Spectral Simplification and "Pseudo Decoupling" in One-Dimensional Proton NMR Spectra Using Pulsed-Field Gradients

High-resolution proton NMR spectra of complex molecules are generally crowded due to the homonuclear spin–spin couplings. Knowledge of chemical-shift frequencies from such crowded spectra are of utmost importance in identifying the spin system. These shifts would have been easily measurable had it been possible to achieve proton decoupling. Since the pioneering work of Aue et al. (1) , two- dimensional J-resolved spectroscopy seems to have offered the best route to ‘‘broadband-decoupled’’ proton spectra. Subsequently, various attempts have been made to improve upon the technique (2–9) , of which the most promising solution seems to be ‘‘purged J spectroscopy’’ ( 8, 9) . Recently, a new proton ‘‘decoupling’’ method has been proposed (10) which uses the classic spin-echo sequence without any purging pulse. A new era has started in the field of NMR with the use of pulsed-field gradients, which in turn, is a result of technological progress in the design of high- doublequality shielded gradients. We have reported the use of these gradients for editing one-dimensional proton NMR spectra (11) , where the gradients are used for suppressing artifacts arising from $180^o$ pulse imperfections and for z filtering. It was shown that the experiment can be used to identify unambiguously singlets and triplets overlapping with other multiplets. This was done by suppressing the signals arising from doublets and quartets by matching the interval between the two $90^o_x$ pulses with \tau= 1/2J and with the use of gradient pulses (Fig. 1a) . This process leaves the coherences of the singlets unaffected. For triplets, the central line is unaffected and the outer lines are inverted (Fig. 2a) . We report here the extensions of this pulse sequence to filter singlets and triplets without any distortions (Fig. 1b) and to achieve ‘‘pseudo-decoupling’’ of the triplets in the proton NMR spectra (Figs. 1c and 1d)

Anisotropic contributions of indirect proton-phosphorus couplings in oriented phenylphosphonous dichloride and dibromide

The spectral parameters for the title compds. were measured in ZLI-1167, Phase 914, and S-1115 liq. crystal solvents. The existence was shown of the anisotropic contribution of the P-H indirect coupling in both the Cl and Br compd

The Exchange Processes in Zinc-Complexes of o-Vanillin Salicyloylhydrazone as studied by 2-Dimensional NMR

Zinc forms two types of complexes with o-vanillin salicyloylhydrazone. The H-1 and C-13 nmr studies suggest that it coordinates with azomethine nitrogen, the carbonyl oxygen and with one or both of the phenolic oxygens. The H-1-H-1 and H-1 decoupled C-13-C-13 two-dimensional nuclear Overhauser and exchange spectra show that there is an exchange between the two types of complexes

Multinuclear Magnetic Resonance Studies of Phenylphosphonic Dichloride Oriented in Nematic Phases

Detailed multinuclear magnetic resonance studies on phenyl-phosphonic dichloride oriented in mixed and individual nematic liquid crystals have been carried out. Analysis of the $^1H,^{13}C$, and $^{31}P$ NMR spectra provided precise geometrical and orientational parameters and the signs and the magnitudes of the indirect spin–spin couplings between heteronuclei

De novo design of a five-stranded beta-sheet anchoring a metal-ion binding site

A five-stranded beta-sheet bearing two histidine residues as part of a metal-binding site has been designed, synthesized and characterised using NMR and electrospray ionization mass spectrometry techniques

Unidentate coordination of 2,2′−bipyridine2,2^\prime-bipyridine and 1,10-phenanthroline in a cyclometallated rhodium(III) complex. Evidence from 1Hand13C^1H and ^{13}C NMR spectra

The binuclear cyclometallated complex $[RhCl (\mu -Cl)(bBzlH_2bz)]_2 (bBzlH_2bzH D 1,3-bis(benzimidazolyl)(benzene))$ undergoes a dichloro bridge cleavage reaction with 2,20-bipyridine (bipy) or 1,10-phenanthroline (phen) in the presence of perchlorate to yield a mononuclear complex of the type $RhCl(OClO_3)(bBzlH_2bz)(N-N)(N-N D bipy or phen)$. Surprisingly, the N-heterocycle, bipy or phen, is neither chelating nor bridging bidentate in the complex. Such a monodentate coordination of bipy or phen was detected using two-dimensional 1H–1H correlated and NOE experiments (DQF-COSY and ROESY), $^1H -^{13}C$ single- and multiple-bond correlated two-dimensional NMR experiments (PFG-HSQC and PFG-HMBC) and $^1H$, $^{13}C$ spin-lattice relaxation time measurements. The noncoordination of the pendant nitrogen of the heterocycle bipy or phen is evidenced by the observation of two sets of signals together with the presence of interligand NOEs only between the coordinated part of the heterocycle and the bisbenzimidazole as seen in the corresponding ROESY spectrum. Further, the $^1H$ and $^{13}C$ spin–lattice relaxation times show lower values for the nuclei in the coordinated part of the heterocycle, bipy or phen, than for the uncoordinated parts, supporting the fact that only one of the two nitrogens of the heterocycle has coordinated to the metal and thus behaves as monodentate ligand

Designed beta-Hairpin Peptides with Defined Tight Turn Stereochemistry

The conformational analysis of two synthetic octapeptides, Boc–Leu–Val–Val–D-Pro–L-Ala–Leu–Val–Val–OMe (1) and Boc–Leu–Val–Val–D-Pro–D-Ala–Leu–Val–Val–OMe (2) has been carried out in order to investigate the effect of beta-turn stereochemistry on designed beta-hairpin structures. Five hundred megahertz 1HNMR studies establish that both peptides 1 and 2 adopt predominantly beta-hairpin conformations in methanol solution. Specific nuclear Overhauser effects provide evidence for a type II’ beta-turn conformation for the D-Pro–L-Ala segment in 1, while the NMR data suggest that the type I’ D-Pro–D-Ala b-turn conformation predominates in peptide 2. Evidence for a minor conformation in peptide 2, in slow exchange on the NMR time scale, is also presented. Interstrand registry is demonstrated in both peptides 1 and 2. The crystal structure of 1 reveals two independent molecules in the crystallographic asymmetric unit, both of which adopt beta-hairpin conformations nucleated by D-Pro–Lala type II’ beta-turns and are stabilized by three cross-strand hydrogen bonds. CD spectra for peptides and 2 show marked differences, presumably as a consequence of the superposition of spectral bands arising from both beta-turn and beta-strand conformations

Physicochemical studies of (o-vanillin thiosemicarbazonato)-nickel(Ii) chelate

Five- and six-membered rings result from the chelation of nickel(II) by the dibasic tridentate Schiff base ligand, o-vanillin thiosemicarbazone(o-VTSC), a new chelate prepared and characterized. The structural results are discussed in the light of spectroscopic and other data