Complete 1H and 13C assignments of 8-C-beta-D-[2-O-(E)-p-coumaroyl]glucopyranosyl-2-(2-hydroxy)propyl-7-methoxy-5-methylchromone

1H and 13C NMR spectra of 8-C-beta-D-[2-O-(E)-p-coumaroyl] glucopyranosyl-2-(2-hydroxy)propyl-7-methoxy-5-methylchromone were completely assigned by 2D NMR observations. Especially the 1H assignments of the glucosyl and hydroxyl protons were achieved by utilizing HMQC, HMBC, 1H-1H COSY and DEPT techniques together with a heavy water exchange 1H NMR experiment.     

Structural and variable-temperature NMR studies of 9-diisopropylphosphanylanthracenes and 9,10-bis(diisopropylphosphanyl)anthracenes and their oxidation products

The diisopropylphosphanyl-substituted anthracenes i-Pr2P(C14H9) (1a), i-Pr2P(C14H8)Br (2a), and (i-Pr2P)2(C14H8) (3a) and some of their oxidation products were prepared from 9-bromoanthracene and 9,10-dibromoanthracene, respectively. Low-temperature (1)H NMR spectra of the 9-monophosphanyl-substituted anthracenes 1a and 2a are in accordance with a staggered conformer, while above room temperature dynamic processes occur. The low-temperature NMR spectrum of the 9,10-diphosphanylanthracene 3a indicates the presence of two different rotational isomers. The rotational barrier for 1a was determined from variable-temperature (1)H NMR spectra to be 56 kJ mol(-1) (DeltaG(298K)). The crystal structure determinations show the solid-state conformers to be consistent with the prevailing conformer at low temperature.     

Hartree-Fock, Møller-Plesset calculations and dynamic NMR study of 3,3-dimethoxy-1-(imidazolidin-2-ylidene)propan-2-one

The experimental (1)H, (13)C NMR spectra of 3,3-dimethoxy-1-(imidazolidin-2-ylidene)propan-2-one were recorded in CDCl(3) at temperature range 213-323 K. The variable temperature spectra revealed a dynamic NMR effect which is attributed to restricted rotation around the C=C double bond. Fast exchange processes of deuterium atoms between CDCl(3) and 3,3-dimethoxy-1-(imidazolidin-2-ylidene)propan-2-one or fast exchange of proton between nitrogen and oxygen atoms of carbonyl group is also revealed by broadening of N-H (singlet) proton NMR signals. Proton and carbon theoretical chemical shifts of the title molecule were calculated by using RHF and MP2-GIAO levels and different basis sets in gas phase at 298 K. The calculated proton chemical shifts show that the experimental values have no agreement with theoretical values, but for carbon chemical shifts a good agreement achieved by using RHF with 6-31G basis set and MP2/3-21G, 6-31G basis sets. Discrepancies are attributed to either the limitations of calculating program, because the change of the structure while rotation are not considered. The results showed that to select of basis set has more important rule, because RHF-GIAO level calculation with 6-31G basis set in gas phase can excellently reproduce the (13)C NMR spectrum. Moreover, MP2/3-21G, 6-31G calculation has not significant influence on (13)C NMR chemical shifts with respect to RHF-6-31G.     

Structure and dynamics of a dihydrogen/hydride ansa molybdenocene complex

In contrast to [Cp(2)MoH(3)](+), which is a thermally stable trihydride complex, the ansa-bridged analogue [(eta-C(5)H(4))(2)CMe(2)MoH(H(2))](+) (1) is a thermally labile dihydrogen/hydride complex. Partial deuteration of the hydride ligands allows observation of J(H)(-)(D) = 11.9 Hz in 1-d(1) and 9.9 Hz in 1-d(2) (245 K), indicative of a dihydrogen/hydride structure. There is a slight preference for deuterium to concentrate in the dihydrogen ligand. A rapid dynamic process interchanges the hydride and dihydrogen moieties in complex 1. Low temperature (1)H NMR spectra of 1 give a single hydride resonance, which broadens at very low temperature due to rapid dipole-dipole relaxation (T(1) = 23 ms (750 MHz, 175 K) for the hydride resonance in 1). Low temperature (1)H NMR spectra of 1-d(2) allow the observation of decoalescence at 180 K into two resonances. The bound dihydrogen ligand exhibits hindered rotation with DeltaG(150) = 7.4 kcal/mol, but H atom exchange is still rapid at all accessible temperatures (down to 130 K). Density functional calculations confirm the dihydrogen/hydride structure as the ground state for the molecule and give estimates for the energy of two hydrogen exchange processes in good agreement with experiment. The presence of the C ansa bridge is shown to decrease the ability of the metallocene fragment to donate to the hydrogens, thus stabilizing the (eta(2)-H(2)) unit and modulating the barrier to H(2) rotation.     

Interaction of hydronium ion with dibenzo-18-crown-6: NMR, IR, and theoretical study

Interaction of dibenzo-18-crown-6 (DBC) with H 3O (+) (HP) in nitrobenzene- d 5 and dichloromethane- d 2 was studied by using (1)H and (13)C NMR spectra and relaxations, FTIR spectra, and quantum chemical DFT calculations. NMR shows that the DBC*HP complex is in a dynamic equilibrium with the reactants, the equilibrium constant K being 0.66 x 10 (3), 1.16 x 10 (4), and 1.03 x 10 (4) L x mol (-1) in CD 2Cl 2, nitrobenzene, and acetonitrile, respectively. The complex appears to have a C 2 v symmetry in NMR, but FTIR combined with DFT normal mode calculations suggest that such high symmetry is only apparent and due to exchange averaging of the structure. FTIR spectra as well as energy-optimized DFT calculations show that the most stable state of the complex in solution is that with three linear hydrogen bonds of HP with one CH 2-O-CH 2 and two Ar-O-Ar oxygen atoms. The structure is similar to that found in solid state but adopts a somewhat different conformation in solution. The dynamics of exchange between bound and free DBC was studied by NMR transverse relaxation. It was found to be too fast to give reproducible results when measured with the ordinary CPMG sequence or its variant DIFTRE removing residual static dipolar interaction, but it could be established by rotating-frame measurements with high intensity of the spin-lock field. The correlation time of exchange was found to be 5.6 x 10 (-6) and 3.8 x 10 (-6) s in dichloromethane and nitrobenzene, respectively. Such fast exchange can be explained by cooperative assistance of present water molecules.     

Amide-Exchange-Rate-Edited NMR （AERE-NMR） Experiment： A Novel Method for Resolving Overlapping Resonances

This paper describes an amide-exchange-rate-edited （AERE） NMR method that can effectively alleviate the problem of resonance overlap for proteins and peptides. This method exploits the diversity of amide proton exchange rates and consists of two complementary experiments： （1） SEA （solvent exposed amide）-type NMR experiments to map exchangeable surface residues whose amides are not involved in hydrogen bonding, and （2） presat-type NMR experiments to map solvent inaccessibly buried residues or nonexchangeable residues located in hydrogen-bonded secondary structures with properly controlled saturation transfer via amide proton exchanges with the solvent. This method separates overlapping resonances in a spectrum into two complementary spectra. The AERE-NMR method was demonstrated with a sample of ^15N/^13C/^2H（70%） labeled ribosome-inactivating protein trichosanthin of 247 residues.     

Solid State Dynamics of Tricarbonyl(eta-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(eta-1,5-cycloheptadienylium)iron Tetrafluoroborate

The dynamic behavior of [(C(6)H(7))Fe(CO)(3)]BF(4) (I) and [(C(7)H(9))Fe(CO)(3)]BF(4) (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature (1)H and (13)C NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). (57)Fe M?ssbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from (1)H spin-lattice relaxation time measurements to be 17.5 kJ mol(-)(1). Static (13)C NMR measurements of the solid complexes in the high-temperature phase indicate that the (13)C chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) ?, c = 21.761(3) ?, V = 2449.7(5) ?(3), rho(calc) = 1.734 g cm(-)(3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.     

Sites of hydroxyl radical reaction with amino acids identified by (2)H NMR detection of induced (1)H/(2)H exchange.

Hydroxyl radical reacts with the aliphatic C-H bonds of amino acids by H atom abstraction. Under anaerobic conditions inclusion of a (2)H atom donor results in (1)H/(2)H exchange into these C-H bonds [Goshe et al. Biochemistry 2000, 39, 1761--1770]. The site of (1)H/(2)H exchange can be detected and quantified by (2)H NMR. Integration of the (2)H NMR resonances within a single spectrum permits the relative rate of H atom abstraction from each position to be determined. Analysis of the aliphatic amino acid spectra indicates that the methine and methylene positions were more reactive than the methyl positions. The (2)H NMR spectra of isoleucine and leucine show that H-atom abstraction distal to the alpha-carbon occurs preferentially. Significant (1)H/(2)H exchange was observed into the delta positions of proline and arginine and into the epsilon-methylene of lysine, indicating that a positive charge on a geminal N does not inhibit the (1)H/(2)H exchange. Comparisons of (2)H NMR integrations between amino acid spectra indicated that (1)H/(2)H exchange occurred in the following descending order: L > I > V > R > K > Y > P > H > F >M> T > A > [C, S, D, N, E, Q, G, W]. The extent of (1)H/(2)H exchange into methionine, N-glycyl-methionine, and methionine sulfoxide suggests that a prominent solvent exchange pathway involving hydroxyl radical mediated oxidation of methionine exists to account for the large (2)H incorporation into the gamma-methylene of methionine sulfoxide that is absent for N-glycyl-methionine. Analysis of the (1)H NMR spectra of the reactions with phenylalanine and tyrosine indicated that hydroxyl radical addition to the phenyl ring under the anaerobic reductive reaction conditions did not result in either exchange or hydroxylation.     

Investigations of polypeptide rotational diffusion in aligned membranes by 2H and 15N solid-state NMR spectroscopy

Transmembrane and in-plane oriented peptides have been prepared by solid-phase peptide synthesis, labeled with 3,3,3-2H3-alanine and 15N-leucine at two selected sites, and reconstituted into oriented phophatidylcholine membranes. Thereafter, proton-decoupled 15N and 2H solid-state NMR spectroscopy at sample orientations of the membrane normal parallel to the magnetic field direction have been used to characterize the tilt and rotational pitch angle of these peptides in some detail. In a second step the samples have been tilted by 90 degrees . In this setup the spectral line shapes are sensitive indicators of the rate of rotational diffusion. Whereas monomeric transmembrane peptides exhibit spectral averaging and well-defined resonances, larger complexes are characterized by broad spectral line shapes. In particular the deuterium line shape is sensitive to association of a few transmembrane helices. In contrast, the formation of much larger complexes affects the 15N chemical shift spectrum. The spectra indicate that in liquid crystalline membranes an amphipathic peptide of 14 amino acids exhibits fast rotational diffusion on both the 2H and 15N time scales (>10(-5) s). Extending the sequences to 26 amino acids results in pronounced changes of the 2H solid-state NMR spectrum, whereas the signal intensities of 15N solid-state NMR spectra degrade. Below the phase transition temperature of the phospholipid bilayers, motional averaging on the time scale of the 2H solid-state NMR spectrum ceases for transmembrane and in-plane oriented peptides. Furthermore at temperatures close to the phase transition the total signal intensities of the deuterium solid-state NMR spectra strongly decrease.     

Stereodynamics of N-isopropyl-N-methylpropargylamine. Dynamic NMR studies. Molecular mechanics calculations.

N-Isopropyl-N-methylpropargylamine (N-isopropyl-N-methyl-2-propyn-1-amine; IMPA) is chiral at the pyramidal nitrogen. Racemization occurs via an inversion-rotation process. Both 13C(1H) and 1H dynamic NMR (DNMR) spectra decoalesce in response to slowing inversion-rotation (delta G++ = 7.7 +/- 0.1 kcal/mol). While aspects of the DNMR spectra suggest the presence of minor conformations, the spectrum at 100 K shows a strong preference for one conformation. The NMR data suggest that the preferred conformation has both the isopropyl methine proton and the ethynyl group anti to the lone pair. Both isopropyl methyl groups are gauche to the lone pair. This conformational preference is in significant contrast to N-ethyl-N-methyl-2-aminopropane in which the population of that conformation having the ethyl methyl group and the isopropyl methine proton both anti to the lone pair is only 5% at 95 K. The NMR data, supported by molecular mechanics (MMX) calculations, suggest a special stabilization for the ethynyl group being oriented anti to the lone pair.     

Synthetic and spectral studies of some 4ƒ-metal complexes

Salicylaldehyde(N-benzoyl)glycyl hydrazone (SalBzGH) has been synthesized and characterized. Metal complexes of the empirical formula [Ln(SalBzGH-2H)OH(H₂O)₂] where Ln = Y, Pr, Nd, Sm, Eu, Gd and Dy, have been synthesized and their structures studied by infrared, far-infrared, absorption, emission and NMR spectra. ¹H and ¹³C NMR spectra indicate the presence of dynamic equilibrium between two isomers of SalBzGH over the 298–368 K temperature range and stabilization of a single isomer upon complexation with Ln(III). The hypersensitive band profile of the Nd(III) complex indicates a change in metal coordination number due to solvation, while an increase of temperature results in a decrease of ε_max of the hypersensitive band. The emission spectrum of [Eu(SalBzGH-2H)OH(H₂O)₂] at 77 K indicates a low site symmetry. SalBzGH coordinates through imidolic oxygen, azomethine nitrogen and phenolate oxygen.     

RUBIDIUM, a program for computer-aided assignment of two-dimensional NMR spectra of polypeptides.

Taking advantage of the rule-based expert system technology, a program named RUBIDIUM (Rule-Based Identification In 2D NMR Spectrum) was developed to accomplish the automatic 1H NMR resonance assignments of polypeptides. Besides noise elimination and peak selection capabilities, RUBIDIUM detects the cross-peak patterns of amino acid residues in the COSY spectrum, assigning these patterns to amino acid types, performing sequential assignments using combined COSY/NOESY spectra, and finally, achieving the total assignment of the 1H NMR spectrum.     

Cation exchange at the mineral-water interface: H3O+/K+ competition at the surface of nano-muscovite

This article describes a (39)K nuclear magnetic resonance (NMR) spectroscopic study of K (+) displacement at the muscovite/water interface as a function of aqueous phase pH. (39)K NMR spectra and T 2 relaxation data for nanocrystalline muscovite wet with a solid/solution weight ratio of 1 at pH 1, 3, and 5.5 show substantial liquid-like K (+) only at pH 1. At pH 3 and 5.5, all K (+) appears to be associated with muscovite as inner- or outer-sphere complexes, indicating that H 3O (+) does not displace basal surface K (+) beyond the (39)K detection limit under these conditions. In our pH 1 mixture, only approximately 1/3 of the initial basal surface K (+) population is located more than 3-4 A from the surface. (29)Si and (27)Al MAS NMR spectra and SEM images show no evidence of dissolution during the (39)K experiments, consistent with the liquid-like (39)K fraction originating from displaced basal surface K (+). Assuming no muscovite dissolution or interlayer exchange, the K (+)/H 3O (+) ratio relevant to the solution/surface exchange equilibrium is controlled by the total amount of K (+) on the surface and H 3O (+) in solution (K (+) surf/H 3O (+) aq). These parameters, in turn, depend on the basal surface area, solution pH, and the solid/solution ratio. The results here are consistent with significant displacement of surface K (+) only under conditions where the initial K (+) surf/H 3O (+) aq ratio is less than approximately 1. Computational molecular models of the muscovite/water interface should account for both K (+) and H 3O (+) in the near-surface region.     

Magic-angle spinning solid-state NMR spectroscopy of the beta1 immunoglobulin binding domain of protein G (GB1): 15N and 13C chemical shift assignments and conformational analysis

Magic-angle spinning solid-state NMR (SSNMR) studies of the beta1 immunoglobulin binding domain of protein G (GB1) are presented. Chemical shift correlation spectra at 11.7 T (500 MHz 1H frequency) were employed to identify signals specific to each amino acid residue type and to establish backbone connectivities. High sensitivity and resolution facilitated the detection and assignment of every 15N and 13C site, including the N-terminal (M1) 15NH3, the C-terminal (E56) 13C', and side-chain resonances from residues exhibiting fast-limit conformational exchange near room temperature. The assigned spectra lend novel insight into the structure and dynamics of microcrystalline GB1. Secondary isotropic chemical shifts report on conformation, enabling a detailed comparison of the microcrystalline state with the conformation of single crystals and the protein in solution; the consistency of backbone conformation in these three preparations is the best among proteins studied so far. Signal intensities and line widths vary as a function of amino acid position and temperature. High-resolution spectra are observed near room temperature (280 K) and at <180 K, whereas resolution and sensitivity greatly degrade substantially near 210 K; the magnitude of this effect is greatest among the side chains of residues at the intermolecular interface of the microcrystal lattice, which we attribute to intermediate-rate translational diffusion of solvent molecules near the glass transition. These features of GB1 will enable its use as an excellent model protein not only for SSNMR methods development but also for fundamental studies of protein thermodynamics in the solid state.     

The anisotropic effect of functional groups in 1H NMR spectra is the molecular response property of spatial nucleus independent chemical shifts (NICS)--conformational equilibria of exo/endo tetrahydrodicyclopentadiene derivatives

The inversion of the flexible five-membered ring in tetrahydrodicyclopentadiene (TH-DCPD) derivatives remains fast on the NMR timescale even at 103 K. Since the intramolecular exchange process could not be sufficiently slowed for spectroscopic evaluation, the conformational equilibrium is thus inaccessible by dynamic NMR. Fortunately, the spatial magnetic properties of the aryl and carbonyl groups attached to the DCPD skeleton can be employed in order to evaluate the conformational state of the system. In this context, the anisotropic effects of the functional groups in the (1)H NMR spectra prove to be the molecular response property of spatial nucleus independent chemical shifts (NICS).     

1H NMR studies of maltose,maltoheptaose, alpha-, beta-, and gamma-cyclodextrins,and complexes in aqueous solutions with hydroxy protons as structural probes

The (1)H NMR chemical shifts, coupling constants, temperature coefficients, and exchange rates have been measured for the hydroxy protons of aqueous solutions of alpha-, beta-, and gamma-cyclodextrins, maltose, and maltoheptaose. In cyclodextrins (CDs), the high chemical shift of the O(3)H signal and its small (3)J(OH,CH) value suggest that O(3)H is involved in a hydrogen bond. The small temperature coefficients and rate of exchange values of O(2)H and O(3)H confirm the involvement of O(3)H in hydrogen bonding and indicate that O(2)H is the hydrogen bond partner. In maltose, two distinct NMR signals with two different vicinal coupling constants are found for O(2')H. A cross-peak in the ROESY spectrum indicates chemical exchange between the O(2')H and O(3)H protons. The existence of two distinct NMR signals with different J values for O(2')H shows the influence of anomeric configuration on the O(2')H-O(3)H interaction. The effect of complexation with methyl benzoate, adamantane-1-carboxylic acid, adamantane-1-ol, and l- and d-tryptophane on the NMR spectra of the hydroxy protons of alpha-, beta-, and gamma-cyclodextrins and of maltose has been investigated. No significant spectral changes were observed upon addition of methyl benzoate and adamantane-1-carboxylic acid. The addition of adamantane-1-ol resulted in an upfield shift and a strong broadening of the O(2)H signal from alpha-CD, and a small temperature coefficient was measured upon complexation. The O(2)H and O(3)H signals in beta-CD were broadened and shifted downfield upon addition of l- and d-tryptophane.     

NMR characterization of the host-guest inclusion complex between beta-cyclodextrin and doxepin

The interaction between doxepin, a member of the tricyclic antidepressant (TCA) class of drugs, with beta-cyclodextrin (beta-CD) was investigated using NMR. Several TCAs have been reported to form a complex with beta-CD having 1:1 stoichiometry. Previous results from UV-visible spectroscopy, fluorescence measurements, and molecular modeling indicated that for imipramine, desipramine, and amitriptyline, the TCA aliphatic tail is included in the cyclodextrin cavity with apparently no interaction of the tricyclic ring. An alternative view of the doxepin-beta-CD complex is presented in this work using analysis of complexation-induced chemical shifts (CICSs), the method of continuous variation (Job's analysis), and analysis of ROESY spectra. The Job's plot derived from the NMR spectral data confirms that the complex formed has 1:1 stoichiometry. The largest changes in the CICS data were observed for the aromatic protons of one of the doxepin rings, with much smaller chemical shift changes observed for the protons of the other aromatic ring and the doxepin tail. Perhaps the most significant evidence for inclusion of the doxepin tricyclic ring is the strong ROESY cross peaks between the doxepin aromatic resonances and the protons located inside the beta-CD cavity. Changes in the doxepin (1)H NMR spectrum and the behavior of ROESY exchange cross peaks suggest that inclusion complex formation decreases the rate of internal motions of doxepin.     

Cooperative interaction of H3O+ with 1,3-alternate tetrapropoxycalix[4]arene: NMR and theoretical study

Interaction of H3O+ or H5O2+ with 1,3-alternate tetrapropoxycalix[4]arene (1) was studied in nitrobenzene and dichloromethane using 1H and 13C NMR including transverse and rotating-frame relaxations and density functional level of theory (DFT) quantum calculations. According to NMR, the ion forms an equimolecular complex with 1 with the equilibrium constant K being 3.97 x 10(3) L.mol(-1) at 296 K. The ions are bound by strong hydrogen bonds to the phenoxy-oxygen atoms of one half of 1 and by a medium-strong hydrogen bond to the pi system of the aromatic rings of the other half. The complex appears to have C(4h) symmetry in NMR even when cooling its solution down to 213 K, which could be due either to a genuine symmetry of the complex (if the ion is H5O2+) or to fast structure averaging by ion exchange processes (if the ion is H3O+). Therefore, the dynamics of the system was studied. Using two independent NMR methods (transverse and rotating-frame relaxation), two different exchange processes were discerned with correlation times 25 x 10(-6) and 5 x 10(-6) s, the first being clearly intermolecular and the other being apparently intramolecular. The energetic aspects of the possible exchange processes were examined by DFT quantum calculations. Rotation of H3O+ ion within one binding site with the energy barrier 8.13 kcal/mol is easily possible. Intermolecular exchange by freeing the ion from the complex has too high a barrier but cooperative interaction of the ion with additional water molecules makes it viable. The intramolecular exchange (or hopping) of the H3O+ ion between the two sites of the molecule is not viable in the classical manner, the barrier being 25.6 kcal/mol. Quantum tunneling of the ion is highly improbable, too. Alternative mechanisms including concerted two-ion intermolecular exchange and cooperative interaction with another bound water molecule including complexation with proton dihydrate H5O2+ are discussed.