Solid-state 15N Research Articles

The interaction of helical polypeptides with biological model membranes

Proton-decoupled solid-state 15N NMR spectroscopy was used to investigate helical peptides reconstituted into oriented phospholipid bilayers. Hydrophobic channel peptides such as the N-terminal region of Vpu of human immunodeficiency virus (HIV-1) adopt transmembrane orientations, whereas amphipathic peptide antibiotics are oriented parallel to the bilayer surface. The alignment of helical peptides in lipid membranes was analysed in some detail using model peptides. In particular, peptides with pH-dependent topology and a series of peptides that allow one to study the contributions of specific interactions were designed. The energies of transfer of several amino acids from the in-plane to transmembrane localisation were determined. In addition, the alignment of peptides and phospholipids under conditions of hydrophobic mismatch have been investigated in considerable detail.

Relation between ductility and segmental mobility of nylon-6

High-resolution solid-state 15N and 13C NMR and dynamic mechanical measurements were carried out for solution grown crystals of the α- and γ-forms of nylon-6 to understand the relation between segmental mobility including interchain interactions and ductility of polyamides. The ductility, in the temperature range of 100–180 °C, was lower for the α-crystals than for γ-crystals. 15N chemical shift revealed that the hydrogen bonding was stronger for the γ-crystals than for α-crystals. In addition, lower mobility of NH group in the γ-crystals than in α-crystals was shown by 15 NT 1 results. The results suggest that the ductility of nylon-6 could not be simply related to the strength of hydrogen bonding. Highly crystalline films of aggregates of solution grown α-crystals and γ-crystals showed no crystalline relaxation in the temperature dependence of dynamic mechanical loss factor, suggesting an existence of strong intermolecular interactions in the crystalline regions although 13 CT 1 data indicated that the mobility of methylene units was higher in the γ-crystals than in α-crystals. Information on the large-scale chain dynamics in the crystalline regions might be necessary to understand the low ductility of polyamides.

Solid state 15N and 13C NMR study of dioxomolybdenum (VI) complexes of Schiff bases derived from trans-1,2-cyclohexanediamine

The 13C, 15N CP MAS NMR and FT-IR spectra of dioxomolybdenum (VI) complexes of trans- N, N′-bis-(R-salicylidene)-1,2-cyclohexanediamine (R=H, R=3,5-diCl, R=3,5-diBr, R=4,6-diOCH 3), trans- N, N′-bis-(2-OH-naphthylidene)-1,2-cyclohexanediamine and trans- N-(salicylidene)- N′-(2-OH-naphthylidene)-1,2-cyclohexanediamine have been measured. Comparative analysis of the NMR and IR spectra of the complexes with those of the corresponding ligands has shown that the complexation of the di-Schiff bases leads to changes in the conformation of the ligands and the charge redistribution. The asymmetric structure and non-planar structure of the complexes have been suggested.

An Enzymatically Synthesized Polyaniline: A Solid-State NMR Study

The variation in molecular and electronic structure of doped (as synthesized) conducting, dedoped base, and redoped conducting forms of polyaniline (PANI) prepared by oxidative enzymatic polymerization of aniline with and without the presence of polyelectrolyte template was analyzed by solid-state 13C and 15N CP/MAS NMR spectroscopy. Polyanilines were enzymatically synthesized using a natural biocatalyst such as horseradish peroxidase (HRP) at pH 4.3 and a synthetic biomimetic catalyst, specifically poly(ethylene glycol)-coupled hematin (PEG-hematin), at pH 1.0. The HRP-catalyzed enzymatic polymerization of PANI was carried out in the presence of macromolecular polyelectrolyte templates such as poly(vinylphosphonic acid) (PVP), poly(4-styerenesulfonate) (SPS), and dodecylbenzenesulfonic acid (DBSA). 13C and 15N CP/MAS NMR spectral features indicate that the PANI obtained by PEG-hematin-catalyzed enzymatic polymerization resembles most with chemically synthesized analogues in its doped (as synthesized) con...

Solid state 15N and 13C NMR study of dioxomolybdenum (VI) complexes of Schiff bases derived from trans-1,2-cyclohexanediamine

The 13C, 15N CP MAS NMR and FT-IR spectra of dioxomolybdenum (VI) complexes of trans-N,N′-bis-(R-salicylidene)-1,2-cyclohexanediamine (R=H, R=3,5-diCl, R=3,5-diBr, R=4,6-diOCH3), trans-N,N′-bis-(2-OH-naphthylidene)-1,2-cyclohexanediamine and trans-N-(salicylidene)-N′-(2-OH-naphthylidene)-1,2-cyclohexanediamine have been measured. Comparative analysis of the NMR and IR spectra of the complexes with those of the corresponding ligands has shown that the complexation of the di-Schiff bases leads to changes in the conformation of the ligands and the charge redistribution. The asymmetric structure and non-planar structure of the complexes have been suggested.

Highly resolved spin-density distribution in the Prussian-blue precursors Cs2K[Fe(CN)6] and Cs2K[Mn(CN)6

In magnetic materials of the Prussian-blue type, what proportion of the unpaired electrons in the simple hexacyanometalate moves from the metal (red) to the carbon (gray) and nitrogen (green) atoms? Based on the solid-state 13C and 15N NMR spectra, the amount, the sign, and the shape of the spin densities can be distinguished for axial and equatorial CN ligands. There is surprisingly little spin on the N atoms.

15N NMR Chemical Shift Tensors of Substituted Hexaazaisowurtzitanes: The Intermediates in the Synthesis of CL-20

Several cage-substituted hexaazaisowurtzitanes [2,4,6,8,10,12-hexabenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (HBIW); 2,6,8,12-tetraacetyl-4,10-dibenzyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADB); and 2,6,8,12-tetraacetyl-4,10-diformyl-2,4,6,8,10,12-hexaazaisowurtzitane (TADF)] have been synthesized as intermediate steps in the synthesis of CL-20. The 15N NMR isotropic and chemical shift tensor principal values have been obtained for these intermediates using CP/MAS and FIREMAT methods. Solid-state 15N NMR offers unique advantages in the study of these materials, due to solubility issues and lack of high-quality X-ray crystallographic data. In addition, the caged structures possess unique electronic properties that can be useful in further refining theoretical methods. The effects of the different functional groups on the chemical shift tensor principal values of these caged amines have been evaluated and are shown to be reflected in the total anisotropy. The position of the amine within the cage is manifested in the δ22 values. Nitrogen-15 NMR shielding calculations using the embedded ion method (EIM) to account for long-range intermolecular interactions show a good correlation, rmsd = 4 ppm, with the observed chemical shift tensor values.

15N-CPMAS nuclear magnetic resonance spectroscopy and biological stability of soil organic nitrogen in whole soil and particle-size fractions

Soil organic nitrogen was quantified by solid-state 15N cross-polarization nuclear magnetic resonance spectroscopy (NMR) during a 14-month laboratory incubation of a sandy loam soil amended with 15N-clover. In whole soil and particle-size fractions, the clover-derived N was always 85–90% amide, 5–10% guanidinium N of arginine, and 5% amino. Quantitativeness of these results was suggested by (1) analysis of a standard containing a complex mixture of organic 15N and (2) correlation of spectral intensities with 15N concentrations. Based on the unchanging proteinaceous NMR signature of clover-derived N throughout the incubation, differences in the mineralization/immobilization kinetics of clover-N among the different particle-size fractions appeared not to be linked to organic functional group. Kinetic analysis of the mineralization of 15N, with correction of rate constants for field temperatures, suggested that the proteinanceous 15N in the clay and fine silt fractions observed here had a mean residence time of 7 years in the field.

Investigation of structural and dynamic properties of NH 4[N(CN) 2] by means of X-ray and neutron powder diffraction as well as vibrational and solid-state NMR spectroscopy

The crystal structure, spectroscopic and thermal properties of ammonium dicyanamide NH 4[N(CN) 2] have been thoroughly investigated by means of temperature-dependent single-crystal X-ray and neutron powder diffraction, vibrational and MAS-NMR spectroscopy as well as thermoanalytical measurements. The comprehensive elucidation of structural details is of special interest with respect to the unique solid-state transformation of ammonium dicyanamide into dicyandiamide. This reaction occurs at temperatures >80°C and it represents the isolobal analogue of Wöhler's historic transformation of ammonium cyanate into urea. NH 4[N(CN) 2] crystallizes in the monoclinic space group P2 1/ c with lattice constants a=3.7913(8), b=12.412(2), c=9.113(2) Å, β=91.49(2)° and Z=4 (single-crystal X-ray data, T=200 K). The temperature dependence of the lattice constants shows anisotropic behavior, however, no evidence for phase transitions in the investigated temperature range was observed. The hydrogen positions could be localized by neutron diffraction (10–370 K), and the temperature-dependent behavior of the ammonium group has been analyzed by Rietveld refinements using anisotropic thermal displacement parameters. They were interpreted by utilizing a rigid body model and extracting the libration and translation matrices of the ammonium ion by applying the TLS formalism. The results obtained by the diffraction methods were confirmed and supplemented by vibrational spectroscopy and solid-state 15N and 13C MAS-NMR investigations.

Binding of trinitrotoluene (TNT) to water extractable humus

Solid-state 15N NMR was applied to the aqueous extracts of a 13C-enriched plant slurry (Lolium perenne), anaerobically incubated with 15N3-trinitrotoluene (TNT). Almost all 15N3-TNT transformation products became covalently bound to the plant-derived organic material extractable with water. DCPMAS 15N 13C NMR revealed a three-step reaction scheme. After reduction of TNT, the aryl amines are acetylated. Subsequent alkylation of the resulting amides strengthens the incorporation of TNT-transformation products into humic material. Comparable results have been recently obtained under aerobic conditions, which indicates that this pathway is a common process during biological TNT transformation.

Effect of N content and soil texture on the decomposition of organic matter in forest soils as revealed by solid-state CPMAS NMR spectroscopy

N has a controlling effect on litter biodegradation in the forest floor, while stabilization of organic matter in the mineral soil may be influenced by physical parameters related to soil texture. In this study, in order to understand the processes involved in soil organic matter (SOM) formation, the chemical composition of SOM was followed and evaluated with regards to N contents and soil texture. Samples were taken on sites covered with Norway spruce and displaying contrasting values of C/N ratios in the forest floor. The chemical structure of OM was characterized using solid-state CPMAS 13C and 15N nuclear magnetic resonance (NMR) spectroscopy, along with Proton Spin Relaxation Editing (PSRE) sequences. Four groups of sampling sites were defined based on the NMR spectra of Oh and A horizons. In each group displaying similar NMR characteristics, N content and soil texture could be highly different among sites. Some Oh horizons with similar NMR spectra had very different N contents. Highly humified OM in Oh horizons were observed mainly on sites with low N contents. Some A horizons with different soil texture displayed similar OM chemical structure. High contents of O-alkyl C in some A horizons could originate from higher fresh root material input.

Isomorphism of bis(diethyldithiocarbamato)zinc(II) adduct with pyridine, [Zn(Py)(EDtc) 2]: hysteresis in the reaction of the adduct formation

The structure of the polycrystalline adduct bis(diethyldithiocarbamato)-pyridine zinc(II) depends on the pathway of physico-chemical conditions during the preparation procedure, as was revealed by solid state 15N CP/MAS spectroscopy in good correlation with known single crystal X-ray diffraction structures of this adduct. Two isomorphs of the adduct, namely α-[Zn(Py)(S 2CNEt 2) 2] and β-[Zn(Py)(S 2CNEt 2) 2], are the two molecules in the asymmetric unit of a single crystal (or polycrystalline) sample that can be obtained by recrystallization from toluene of the equimolar solution of the initial diethyldithiocarbamate zinc(II) complex and pyridine. The third isomorph, γ-[Zn(Py)(S 2CNEt 2) 2], can be obtained by recrystallization from pure pyridine of the diethyldithiocarbamate zinc(II) complex, or by its equimolar absorption of pyridine, or by desorption of pyridine from the clathrated adduct, [Zn(Py)(S 2CNEt 2) 2]·Py. Finally, the γ-[Zn(Py)(S 2CNEt 2) 2] isomorph recrystallizes from the melt into α/β-isomorphs of the adduct.

Room‐Temperature Ammonia Formation from Dinitrogen on a Reduced Mesoporous Titanium Oxide Surface with Metallic Properties.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Syntheses and characterization of aqua-bridged dimetallic complexes, M 2(μ-H 2O)(μ-OAc) 2(Im) 4(OAc) 2 (M=Mg 2+, Mn 2+ and Ni 2+) : Structural models for the active sites of dimetallic hydrolases

Four novel aqua-bridged dinuclear complexes with a formula M 2(μ-H 2O)(μ-OAc) 2(Im) 4(OAc) 2 (where Im=imidazole, M=Mg 2+ 1, Mn 2+ 2, Ni 2+ 3 and Co 2+ 4) have been synthesized and characterized. Complexes 1, 2 and 3 have been characterized by X-ray crystallography. Two M 2+ ions are bridged by an aqua molecule and two carboxylate anion with M⋯M=3.635–3.777 Å, M–OH 2=2.109–2.246 Å and M–OH 2–M=114.4–119.0°, respectively. Each complex is further stabilized by two intramolecular hydrogen bonds between the hydrogens of the bridging aqua and the oxygens of the terminal monodentate acetates with a distance of O⋯O=2.6 Å. The terminal monodentate acetates display ‘reversed’ C–O distances, namely the C–O(free) distances are actually longer than the C–O(coordinating) distances. This abnormal geometry of a monodentate carboxylate would be caused by the strong ‘pulling effect’ on the terminal carboxylates by intra- and intermolecular hydrogen bonds. The O–H stretching vibration of the bridging water was identified at ca. 2328 cm −1 in IR spectra based on the deuterium isotope shift. The solid state 13C and 15N NMR spectra of 1 displayed two sets of peaks for acetate and Im ligands, respectively, consistent with the presence of two types of coordination modes of acetate and the two symmetrically non-equivalent Im as revealed by X-ray structure. 15N chemical shift of NH in Im ligands underwent about 6 ppm downfield shift due to its involvement in an intermolecular hydrogen bond.

XPS and 15N NMR Study of Nitrogen Forms in Carbonaceous Solids

A combination of XPS and solid-state 15N NMR have been used to characterize the nitrogen forms in a variety of different carbonaceous samples having high natural nitrogen abundance. It is currently difficult to unequivocally interpret and quantify individual 15N NMR and XPS spectra. The advantages of using a multiple technique approach for nitrogen speciation as well as the limitation of XPS nitrogen (1s) and 15N NMR spectroscopy are discussed. XPS and 15N NMR results show that pyridinic and pyrrolic nitrogen are present in quinoline and isoquinoline pyrolysis chars. Pyridinic nitrogen is the most abundant form in quinoline pitch while isoquinoline pitch produced the most pyrrolic nitrogen. A small amount of quaternary nitrogen (∼10 mol %) appeared in the XPS spectrum of the chars, however, no additional nitrogen forms were identified above the noise level in the 15N NMR spectra. The 15N NMR spectrum of Green River kerogen shows a single broad feature centered around −245 ppm in the 15N NMR, consistent wi...

15N Chemical Shifts in Energetic Materials: CP/MAS and ab Initio Studies of Aminonitropyridines, Aminonitropyrimidines, and Their N-Oxides

Solid state 15N NMR chemical shift measurements have been performed on a series of nitro- and amino-substituted nitrogen-containing heterocycles that are of interest as potential new insensitive explosives. Due to low solubilities, many of these compounds are not amenable to study by solution state methods. Theoretical calculations of 15N chemical shift parameters have been performed on the structures of interest and are reported herein. The calculated and experimental values are in good agreement. The use of a model that includes intermolecular effects and allows the proton positions of the nearest neighbors to be optimized leads to the best agreement between calculated and experimental values. The theoretical models accurately predict the effects of nitro and amino substituents on ringnitrogen chemical shifts, explaining a seeming reversal in trend that is noted in the pyridine and pyridine-1-oxide chemical shifts of the highly substituted compounds.

High resolution solid state NMR investigation of various boron nitride preceramic polymers

Hexagonal boron nitride (h-BN) ceramics can be prepared from polymeric precursors by heat treatment under inert atmosphere. The purpose of this paper is to illustrate the strategy that can be followed to characterize by 15N and 11B solid state NMR the polymers as well as their pyrolysis derivatives. A suitable choice of NMR techniques, combined with isotopic enrichment, can lead to a good qualitative as well as quantitative description of the local environment of B and N sites in such complex samples.

Change of nitrogen functionality of 15N-enriched condensation products during pyrolysis

Changes in the nitrogen functionality of 15N-enriched condensation products prepared from glucose and 15N-glycine were investigated during pyrolysis at 600–1000 °C. The structural changes in the condensation products were studied by means of solid-state 13C and 15N NMR spectroscopies. During pyrolysis, the aliphatic moieties of the condensation products decomposed and evolved as gas and tar. At pyrolysis temperatures above 600 °C, almost all the carbon in the chars were converted to aromatic carbon. After pyrolysis, large amounts of nitrogen remained in the chars as char nitrogen (char-N), and about 30% of the nitrogen was eliminated from the chars as HCN and NH 3. With increasing temperature, the production of HCN and NH 3 increased and the amount of char-N decreased. By combining X-ray photoelectron spectroscopy and NMR results, detailed results for nitrogen fractions in chars were obtained. During pyrolysis, the fraction of unsubstituted pyrrole-N decreased and the fraction of quaternary-N increased. The fraction of pyridine-N remained almost constant at temperatures below 800 °C, but at 900 °C and above, the fraction of pyridine-N decreased. The fraction of substituted pyrrole-N showed minimum at 800 °C. On the basis of these results, structural changes of nitrogen functional groups during pyrolysis are discussed.

Observation of the glycines in elastin using (13)C and (15)N solid-state NMR spectroscopy and isotopic labeling.

We report the solid-state 13C and 15N NMR of insoluble elastin which has been synthesized in vitro with isotopically enriched glycine. Most of the glycines reside in a domain with good cross-polarization (CP) efficiencies, although surprisingly, a portion resides in an environment that is not detectable using CP. Our data indicate that much of the 13C population resides in regions of significant conformational flexibility. To support these conclusions, we present 13C and 15N cross-polarization with magic-angle-spinning (CPMAS) data in conjunction with "direct-polarization", nonspinning CP, and T1 measurements.

Syntheses, crystal structures and antimicrobial activities of polymeric silver(i) complexes with three amino-acids [aspartic acid (H2asp), glycine (Hgly) and asparagine (Hasn)]Note: For ease of reference during discussion of their anions, H2asp, Hgly and Hasn have been used as the abbreviations for the neutral amino-acids, rather than the conventional Asp, Gly and Asn, respectively.

As model compounds for silver(I)–protein interactions, silver(I) complexes with three amino-acid ligands, i.e. aspartic acid (H2asp), glycine (Hgly) and asparagine (Hasn), were prepared, characterized by elemental analysis, FTIR, TG/DTA, solution (1H and 13C) and solid-state (13C and 15N) NMR, and X-ray crystallography. Such complexes showed a wide spectrum of effective antimicrobial activities against Gram-negative (E. coli, P. aeruginosa) and -positive (B. subtilis and S. aureus) bacteria, yeasts (C. albicans and S. cerevisiae) and more than 11 tested molds. The crystal structures of four silver(I) complexes, {[Ag2(D-Hasp)(L-Hasp)]·1.5H2O}n1, {[Ag(gly)]2·H2O}n2, [Ag(L-asn)]n3 and [Ag(D-asn)]n4, were determined. The bonding modes of the silver(I) center were different for each complex, and different to those of the two recently reported silver(I) histidinates with only Ag–N bonds, i.e. water-soluble powder [{[Ag(Hhis)]·0.2EtOH}2]n6 and water-insoluble crystals [Ag(Hhis)]n7. Silver(I) complexes formed by amino-acids with N and O donor atoms and without an S atom can be classified into four types (I–IV) based on the bonding modes of the silver(I) center. Complex 1 belongs to type I which contains only Ag–O bonds, complex 2 belongs to type II, in which the two-coordinate O–Ag–O and N–Ag–N bonding units are alternately repeated, complexes 3 and 4 belong to type III, in which the two-coordinate N–Ag–O bonding units are repeated, and complexes that contain only Ag–N bonds, as recently found in the two silver(I) histidinates 6 and 7, belong to type IV.