Double Pulsed Field Gradient Spin Research Articles

NMR‐based conformational analysis of perezone and analogues

Complete assignment of the (1)H NMR chemical shift and coupling constant values of perezone (1), O-methylperezone (2) and 6-hydroxyperezone (3) was carried out by total-line-shape-fitting calculations using the PERCH iterative spectra analysis software (PERCH Solutions Ltd., Kuopio, Finland). The resulting simulated spectra for the three compounds showed strong similarity to their corresponding experimental spectra. Particularly, all vicinal, allylic and homoallylic coupling constant values for the side chain of the three compounds were very similar, thus revealing that the conformation of these three molecules in solution is indeed almost identical. This fact is in agreement with extended side chain conformations over folded chain conformations because 1, 2 and 3 undergo completely different intramolecular cycloaddition reactions. In addition, results of double pulsed field gradient spin echo NOESY 1D experiments performed on perezone (1) were unable to provide evidence for folded conformers.

Structural analyses of mannose pentasaccharide of high mannose type oligosaccharides by 1D and 2D NMR spectroscopy

NMR spectroscopy is a very important and useful method for the structural analysis of oligosaccharides, despite its low sensitivity. We first applied conventional measuring methods, 2D DQF COSY, (1)H-(13)C HSQC, and (1)H-(13)C HMBC, and also the Double Pulsed Field Gradient Spin Echo (DPFGSE)-TOCSY and DPFGSE-NOESY/ROESY techniques to analyze a branched mannose pentasaccharide as a model of high mannose type N-glycans in natural abundance. The NMR spectra of the model compound are very complex and difficult to analyze owing to overlapping signals. The superior selective irradiation capability of the DPFGSE technique is useful for fine structural and conformational analyses of such complex oligosaccharides. We here introduce a novel technique called DPFGSE-Double-Selective Population Transfer (SPT)-Difference and DPFGSE-NOE/ROE-SPT-Difference spectroscopy. The DPFGSE-Double-SPT-Difference method involves irradiation of two peaks from one proton and the subtraction of higher and lower peaks from each spectrum. The DPFGSE-NOE/ROE-SPT-Difference method involves the transfer of the magnetization polarized by NOE/ROE from the nuclei to the spin-coupled nuclei through scalar spin-spin interaction using the SPT method. Even if the signals in the NMR spectra overlap, each signal can be accurately assigned. In particular, DPFGSE-NOE/ROE-SPT-Difference is very useful for identifying sugar connectivity.

A theoretical study of the biselective double pulsed field gradient spin-echo NMR experiment

The application of a biselective radiofrequency pulse to a coupled homonuclear spin system produces effects that are exploited in liquid-state NMR experiments. The insertion of such an RF pulse in a double pulsed field gradient spin echo sequence results in a module for the biselective production of antiphase magnetization states. Biselectivity is obtained through cosine modulation of a singly selective pulse profile. A full description of the module behaviour is possible by means of four parameters, of which two are related to a property called ‘pulse ellipticity’. The theory is in good agreement with the experimental results and numerical simulations. It is thus possible to predict how a maximum of antiphase magnetization can be produced.

SOGGY: Solvent-optimized double gradient spectroscopy for water suppression. A comparison with some existing techniques

Excitation sculpting, a general method to suppress unwanted magnetization while controlling the phase of the retained signal [T.L. Hwang, A.J. Shaka, Water suppression that works. Excitation sculpting using arbitrary waveforms and pulsed field gradients, J. Magn. Reson. Ser. A 112 (1995) 275–279] is a highly effective method of water suppression for both biological and small molecule NMR spectroscopy. In excitation sculpting, a double pulsed field gradient spin echo forms the core of the sequence and pairing a low-power soft 180°(− x) pulse with a high-power 180°( x) all resonances except the water are flipped and retained, while the water peak is attenuated. By replacing the hard 180° pulse in the double echo with a new phase-alternating composite pulse, broadband and adjustable excitation of large bandwidths with simultaneous high water suppression is obtained. This “Solvent-Optimized Gradient–Gradient Spectroscopy” (SOGGY) sequence is a reliable workhorse method for a wide range of practical situations in NMR spectroscopy, optimizing both solute sensitivity and water suppression.

Solution structure of a pentasaccharide representing the repeating unit of the O-antigen polysaccharide from Escherichia coli O142: NMR spectroscopy and molecular simulation studies.

Conformational studies have been performed of a pentasaccharide derived from the O-polysaccharide from Escherichia coli O142. The polymer was selectively degraded by anhydrous hydrogen fluoride and reduced to yield an oligosaccharide model of its repeating unit, which in the branching region consists of four aminosugars. A comparison of (1)H and (13)C chemical shifts between the pentasaccharide and the polymer showed only minor differences, except where the cleavage had taken place, indicating that the oligomer is a good model of the repeating unit. Langevin dynamics and molecular dynamics simulations with explicit water molecules were carried out to sample the conformational space of the pentasaccharide. For the glycosidic linkages between the hexopyranoside residues, small but significant changes were observed between the simulation techniques. One-dimensional (1D) (1)H,(1)H double pulsed field gradient spin echo (DPFGSE) transverse rotating-frame Overhauser effect spectroscopy (T-ROESY) experiments were performed, and homonuclear cross-relaxation rates were obtained. Subsequently, a comparison of interproton distances from NMR experiment and the two simulation approaches showed that in all cases the use of explicit water in the simulations resulted in better agreement. Hydrogen-bond analysis of the trajectories from the molecular dynamics simulation revealed interresidue interactions to be important as a cluster of different hydrogen bonds and as a distinct highly populated hydrogen bond. NMR data are consistent with the presence of hydrogen bonding within the model of the repeating unit.

Structural elucidation and complete assignment of a novel class of sulfonamides: bridgehead tricyclic sultams

AbstractEndocyclic sulfonamide templates bearing both an exocyclic ketone function and an internal olefin underwent reaction with a variety of hydroxylamines to effect an intramolecular nitrone–olefin cycloaddition to afford a new class of compounds suitable for derivatization by high‐throughput medicinal chemistry. Structural elucidation via complete assignment of the 1H and 13C NMR spectra of this new class of compounds was achieved using gradient‐COSY, gradient heteronuclear multiple quantum‐coherence spectroscopy and gradient heteronuclear multiple bond correlation spectroscopy. Additionally, double pulsed field gradient spin echo–nuclear Overhauser effect experiments were carried out in order to study the spatial conformation of this new type of molecule and assess the stereo‐ and regio‐selectivity of the chemical transformation. The unequivocal molecular framework and structural conformation was confirmed by X‐ray diffraction. Copyright © 2002 John Wiley & Sons, Ltd.

NMR Spectra of Nitrobenzanthrones Obtained by Using Double-Pulsed Field-Gradient Spin Echo

NMR Spectra of Nitrobenzanthrones Obtained by Using Double-Pulsed Field-Gradient Spin Echo

NMR Spectra of Nitrobenzanthrones Obtained by Using Double-Pulsed Field-Gradient Spin Echo

We have synthesized several nitrobenzanthrones (NBAs) and investigated their structures in solution using NMR spectroscopy. In our study, however, low solubility of NBAs in organic solvents always became a serious problem and led to NMR spectra with low signal-to-noise ratios. To solve this problem, we adopted a new method of conventional coherence transfer pathway selection using pulsed field-gradients. The double-pulsed field-gradient spin echo (DPFGSE) sequence was used for selecting natural-abundance carbon-13 isotopomers by using a bilinear rotation decoupling (BIRD) sequence and a so-called gradient-BIRD pulse for the excitation sculpting technique. We performed NMR measurements on 3,9-dinitro-7 H -benz[ d,e ]anthracene-7-one (compound 1), 3,9,11-trinitro-7 H -benz[ d,e ]anthracene-7-one (compound 2), and 2-nitro-7 H -benz[ d,e ]anthracene-7-one (compound 3) by this method and obtained spectra of high quality. Combining the results from phase-sensitive 2D NOESY, HOHAHA, and DPFGSE-HMQC, DPFGSE-HMQC TOCSY, and phase-sensitive FG-HMBC permitted us to complete NMR assignments. It was confirmed that the present method enables us to improve the sensitivity and to assign the NMR signal readily, even for compounds with low solubility.

NMR structural study of cycloadducts obtained by hetero-Diels- Alder reaction of chiral 1-oxa-1,3-butadienes

Chiral pyran derivatives were prepared by hetero-Diels– Alder reaction between ethyl vinyl ether and 1-oxa-1,3-butadienes bearing a chiral substituent in position 4 to obtain more information about the stereoselectivity of this reaction. The structure of reaction products was studied by one-dimensional NMR methods based on a double pulsed field gradient spin echo. Two novel NMR pulse sequences based on a combination of COSY and NOESY/ROESY are described, relieving the problem of heavy overlaps of target resonances in NOE experiments. The NMR studies in combination with further synthetic transformations allowed us to assign the configurations of the isolated cycloadducts. The results indicate that the configuration of the chiral substituent in position 4 is not the principal factor which controls the approach of the ethyl vinyl ether to the planar 1-oxa-1,3-butadiene system. Copyright © 2000 John Wiley & Sons, Ltd.

Structural Studies of 5-Ethyl-2′-Deoxyuridine by Selective Pulse 1H DPFGSE NOE Spectroscopy and PM3 Calculations

The solution conformation of 5-ethyl-2′-deoxyuridine (EDU) has been calculated from the vicinyl proton-proton NMR coupling constants and nuclear Overhauser (NOE) distances using excitation sculpting of selective pulses (Double Pulsed Field Gradient Spin Echo NOE) at 500 MHz and molecular modelling (PM3) studies.

Pentacoordinate Siliconate Tetraanionic Molecular Squares. Synthesis, Structure, and Solution Conformation of a Novel Tetraanion−Tetracation Ion Pair

A tetraanionic molecular square containing four pentacovalent siliconates is produced in high yield by the condensation of phenyltriethoxysilane and spirocatechol 1. A single isomer, meso 2, is produced from this diastereoselective condensation reaction. The steps leading up to the formation of meso-2 are not known, but the reaction appears to be driven by the insolubility of the tetrakistriethylammonium salt 2a. The tetraanionic macrocycle is a stable salt that undergoes ion-exchange reactions. An ion pair composed of the tetraanionic siliconate macrocycle and cyclobis(paraquat-p-phenylene) tetracation (2e) has been synthesized by ion exchange. Following complete assignment of all proton resonances in this 4 × 4 complex, the solution conformation was analyzed by an intermolecular double pulsed field gradient spin−echo (DPFGSE) NOE study. These results, aided by a molecular mechanics modeling study, have led to a proposed dynamic solution structure that features the cyclic bisparaquat tetracation selectively intercalating around a phenyl ring of the C2 face of the macrocyclic tetraanion which in turn is engulfed by the interior of the tetraanion (Figure 10). This novel host−host complexation calls attention to the conformational flexibility of the macrocyclic tetrasiliconates and, in all probability, all macrocycles be they charged or neutral.

Stabilization of a C7 equatorial gamma turn in DMSO-d6 by a ditryptophan crosslink

Covalent crosslinks can control local peptide conformation. In tripeptide sequences of the general formula Cys-Xxx-Cys, cysteine disulfides have been previously shown to enforce a C7 equatorial γ-turn conformation (also referred to as an inverse γ-turn). Much less is known about the effects of dityrosine and ditryptophan crosslinks on local peptide structure. In a series of tripeptides, ditryptophan crosslinks were formed using the two-step process of acid-promoted Mannich dimerization followed by oxidative aromatization. In these peptides, with the general formula Trp-Xxx-Trp (Xxx≠Gly), ditryptophan crosslinks were found to stabilize a C7 equatorial γ-turn conformation in DMSO-d6. Rigorous support for a C7 equatorial conformation in the crosslinked sequence Trp-Pro-Trp came from a variety of 1H NMR experiments and molecular modelling. Interproton distances were derived from NOE buildups that were determined through a series of double pulsed field gradient spin echo (DPFGSE) experiments. In addition, the small temperature dependence of the i+2 NH chemical shifts (Δδ/ΔT<2ppm/°C) provided further support for the intramolecular hydrogen bond which defines a γ-turn.

An ω1-band-selective, ω1-homonuclear decoupled ROESY experiment: application to the assignment of1H NMR spectra of difficult-to-assign peptide sequences

Application of an ω1-band-selective, ω1-homonuclear decoupled ROESY (BASHD-ROESY) experiment to the assignment of 1H NMR spectra of peptides is demonstrated. Band selection in the ω1 dimension is achieved with the double pulsed field gradient spin echo (DPFGSE) technique; homonuclear decoupling in the ω1 dimension is achieved by placing a non-selective 180° pulse together with the first half of the DPFGSE in the middle of the evolution period. Application of the BASHD-ROESY experiment is demonstrated with the complete assignment of the proton resonances of the synthetic 19 amino acid peptide N-Ac–Ala–Glu–Ala–Ala– Ala–Arg–Ala–Ala–Ala–Arg–Arg–Ala–Ala– Arg–Arg–Ala–Ala–Ala–Arg–NH2. Critical to making the assignments was the significantly increased resolution in the CαH–NH region of the ROESY spectrum measured with the BASHD-ROESY pulse sequence with band selection and homonuclear decoupling in the CαH region. NOEs observed for the peptide indicate it has a helical secondary structure in solution. © 1998 John Wiley & Sons, Ltd.

In-Phase Double Selective Excitation of Coupled Spin Systems Using Excitation Sculpting

An experiment that excites exclusively coupled nuclei pairsis presented. It involves the biselective defocusing and refocusing of coupled transverse magnetization under double pulsed field gradient spin echo conditions. Application to the extraction of subspectra from crowded COSY spectra is presented, as well as a doubly selective version of the homonuclearJ-resolved experiment.

Structural assignment of a molybdenum-containing silsesquioxane which catalyzes the metathesis of olefins: DPFGSE-NOE and x-ray diffraction studies

The reaction of [(c-C6H11)7Si7O9(OSiMe3)(OTl)2] with [Mo(CHCMe2Ph)(NAr)(OSO2CF3)2(dme)] (Ar=2,6-diisopropylphenyl) affords 4a as the predominant (>99%) molybdenum-containing product. Assignment of 4a as the syn-isomer with juxtaposed arylimido and Me3Si groups was made on the basis of double pulsed field gradient spin echo (DPFGSE) NOE experiments and confirmed by a single-crystal x-ray diffraction study. © 1997 John Wiley & Sons, Ltd.