Labile NH Research Articles

On the Observation of 14N Quadrupole Resonance Transitions in Water Proton NMR Relaxometry Dispersion Curves: The Case of a Labile NH Grouping in a Semirigid Molecular Moiety.

The electric field gradient tensor (considered here at the level of a nitrogen nucleus) can be described by two parameters: the largest element in the (X,Y,Z) principal axis system, denoted by VZZ (leading to the nuclear quadrupole coupling), and the asymmetry parameter η = (|VYY| - |VXX|)/|VZZ| with |VZZ| > |VYY| > |VXX|. The frequencies of the three nitrogen-14 nuclear quadrupole resonance (NQR) transitions depend on both parameters but, for sensitivity reasons, their determination may be especially difficult and time consuming. For a partly rigid NH grouping with a labile proton, water nuclear magnetic resonance (NMR) relaxometry curves may exhibit these three transitions (dubbed quadrupolar dips or quadrupole relaxation enhancement (QRE)), provided that the NH grouping belongs to a moiety possessing a sufficient degree of ordering. Their line shape leads to the correlation time describing mainly the motion of the NH grouping (the proton of which being in exchange with water protons), and their amplitude can be interpreted in terms of an effective NH distance. This approach is applied to a hydrogel, where separate NQR lines are observed for the different types of water existing in this system. Furthermore, the analysis of experimental data allows one to determine the nuclear quadrupole coupling in the protonated and deprotonated forms of this molecular moiety involving a labile NH grouping.

Cyclometalation of (2R,5R)-2,5-Diphenylpyrrolidine and 2-Phenyl-2-imidazoline Ligands with Half-Sandwich Iridium(III) and Rhodium(III) Complexes

Cyclometalation of (2R,5R)-2,5-diphenylpyrrolidine (1) with [(η5-Cp*)MCl2]2 using NaOAc in CH2Cl2 at room temperature, followed by cationization with KPF6 in CH3CN, cleanly yielded the cationic cyclometalated amines 2a,b (a, M = Ir; b, M = Rh). This constituted an improvement with regard to another cyclometalation pathway, which had led to unwanted oxidation of the ligand to an imine. Compounds 2a,b were characterized by elemental analysis and X-ray diffraction of single crystals; the latter gave evidence of an R configuration of the metal center, and a λ envelope conformation of the five-membered metallacycle. Successful application of the NaOAc/CH2Cl2 methodology to the cycloiridation and cyclorhodation of the imidazolines 3–5 led to the neutral half-sandwich chloro complexes 6–8, which were completely characterized, with yields ranging from 44% to 82%. In these complexes, the labile NH proton was localized in a γ position with regard to the metal. From the enantiopure chiral ligands 4 and 5, both diastereoisomers of the four compounds 7a,b and 8a,b were identified in solution by NMR; they were in equilibrium in solution. Complexes 6b, (SM)-7a,b, and (RM)-8a,b were characterized by radiocrystallography, which showed that the metallacycles were planar. Complexes 6–8 were tested as catalysts for the reduction of acetophenone and imines by hydrogen transfer.

Multivalency in the Gas Phase: H/D Exchange Reactions Unravel the Dynamic “Rock ’n’ Roll” Motion in Dendrimer-Dendrimer Complexes

Noncovalent dendrimer-dendrimer complexes were successfully ionized by electrospray ionization of partly protonated amino-terminated polypropylene amine (POPAM) and POPAM dendrimers fully functionalized with benzo[21]crown-7 on all branches. Hydrogen/deuterium exchange (HDX) experiments conducted on dendrimer-dendrimer complexes in the high vacuum of a mass spectrometer give rise to a complete exchange of all labile NH hydrogen atoms. As crown ethers represent noncovalent protective groups against HDX reactions on the ammonium group to which they are coordinated, this result provides evidence for a very dynamic binding situation: each crown is mobile enough to move from one ammonium binding site to another. Schematically, one might compare this motion with two rock 'n' roll dancers that swirl around each other without completely losing all contact at any time. Although the multivalent attachment certainly increases the overall affinity, the "microdynamics" of individual site binding and dissociation remains fast.

Photosensitized oxidation of 13C,15N-labeled imidazole derivatives.

An efficient synthesis of imidazoles with isotope labeling at different positions of the five-membered ring was developed. We carried out a detailed mechanistic study of the photosensitized oxidation of isotope-labeled imidazole derivatives. A new product, CO(2), was observed in the photooxidation of 2-H,N1-H imidazoles, but not in 2-substitituted imidazoles. The carbon of CO(2) derives from the 2C of imidazole. As shown by 18O experiments, both oxygen atoms of CO(2) originate mainly from one molecule of oxygen. Transient intermediates were detected by low-temperature NMR in the photosensitized oxidation of the isotope-labeled imidazoles. Quantitative analysis of the 13C NMR at different temperatures and times correlates the formation of one intermediate with the loss of another, thus allowing the complete decomposition pathway of the transient intermediates to be established. Singlet oxygen reacts with 4,5-diphenylimidazole via a [4 + 2] cycloaddition to form a 2,5-endoperoxide, which, upon warming, decomposes to a hydroperoxide. The hydroperoxide in one pathway loses water to form an imidazolone 7, which is hydrolyzed to a hydroxyimidazol-2-one 11. In another pathway, the hydroperoxide rearranges to diol 8. The diol rearranges to a carbamate 9 by opening and reclosing the five-membered ring. 9 decomposes to CO(2) and benzil diimine. A labile NH in the imidazole is crucial for the decomposition of the initially formed endoperoxide, otherwise the endoperoxide decomposes to regenerate starting material. Many similarities exist between the photooxidations of imidazole and guanosine in organic solvent, suggesting that the two reactions share a similar reaction mechanism with singlet oxygen.

Gradient and sensitivity enhancement of 2D TROSY with water flip-back, 3D NOESY-TROSY and TOCSY-TROSY experiments.

Previously we demonstrated a sensitivity enhancement of the original TROSY experiment by a factor of [Formula: see text] by the use of the sensitivity enhanced TROSY (en-TROSY) scheme. Here, we develop a gradient and sensitivity enhanced TROSY experiment (gs-TROSY), which is designed to select magnetization transfer pathways that suppress spectral artifacts and reduce the number of required phase cycles while having minimal loss of sensitivity. Both of these experimental methods (en-TROSY and gs- TROSY) have been combined with a water flip-back scheme which provides a further increase in sensitivity for labile NH groups by avoiding water saturation. We also apply these TROSY schemes to 3D NOESY-TROSY and 3D TOCSY-TROSY experiments.

Quantity‐Intensity Relationships of Soil Ammonia in Long‐Term Rotation Plots

AbstractReports of release of nonexchangeable NH+4 from soils under some conditions and reports that management practices can change the affinity of the exchange complex for NH+4 indicate a need to reevaluate the commonly accepted practice of determining exchangeable NH+4 to assess the availability of NH+4 to plants. To address this need, quantity‐intensity (Q/I) relationships were studied in soil samples from two Iowa soils, a Webster clay loam (fine‐loamy, mixed, mesic Typic Haplaquoll) and a Galva silty clay loam (fine‐silty, mixed, mesic Typic Hapludoll) in long‐term experiments having various crop‐rotation and N‐fertilizer treatments. The observed Q/I relationships gave no evidence for release of nonexchangeable NH+4. Potassium‐exchangeable NH+4 concentrations were well correlated with measurements of labile NH+4 (r = 0.91, P < 0.01) and equilibrium activity ratios for NH+4 (r = 0.89, P < 0.01). No significant relationship was found between relative affinity of the exchange complex for NH+4 and organic C within each soil. Soil management practices influenced NH+4 availability as evaluated by KCl‐exchangeable NH+4 and Q/I parameters, but did not change the relative affinity of the exchange complex for NH+4. Overall, indexes of NH+4 availability based on determinations of K‐exchangeable NH+4 were similar to those derived from studies of Q/I relationships.

Proton NMR structural characterization of a recombinant kringle 2 domain from human tissue-type plasminogen activator

The kringle 2 domain of human tissue-type plasminogen activator (t-PA) has been characterized via 1H NMR spectroscopy at 300 and 620 MHz. The experiments were performed on the isolated domain obtained by expression of the 174-263 portion of t-PA in Escherichia coli [Cleary et al. (1989) Biochemistry 28, 1884-1891]. The spectrum of t-PA kringle 2 is characteristic of a globular structure and shows overall similarity to that of the plasminogen (PGN) kringle 4. Spectral comparison with human and bovine PGN kringle 4 identifies side-chain resonances from Leu46, which afford a fingerprint of kringle folding, and from most of the aromatic ring spin systems. Assignment of signals arising from the His13, His48a, and His64 side chains, which are unique to t-PA kringle 2, was assisted by the availability of a His64----Tyr mutant. Ligand-binding studies confirm that t-PA kringle 2 binds L-lysine with an association constant Ka approximately 11.9 mM-1. The data indicate that homologous or conserved residues relative to those that compose the lysine-binding sites of PGN kringles 1 and 4 are involved in the binding of L-lysine to t-PA kringle 2. These include Tyr36 and, within the kringle inner loop, Trp62, His64, Trp72, and Tyr74. Acid/base titration of aromatic singlets in the presence of L-lysine yields pKa* approximately 6.25 and approximately 4.41 for His13 and His64, respectively, and shows that the His48a imidazole group does not protonate down to pH* approximately 4.3. Thus, the His48a and His64 side chains are in solvent-shielded locations. As observed for the PGN kringles, the Trp62 indole group titrates with pKa* approximately 4.60, which indicates proximity of the side chain to a titratable carboxyl group, most likely that of Asp57 at the binding site. Several labile NH protons of t-PA kringle 2 exhibit retarded H-exchange kinetics, requiring more than a week in 2H2O for full deuteration in the presence of L-lysine at 37 degrees C. This reveals that kringle 2 is endowed with a compact, dynamically stable conformation. Proton Overhauser experiments in 1H2O, centered on well-resolved NH resonances between 9.8 and 12 ppm, identify signals arising from the His48a imidazole NH3 proton and the three Trp indole NH1 protons. A strong dipolar interaction was observed among the Trp25 indole NH1, the Tyr50 amide NH, and the His48a imidazole CH2 protons, which affords evidence for an aromatic cluster in t-PA kringle 2 similar to that found at the hydrophobic kernel of PGN kringles.(ABSTRACT TRUNCATED AT 400 WORDS)

Utility of 1H/2H isotope shifts for the 13C NMR assignments of indole derivatives

AbstractThe deuterium/hydrogen NMR isotope effects on the 13C chemical shift (DHIECS) of the cycloalkano[b]indoles 1,2,3,4‐tetrahydro‐9H‐carbazole, 3‐tert‐butyl‐1,2,3,4‐tetrahydro‐9H‐carbazole and 1,2,3,4‐tetrahydrocyclopent‐[b]indole, wherein the labile NH was partially exchanged to ND, were measured at a concentration of 0.83 M in CDCI3 and in DMSO‐d6 solutions. The DHIECS values show defined trends that allow definitive 13C NMR assignments. The isotope effects over three and four bonds for the sp3 hybridized carbons are linearly interdependent, even when one of these isotope effects becomes negative.

Unambiguous assignment of the 13C NMR spectra of methylindoles

AbstractThe 13C NMR spectra of all possible indoles substituted with one, two or three methyl groups on the pyrrole ring were assigned from heteronuclear two‐dimensional 13C/1H contour plots and homonuclear hydrogen‐hydrogen decoupling experiments at a specific concentration of 0.83 M in CDCl3 and in DMSO‐d6. The results confirm controversial assignments for some compounds, and provide the assignments for those compounds not previously studied by this technique. In addition, deuterium/hydrogen NMR isotope effect on 13C chemical shifts (DHIECS) for indoles, wherein the labile NH was partially exchanged to ND, are reported. In this way DHIECS values over two, three, four and five bonds were measured. The results in each solvent are discussed in terms of the number of bonds between the exchanged hydrogen and the observed carbon, and also according to their geometric dependence. This provides some evidence of the vibrational origin of the DHIECS.