Chemical Shift Correlation Spectra Research Articles

Fine structure of 5,10,15,20-tetrakis(m-hydroxyphenyl)chlorin (m-THPC): a 1H, 13C and 15N NMR study

The 1H, 13C and natural abundance 15N NMR spectra of tetrakis(m-hydroxyphenyl)chlorin (m-THPC) are assigned through the complementary analysis of the data from one-dimensional, and both homo- and hetero-nuclear two-dimensional NMR experiments. The analysis of the 1H and 13C spectra showed that the symmetry of the structure includes an effective mirror plane, σv, and the 1H–15N coupling constant of 99 Hz (measured from the ‘inverse-mode’1H–15N 2D experiment) showed that the central imino protons undergo slow intramolecular tautomeric exchange. The single 15N chemical shift (δ 111.8 relative to 15NH4+) of the proton-bearing nitrogens is characteristic of a ‘pyrrole’ type, and the long range coupling 1H chemical shift correlation (2D) spectrum showed that, on the NMR time scale, the imino hydrogens are localised on five-membered rings that are not in the σv plane. Taken together these data prove that the fine structure of m-THPC is correctly represented with the imino hydrogens placed on opposite rings which are not reduced, as shown in structure 4.

NMR Studies of Hindered Rotation. The Diels-Alder Adduct of Phencyclone with Maleic Anhydride: Restricted Motion of Bridgehead Phenyls

Abstract 1H NMR studies at 300 MHz have been performed for the Diels-Alder adduct of phencyclone and maleic anhydride in CDCl3 at ambient temperatures. The 1D spectrum shows four equal (2H) intensity doublets in the aryl region (in addition to other absorptions) which is fully consistent with a slow exchange limit (SEL) spectrum of a system in which the unsubstituted bridgehead phenyls exhibit hindered rotation around the C(sp2)-C (sp3) bond on the NMR timescale. These protons are assigned to H-1,8 and H-4,5 of the phenanthrene moiety and to H-2′ and H-6′ of the phenvls based on the two-dimensional (2D) homonuclear chemical shift correlation spectrum (COSY) together with arguments regarding carbonyl and aromatic ring anisotropy. Full proton assignments are given.

NMR Studies of Hindered Rotation. The Diels-Alder Adduct of Phencyclone withp-Benzoquinone: Restricted Motion of Bridgehead Phenyls

Abstract The Diels-Alder adduct of phencyclone with p-benzoquinone has been examined by 1H NMR at 300 MHz in CDCl3 at ambient temperatures for evidence of hindered rotation of the unsubstituted bridgehead phenyl groups. The nearly first-order spectrum exhibits four approximate doublets and five approximate triplets in the aromatic region, of roughly equal intensity (ca. 2H). This is consistent with a slow-exchange limit (SEL) spectrum of the hindered phenyls. For rapidly rotating phenyls, the predicted fast-exchange limit (FEL) spectrum would have shown two 2H doublets and one 4H doublet, in addition to three 2H triplets and one 4H triplet, in the aryl region. Full 1H assignments have been made, based on the two-dimensional 1H-1H homonuclear chemical shift correlation spectrum (COSY) and expected magnetic anisotropy effects.

An EPR and NMR study of some tetramethylisoindolin-2-yloxyl free radicals

Isoindolin-2-yloxyls are stable free radicals whose potential as spin probes/labels has not yet been exploited. We have prepared 1,1,3,3-tetramethylisoindoline, its 5-nitro derivative (paving the way to adding prosthetic groups) and a version with fully deuteriated methyl groups. These isoindolines have been oxidized to give the corresponding free radicals. It was found that the isotropic EPR spectra of the radicals have linewidths much smaller than those of the TEMPO family of radicals. The parent radical has a spectrum characterized by hyperfine interactions from nitrogen and from protons of the methyl groups. By measuring the NMR chemical shift differences between the isoindolines and the isoindolinyloxyls, values and signs of hyperfine splittings have been determined for the aromatic protons (too small to measure even in well resolved EPR spectra) and for some of the carbon-13 nuclei (at natural abundance). An important advantage of the NMR method is that assignments can be made with near certainty. This advantage has been taken further by obtaining 1H–13C chemical shift correlation spectra of the free radicals.For spin probe measurements, accurate values of the anisotropic g-factors and nitrogen hyperfine interactions are needed: these have been derived from the EPR powder spectrum of the deuteriated radical.

Deoxyribose conformation in [d(GTATATAC)]2: evaluation of sugar pucker by simulation of double-quantum-filtered COSY cross-peaks

Exchangeable and nonexchangeable proton and phosphorus resonances (11.75 T) of [d(GTATATAC)]2 in aqueous solution were assigned by using proton two-dimensional nuclear Overhauser effect (2D NOE) spectra, homonuclear proton double-quantum-filtered COSY (2QF-COSY) spectra, proton spin-lattice relaxation time measurements, and 31P1H heteronuclear shift correlation spectra. Due to the large line widths, it was not possible to directly extract vicinal proton coupling constant values from any spectrum including ECOSY or 2QF-COSY. However, comparison of quantitative 2QF-COSY spectral simulations with experimental spectra enabled elucidation of coupling constants. The scope and limitations of this approach were explored by computation and by use of experimental data. It was found that proton line widths exhibit some variability from one residue to the next as well as from one proton to the next within a residue and the exact line width is critical to accurate evaluation of coupling constants. Experimental 2QF-COSY spectra were not consistent with a rigid deoxyribose conformation for any of the nucleotide residues. A classical two-state model, with rapid jumps between C2'-endo (pseudorotation angle P = 162 degrees) and C3'-endo (P = 9 degrees) conformations, was able to account for the spectral characteristics of terminal residue sugars: 60% C2'-endo and 40% C3'-endo. However, the 2QF-COSY cross-peaks from the -TATATA- core could be simulated only if the classical two-state model was altered such that the dominant conformer had a pseudorotation angle at 144 degrees instead of 162 degrees. In this case, the major conformer amounted to 80-85%. Alternatively, the spectral data were consistent with a three-state model in which C2'-endo and C3'-endo conformations had the largest and smallest populations, respectively, but a third conformer corresponding to C1'-exo (P = 126 degrees) was present, consistent with recent molecular dynamics calculations. This alternative yielded populations of 50% (P = 162 degrees), 35% (P = 126 degrees), and 15% (P = 9 degrees) for the -TATATA- sugars. The spectral results indicate little variation of sugar pucker between T and A. Small differences in cross-peak component intensities and characteristic spectral distortions, however, do suggest some unquantified variation. 31P1H heteronuclear chemical shift correlation spectra manifested alternating chemical shifts and coupling constants suggestive of phosphodiester backbone conformational differences between TA and AT junctions.

The first synthesis of thieno[c]isoquinolines and an improved synthesis of phenanthridine and thieno[c]quinolines through pd(0) catalyzed coupling of ortho‐formylarylboronic acids with functionalized aryl halides

AbstractAll three isomeric hitherto unknown thieno[c]isoquinolines have been synthesized in high yields by the Pd(0)‐catalyzed coupling of 2‐formylbenzeneboronic acid with t‐butyl‐N‐(ortho‐halothienyl)carbaniates. When 2‐bromoacetanilide, instead of 2‐bromoaniline, was coupled with orth‐formylarylboronic acids under Pd‐catalysis, phenanthridine and thieno[c]quinolines were obtained in improved yields.Total assignments of 1H nmr spectra of thieno[c]isoquinolines and thieno[c]quinolines are reported. Assignments are based on high resolution 300 MHz 1H nmr spectra, two‐dimentional 1H‐13C chemical shift correlation spectra and one‐dimentional INADEQUATE 13C nmr spectroscopy.

Effects of misoptimization of the BIRD pulse on one‐bond modulation effects in long‐range heteronuclear chemical shift correlation spectra

AbstractBIRD pulses have been shown to provide an effective means of decoupling one‐bond modulation of long‐range response intensity. It is not possible to optimize the BIRD pulse effectively when there is a range of one‐bond coupling constants. To examine the effects of misoptimization of the fixed delays (τ) in the BIRD pulse, the simple alkaloid norharmane was employed. Long‐range heteronuclear correlation spectra were recorded using a modified pulse sequence containing a BIRD pulse midway through the Δ2 interval. The experiment was optimized for 10.3 and 11.1 Hz, which correspond to a valley and a peak in the response intensity curve when the experiment is performed without the BIRD pulse midway through Δ2. The delays in the BIRD pulse were variously optimized for values ranging from 80 to 175 Hz. The results suggest that the BIRD pulse is effective in decoupling one‐bond modulations of response intensity even when the τ delays are grossly misoptimized for the one‐bond coupling constantin question.

Structure elucidation of cordifolin A, a novel cucurbitacin from Fevillea cordifolia, using one and two dimensional n.m.r. techniques

Cordifolin A (3)[3β,16α-dihydroxy-20,25-epoxy-19(10 → 9β)-abeo-10α-lanost-5-en-11,22-dione] was isolated from the methanol-soluble fraction of the 1% tartaric acid extract of dried seeds of Fevillea cordifolia L. (Cucurbitaceae) used in Jamaican folk medicine. Structure elucidation was based primarily on the use of multi-pulse n.m.r. techniques, especially combination of long range and direct 1H,1H INADEQUATE and 1H, 13C chemical shift correlation spectra. Biosynthesis of this novel triterpene apparently involves cyclization of the side chain to give a six-membered ether ring which is unprecedented among cucurbitacins.

DEPT‐related sequences for selective long‐range heteronuclear chemical shift correlation spectroscopy

AbstractDEPT‐related sequences are proposed which suppress the generally unnecessary direct proton‐carbon correlations from the long‐range optimized heteronuclear chemical shift correlation spectra. One‐bond modulation of long‐range response intensities is removed through the use of a BIRD pulse train. It is demonstrated that these modified sequences are superior to the original 2D DEPT experiment for the detection of long‐range connectivities of protonated carbons.

Characterization and chemical properties of phosphoribosylamine, an unstable intermediate in the de novo purine biosynthetic pathway

Incubation of [1-13C]-5-phosphoribosyl pyrophosphate ([1-13C]PRPP) and glutamine with PRPP amidotransferase results in rapid production and disappearance of two new resonances at 89.3 and 85.9 ppm. These resonances coincide with two of the products produced upon incubation of [1-13C]ribose 5-phosphate with NH3. Extensive NMR studies (15N and 1H-13C chemical shift correlation spectra) have allowed assignment of these resonances to beta- and alpha-phosphoribosylamine. These studies represent the first spectral observations of this chemically reactive intermediate. The rate of interconversion of alpha- to beta-phosphoribosylamine as a function of pH has been determined by saturation and inversion-transfer NMR methods. The rate of formation of 5-phosphoribosylamine (PRA) from ribose 5-phosphate and NH3 and its rate of decomposition as a function of pH have been determined with a glycinamide ribonucleotide synthetase trapping system fashioned after earlier studies of Nierlich and Magasanik [Nierlich, D. P., & Magasanik, B. (1965) J. Biol. Chem. 240, 366]. Phosphoribosylamine has a t1/2 = 38 s at 37 degrees C and pH 7.5. The pH-independent equilibrium constant for ribose 5-phosphate and NH3 with phosphoribosylamine has been established, 2.5 M-1, by use of these rate constants as well as by NMR methods. This equilibrium constant and the rates of nonenzymatic interconversion of alpha- and beta-PRA provide essential background for studying the mechanism of glycinamide ribonucleotide synthetase and investigating the possibility of channeling phosphoribosylamine between this enzyme and the first enzyme in the purine pathway.

Modulation of methylene carbon response intensity in long‐range heteronuclear 2D NMR chemical shift correlation spectra

AbstractSeveral recent papers have dealt with the modulation of protonated carbon response intensities in long‐range heteronuclear 2D NMR chemical shift correlation spectra. Expressions describing the one‐bond modulation of the response intensity of the C‐3 methylene resonance of 6,7‐dimethoxy‐3,4‐dihydroisoquinoline, and the decoupling of this modulation afforded by a BIRD pulse located midway through the Δ2 refocusing delay, are presented with experimental confirmation. Long‐range response intensities are related to the conventional heteronuclear chemical shift correlation spectrum optimized for transfer from the directly attached proton.

Elimination of direct responses and one‐bond modulations in long‐range heteronuclear chemical shift correlation spectra through the use of low‐pass J filters and BIRD pulses

AbstractModified long‐range heteronuclear chemical shift correlation pulse sequences which incorporate both BIRD pulses midway through the Δ2 magnetization transfer delay and low‐pass J filters are described. The BIRD pulse provides the means of decoupling one‐bond modulations of response intensity when magnetization is transferred to protonated carbons. The one‐ and three‐step low‐pass J filters utilized eliminate responses due to directly attached protons over a range of one‐bond coupling constants.Results obtained by long‐range optimization of the conventional heteronuclear chemical shift correlation pulse sequence, the previously reported pulse sequence containing only the BIRD pulse midway through Δ2 and the two new pulse sequences described in this work are compared using 2,3‐dihydrofuran as a model compound.The low‐pass J filters have no adverse effects on the ability of the BIRD pulse to decouple one‐bond modulations. Modulation decoupling in the modified pulse sequences was confirmed using the 3J(C‐3, H‐1) coupling pathway of the simple alkaloid norharmane. The response intensities, however, were up to 10% lower with a one‐step low‐pass J filter when compared with the simpler pulse sequence containing only the BIRD pulse midway through Δ2.

Two‐dimensional NMR studies of marine natural products. IV. Isolation of the cembranoid diterpene jeunicin from the mollusc planaxis sulcatus: Assignment of the proton and carbon NMR spectra by two‐dimensional techniques

AbstractNumerous cembranoic diterpenes have been isolated from marine gorgonians and terrestrial plants. There have, however, been only a few reports of the isolation and identification of these interesting compounds from higher animal sources. The isolation of the oxo‐bridged cembranoid diterpene jeunicin from the mollusc Planaxis sulcatus is reported. A comparative discussion of the utility of COSY, homonuclear RELAY (relayed COSY) and proton double quantum coherence techniques is presented, the latter successfully establishing the identities of all of the proton resonances. Acquisition of a two‐dimensional proton‐carbon chemical shift correlation spectrum provided the means of unequivocally establishing 13C resonance assignments.

One- and two- dimensional 15N/ 1H NMR of filamentous phage coat proteins in solution

High resolution 15N NMR studies of proteins in solution can be performed efficiently by combining the use of isotopically enriched proteins and pulse sequences that generate polarization transfer from protons and result in two-dimensional heteronuclear chemical shift correlation spectra. The coat proteins of the filamentous bacteriophages fd and Pf1 solubilized in detergent micelles give one- and two- dimensional NMR spectra with resolved resonances for nearly all of the nitrogen sites in the proteins. The resonances from the amide sites with slowly exchanging protons can be obtained as a subset of the resonances of all amide sites by comparing the spectra of proteins in D2O and H2O solutions at pH = 4.0.

Autocorrelated 13c‐13c double quantum coherence two‐dimensional nmr spectroscopy: Utilization of a modified version of the technique as an adjunct in the total assignment of the 1h‐ and 13c‐nmr spectra of the mutagen phenanthro[3,4‐b]thiophene

AbstractDevelopment of successively higher field nmr spectrometers has facilitated the study of increasingly more complex molecules, although smaller molecules such as phenanthro[3,4‐b]thiophene still offer very substantial assignment problems because of the highly congested nature of their 1H‐ and 13C‐nmr spectra. Assignments of such spectra, if they are to be unequivocal, frequently require the utilization of two‐dimensional nmr spectroscopic techniques. Total assignments of the 1H‐ and 13C‐nmr spectra of phenanthro[3,4‐b]thiophene are reported. Assignments were based on a conventional high resolution 500 MHz 1H‐nmr spectrum, autocorrelated two‐dimensional 1H‐nmr spectra (COSY), two‐dimensional 1H‐13C chemical shift correlation spectra and a modified version of autocorrelated 13C‐13C double quantum coherence two‐dimensional nmr spectroscopy. From NOE measurements, a separation of 1.99 Å between H1 and H11 was computed, suggesting that phenanthro[3,4‐b]thiophene has a pronounced helical conformation in solution.