tetrahedral and octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles as a function of particle size and composition. The results show that all of the as-synthesized samples exhibit spinel-type single phase; the crystallite size of the samples is about 20-50 nm and increases with increasing annealing temperature and decreases with Co-enrichment. Zn(2+) ions are located in large proportions in the tetrahedral sites and in small proportions in the octahedral sites in Co(x)Zn(1-x)Al(2)O(4) nanoparticles. The fraction of octahedral Zn(2+) increases with increasing Co concentration and decreases with increasing particle size. Besides the tetrahedral and octahedral coordinations, the presence of the second octahedrally coordinated Al(3+) ions is observed in the nanoparticles. The change of the inversion parameter (2 times the fraction of Al(3+) ions in tetrahedral sites) with Co concentration and particle size is consistent with that of the Zn fraction in octahedral sites. Analysis of the absorption properties indicates that Co(2+) ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The inversion degree of Co(2+) decreases with increasing particle size.