15N NMR Chemical Shifts Research Articles

Tautomeric equilibrium of pyridoxine in water. Thermodynamic characterization by 13C and 15N nuclear magnetic resonance

A remarkable temperature dependence on the 13C NMR and 15N NMR chemical shifts of pyridoxine in water (pH = 7.0) has been observed. C-3, C-6, and N-1 were the most sensitive nuclei to the temperature effect. This dependence has been explained on the basis of an equilibrium shift thermally induced between the neutral and the dipolar form of this molecule. The thermodynamic characterization of tautomeric equilibria that interconvert quickly on the NMR time scale can be carried out from the observed average 13C NMR and 15N NMR chemical shifts at different temperatures (5-90 degrees C). We have developed a new method for the estimation of the thermodynamic parameters of a given equilibrium by fitting the experimental data to a theoretical curve. This new method allows us to improve the fitting results on our previously proposed methodology. We show that there are linear correlations between the average chemical shifts obtained from different nuclei at the same temperature. This indicates that the parameters of the pure forms are related among them. We have carried out a simultaneous multiple function curve fitting of all data obtained from the most sensitive signals together using these linear correlations as restricted conditions in order to diminish the number of independent parameters to fit. To test the new methodology, we have studied the thermodynamics of the tautomeric equilibrium of pyridoxine in water. We have obtained delta H degree values ranging from -23.6 +/- 1.3 to -25.8 +/- 1.7 kJ/mol for this equilibrium depending on the used data set. This kind of methodology has, among others, the following advantages: It allows the use of a great number of experimental points from different signals in the fitting process, it yields very precise and accurate values of the tautomeric process, and it allows the resolution of the problem with only 13C NMR data in some cases saving NMR time.

Heats of fusion of polynitro derivatives of polyazaisowurtzitane

Relationships were found between the heats of fusion Δ H m,tr of 12 nitramines with rigid molecules, on one hand, and their molecular weights or their lengths of the longest N–N bonds or 15N NMR chemical shifts of their amine and/or nitro group nitrogen atoms, on the other hand. On the basis of these relationships, the Δ H m,tr values have been predicted for 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (HNIW, Δ H m,tr = 42.70 kJ mol −1) and 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazaisowurtzitane (TEX, Δ H m,tr = 36.10 kJ mol −1). It was stated that the HNIW molecule contributes to the intermolecular potential in its molecular crystals mainly by its nitro groups at 2, 6, 8, and 12 positions. The N–N bond length of 0.1377 nm was also predicted for nitramino groupings in 1 and 5 positions of the 1,3,5-trinitro-1,3,5-triazacycloheptane molecule.

Derivatives of hydroxamic acids

Despite the biological importance and rich chemistry of hydroxamic acids (HAs), their organometallic derivatives have received little attention so far. Fully silylated (trimethylsilyl and t-butyldimethylsilyl) derivatives which are promising for medicinal applications have a structure derived from that of the tautomeric hydroximic acid with an E/Z ratio depending on the nature of the acid. Aliphatic acids yield disilyl derivatives in an approximate E/Z ratio of 2:3; aromatic HAs produce solely Z isomer irrespective of the phenyl ring substituent. Silylation of aliphatic dihydroxamic acids proceeds independently on both ends if the two hydroxamic groups are separated by one methylene group at least. It was not possible to determine the E/Z ratio for oxalic and malonic acid derivatives. It is shown that 29Si and 15N NMR chemical shifts are useful for differentiation between hydroxamic and hydroximic structures, but for determination of E and Z configurations spin–spin coupling must be used until more data relating 29Si, 13C and 15N chemical shifts to the configuration are available for ­this class of compounds. The dependence of chemical shifts on Hammett substituent constants in ring-substituted benzhydroxamic acids is discussed. Some errors found in the literature on hydroxamic acids are also explained. Copyright © 2000 John Wiley & Sons, Ltd.

Relationship between detonation characteristics and 15N NMR chemical shifts of nitramines

The paper presents 15N NMR chemical shifts δ of twenty three nitramines out of which 1-nitro-1-azaethylene (DIGEN), 1,3-dinitro-1,3-diazacyclobutane (TETROGEN) and 1,3,5,7,9-pentanitro-1,3,5,7,9-pentaazacyclodecane (DECAGEN) have not been synthesized yet. The corresponding δ values of these three substances have been predicted. The relationship has been confirmed between squares of detonation velocities or, as the case may be, the detonation heats and δ values of nitrogen atoms in nitro group of the nitramines. This relationship represents a certain form of Evans-Polanyi-Semenov equation and such it directly specifies the most reactive nitro group of nitramine molecule in the detonation and, hence, the N-NO2 bond primarily split in this process.

Weak Intermolecular Interactions and Molecular Recognition: Structure and Dynamics of the Benzene and Pyridine p-tert-Butylcalix[4]arene Inclusions

The host cavity of 1:1 host−guest compounds of p-tert-butylcalix[4]arene is well suited for the study of weak interactions in the solid state, as the motional freedom of the guests tests the weak intermolecular interactions in a very direct way. Benzene and pyridine are guests with a controlled number of similar (size, shape) as well contrasting (dipole moment, lone electron pair) properties that allow a meaningful comparative investigation of the structural and dynamic features. The 150 K single-crystal X-ray diffraction studies for the two host−guest compounds show that in both cases the guests occupy essentially the same orientation in the host cavity, with pyridine situated 0.11 Å deeper into the cavity than benzene. The complementarity of the diffraction and solid-state NMR techniques is illustrated, in particular, by incorporating the pyridine structural information obtained from 2H NMR studies into the diffraction data, thus resolving the ambiguity of the nitrogen atom position. Despite similar structural environments, the guests exhibit quite different dynamic behavior. Variable temperature 2H NMR spectra of the perdeuterated pyridine and benzene guests are interpreted in terms of specific motional models; benzene undergoes in-plane rotation followed by reorientation about the compound's 4-fold axis of symmetry. In contrast, pyridine reorients about the pyridine C2 molecular symmetry axis (rather than in-plane rotation), followed by guest reorientation about the compound's C4 axis of symmetry. A significant point is that the pyridine nitrogen has definite orientations in the cavity that cannot be explained by any specific directional electrostatic interactions between the host and guest. Both the dynamically averaged 15N NMR chemical shift tensor components and the absence of short contacts rule out a C−H···N hydrogen bonding interaction of the host to the guest. Despite the fact that the pyridine molecule is tightly docked in the host cavity, intermolecular interactions must be ascribed strictly to steric interactions acting in concert rather than specific directional interactions. Both guests are oriented in the host cavity such that the aromatic plane minimizes rather than maximizes contact with the host CH3 groups. This result questions the ability to ascribe structural features in the solid state to isolated directional interactions, such as the importance and role of CHhost···πguest interactions which have been suggested many times as having a stabilizing influence on this system.

15N NMR and electronic properties of S-nitrosothiols

Investigation of the 15N NMR of S-nitrosothiols showed that primary and tertiary RSNOs have distinct 15N chemical shifts around 730 and 790 ppm, respectively. Using 15N NMR technique, the equilibrium constant of NO transfer between SNAP and GSH was found to be 0.74. For primary RSNOs, linear relationships exist among 15N NMR chemical shifts, reduction potentials, and the p K as of their parent thiols.

Pilus chaperone FimC-adhesin FimH interactions mapped by TROSY-NMR.

The 23 kDa two-domain periplasmic chaperone FimC from Escherichia coli is required for the assembly of type-1 pili, which are filamentous, highly oligomeric protein complexes anchored to the outer bacterial membrane that mediate adhesion of pathogenic E. coli strains to host cell surfaces. Here we identified the contact sites on the surface of the NMR structure of FimC that are responsible for the binding of the 28 kDa mannose-binding type-1 pilus subunit FimH by 15N and 1H NMR chemical shift mapping, using transverse relaxation-optimized spectroscopy (TROSY). The FimH-binding surface of FimC is formed nearly entirely by the N-terminal domain, and its extent and shape indicate that FimC binds a folded form of the pilus subunits.

15N NMR study of isoquinoline alkaloids

15N NMR chemical shifts of 40 tertiary and quaternary isoquinoline alkaloids of six constitutional types are reported. The selected compounds belong to the classes of benzo[c]phenanthridine, protoberberine, benzylisoquinoline, aporphine, pavinane, and β-carboline-proaporphine alkaloids. The 15N chemical shifts were measured by gradient-enhanced inverse-detected single-quantum (GSQMBC) and multiple-quantum (GHMBC) multiple-bond correlation experiments. The 1H–15N long-range coupling constants determined from the phase-sensitive GSQMBC spectra are also reported in order to facilitate optimization of multiple-bond correlation experiments. Copyright © 1999 John Wiley & Sons, Ltd.

15N NMR study of isoquinoline alkaloids

15N NMR chemical shifts of 40 tertiary and quaternary isoquinoline alkaloids of six constitutional types are reported. The selected compounds belong to the classes of benzo[c]phenanthridine, protoberberine, benzylisoquinoline, aporphine, pavinane, and β-carboline-proaporphine alkaloids. The 15N chemical shifts were measured by gradient-enhanced inverse-detected single-quantum (GSQMBC) and multiple-quantum (GHMBC) multiple-bond correlation experiments. The 1H–15N long-range coupling constants determined from the phase-sensitive GSQMBC spectra are also reported in order to facilitate optimization of multiple-bond correlation experiments. Copyright © 1999 John Wiley & Sons, Ltd.

Synthesis and characterization of a series of novel glutamic gamma-15N-anilide dipeptides.

The preparation of a series of novel Cbz-Gln-Gly dipeptide derivatives is reported, wherein the gamma-carboxamide groups of the glutamine side chains have been modified to gamma-15N-anilides which are substituted in the para position with -NO2, -Cl, -H, -CH3, -OCH3, and -N(CH3)2. Characterization of the free anilines (p(kappa)a values and 15N NMR chemical shifts) and corresponding gamma-anilides (15N NMR chemical shifts and FTIR wavenumbers) is also reported. Correlation of these physicochemical data to Hammett substituent parameters ((sigma)para) is discussed. These novel dipeptide derivatives should prove to be generally useful for structure-function enzymology studies of gamma-glutamyl transferring enzymes.

E/Z-Isomerism of semicyclic amidinium salts

1H and 15N NMR chemical shifts of 12 semicyclic amidinium salts are reported. Their E/Z equilibria in solution were analyzed by 1D and 2D 1H and 15N NMR techniques. In the solid state the Z -configuration is observed. © 1999 John Wiley & Sons, Ltd.

1H, 13C, 15N NMR and Theoretical Study of Protonated Carbamic Acids and Related Compounds1

Mono-O-protonated carbamic acid, mono-O-protonated N-methyl carbamate, and mono-O-protonated methyl carbamate and di-O-protonated N,N-bis(carboxyl)-1,2-diaminoethane were prepared in FSO3H/SO2ClF and FSO3H:SbF5/SO2ClF at −78 °C and were characterized by 1H, 13C, and 15N NMR spectroscopy. Persistent diprotonated carbamic acid, diprotonated N-methyl carbamate, and diprotonated methyl carbamate were not observed under these conditions. The structures, energies, and 13C and 15N NMR chemical shifts of mono-, and diprotonated carbamic acids as well as other protonated species were also calculated by ab initio/IGLO/GIAO-MP2 method and compared with the experimental results.

13C and15N NMR chemical shift assignments ofN-1-(2- azidoethyl)-4-R-pyrimidin-2-ones by1H,X HMQ(B)C withz-gradient selection

13C and 15N NMR chemical shift assignments based on z-gradient selected 1H,X (X=13C and 15N) HMQC and HMBC experiments are reported for N-1-(2-azidoethyl)pyrimidin-2-one (ring system of cytosine), its five 4-R derivatives [where R=NH2, OCH3, N(CH2)4, NHCH2CH(CH3)2 and N(CH3)2] and 2-azidoethyl tosylate. The possibilities of detecting all nitrogens in these molecules containing (i) an azido group at N-1 and (ii) an electronegative substituent at C-4 are limited. First, the terminal nitrogen of the azido group is difficult to observe because the nearest proton (in a CH2 group) is located four bonds away from it. Second, in contrast to N-1, N-3 in N-1-(2-azido-ethyl)-4-pyrimidin-2-ones remained undetected. For that reason, an unsubstituted derivative (R=H) was also prepared, where N-3 was easily observed. © 1998 John Wiley & Sons, Ltd.

Determination of the mutual orientation of the 15N and 13C NMR chemical shift tensors of 13- 15N double labeled model peptides for silk fibroin from the dipolar-coupled powder patterns

The 15N and 13C chemical shift tensors, and the orientation of the principal axis system relative to the molecular symmetry axes were determined for 15N and 13C carbonyl carbon sites of 13C 15N double labeled model peptides for Bombyx mori silk fibroin, that is, Boc-[1- 13C]Ala[ 15N]Gly-OMe, Boc-[1- 13C]Ala[ 15N]GlyAlaGly-OPac, Boc-AlaGly[1- 13C]Ala[ 15N]GlyAlaGly-OPac, Boc-[1- 13C]Gly[ 15N]AlaGlyAla-OPac, Boc-GlyAla[1- 13C]Gly[ 15N]AlaGlyAla-OPac and Boc-[1- 13C]Gly[ 15N]ValGlyAla-OPac, where Boc is t-butoxycarbonyl, OMe is methyl ester, OPac is phenacyl ester, Ala is alanine, Gly is glycine and Val is valine. From the comparisons of the 15N chemical shift tensors and the orientations of the principal axis system relative to the molecular symmetry axes among three compounds having [1- 13C]Ala[ 15N]Gly units, it is concluded that the intermolecular interactions such as hydrogen bonding are different between Boc-[1- 13C]Ala[ 15N]Gly-OMe and two compounds, Boc-[1- 13C]Ala[ 15N]GlyAlaGly-OPac and Boc-AlaGly[1- 13C]Ala[ 15N]GlyAlaGly-OPac although the latter two compounds have similar structures. A similar conclusion has also been obtained from the 13C chemical shift tensors of these compounds.

NMR Investigations of Clostridium pasteurianum Rubredoxin. Origin of Hyperfine 1H, 2H, 13C, and 15N NMR Chemical Shifts in Iron−Sulfur Proteins As Determined by Comparison of Experimental Data with Hybrid Density Functional Calculations

High-level, all-electron, density functional calculations on a 104-atom model (B3LYP/6-311G** level) have been used, in conjunction with high-resolution X-ray structural data, to predict the remarkable paramagnetic contact shifts recently measured for 1H, 2H, 13C, and 15N nuclei in Clostridium pasteurianum rubredoxin. Three published X-ray structures for the Fe(III) rubredoxin from C. pasteurianum were employed to construct a 104-atom model for the iron center that included all atoms shown to have strong electronic interactions with the Fe. Each of these models served as a starting point for quantum mechanical calculations at level B3LYP/6-311G**, which, in turn, yielded calculated values for Fermi contact spin densities. The results indicate that the experimental hyperfine shifts are dominated by Fermi contact interactions: calculated Fermi contact spin densities were found to correlate linearly with isotropic hyperfine 1H, 2H, 13C, and 15N NMR chemical shifts determined for Fe(III) rubredoxin. At the c...

Ab initio prediction of 15N‐NMR chemical shift in α‐boron nitride based on an analysis of connectivities

Hydrogen-saturated cut-outs of hexagonal boron nitride have been used to model the solid state. Model compounds have been geometry optimized by means of density functional theory, whereas chemical shift calculations have been carried out at the coupled-perturbed Hartree–Fock level of theory employing gauge-including atomic orbital (GIAO) basis sets. The reliability of results has been tested against experimental values for chemical shifts in stable molecules with similar structural elements. With increasing cluster size, viz. a vanishing influence of the saturating hydrogens on the innermost nitrogen atoms, we find a convergence of 15N chemical shifts. A classification scheme for the chemical environment of a nitrogen atom has been set up according to its bonding graph including the second coordination sphere. For a given connectivity, chemical shifts vary within a few parts per million, thus enabling us to predict a 15N-NMR chemical shift of −285 ± 5 ppm for solid α-boron nitride. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 716–725, 1998

Ab initio prediction of15N-NMR chemical shift in ?-boron nitride based on an analysis of connectivities

Hydrogen-saturated cut-outs of hexagonal boron nitride have been used to model the solid state. Model compounds have been geometry optimized by means of density functional theory, whereas chemical shift calculations have been carried out at the coupled-perturbed Hartree–Fock level of theory employing gauge-including atomic orbital (GIAO) basis sets. The reliability of results has been tested against experimental values for chemical shifts in stable molecules with similar structural elements. With increasing cluster size, viz. a vanishing influence of the saturating hydrogens on the innermost nitrogen atoms, we find a convergence of 15N chemical shifts. A classification scheme for the chemical environment of a nitrogen atom has been set up according to its bonding graph including the second coordination sphere. For a given connectivity, chemical shifts vary within a few parts per million, thus enabling us to predict a 15N-NMR chemical shift of −285 ± 5 ppm for solid α-boron nitride. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 716–725, 1998

Modeling NMR chemical shifts: a comparison of charge models for solid state effects on 15N chemical shift tensors

This paper presents results from applying different point charge models to take into account intermolecular interactions to model the solid state effects on the 15N NMR chemical shifts tensors. The DFT approach with the BLYP gradient corrected exchange correlation functional has been used because it can include electron correlation effects at a reasonable cost and is able to reproduce 15N NMR chemical shifts with reasonable accuracy. The results obtained with the point charge models are compared with the experimental data and with results obtained using the cluster model, which includes explicitly neighboring molecular fragments. The results show that the point charge models can take into account solid state effects at a cost much lower than the cluster methods.

Three-center intramolecular hydrogen bonding in oxamide derivatives. NMR and X-ray diffraction study

This contribution describes the synthesis and structural investigation of the symmetric and non-symmetric oxamides N,N′-bis(2-hydroxyphenyl)oxamide 1, N,N′-bis(5-tert-butyl-2-hydroxyphenyl)oxamide 2, N,N′-bis(3,5-dimethyl-2-hydroxyphenyl)oxamide 3, N,N′-bis(2-hydroxybenzyl)oxamide 4, N,N′-diphenethyloxamide 5, N-(2-hydroxyphenyl)-N′-(2-methoxyphenyl)oxamide 6, N-(2-hydroxyphenyl)-N′-phenethyloxamide 7, (1S,2R)-(–)-N-(2-hydroxyphenylcarbamoylcarbonyl)norephedrine 8, (1R,2S)-(–)-N-(2-hydroxyphenylcarbamoylcarbonyl) 9, ethyl N-(2-hydroxyphenyl)oxalamate 10 and ethyl N-(2-methoxyphenyl)oxalamate 11. The structures were established by 1H, 13C, 15N and variable temperature NMR spectroscopy. Compounds 1–4 and 6–11 are stabilized by intramolecular three-center hydrogen bonding between the amide proton and two oxygen atoms. The 1H NMR Δδ/ΔT value of the amide proton correlates with the 15N NMR chemical shift. The X-ray diffraction molecular structures of 1 and 11 showed a planar conformation with trans configuration in the solid state, corresponding to the preferred conformation found in solution.

ねじれ型アミドの合成，構造，反応

We have found amides of which C (O) -N bonds are extremely twisted. The structures were elucidated by 13C NMR, IR and UV spectroscopies, and X-ray analysis. Since the twisted amides are highly reactive compared to the planar ones, these were used for the acylation of hydroxy groups under neutral conditions. In the reaction of diols containing primary and secondary hydroxy groups with the amides, the primary one was selectively acylated. For diols containing alcoholic and phenolic hydroxy groups, the acylation selectively gave the corresponding alkyl esters. Furthermore, axially chiral twisted amides served as asymmetric acylating agents for sec-alcohols. We also investigated the effects of the C (O) -N bond rotation of the amides on the 13C, 15N, and 17O NMR chemical shifts, and IR carbonyl absorption. As the twist angle increased, the δ13C, δ17O and vC=o values increased, whereas, the δ15N value decreased. It was also found that the 13C (O) -15N coupling constants depend on the twist angles.The 1JC (O) , N value decreased with increasing twist angles.