Long-range Heteronuclear Correlation Research Articles

Selective inverse multiple bond analysis. A simple 1D experiment for the measurement of long-range heteronuclear coupling constants

The general concept of selective excitation is well established and the methodology has been utilized on numerous occasions in a wide range of applications. An interesting application of selective pulses has been in the development of selective analogues of two-dimensional NMR experiments. The first such effort germane to our work was that of Bax and Freeman ( 1)) in which they described the selective heteronuclear 2D J spectrum. Davis applied semiselective proton pulses to the Ha resonances of peptides (2) to establish long-range correlations to carbonyl resonances with high resolution in both frequency domains. In a paper published very shortly thereafter, Bermal and co-workers (3) described a group of two-dimensional pulse sequences intended to provide access to heteronuclear coupling constants that employed semiselective Gaussian 13C pulses. Kessler and colleagues have employed semiselective 13C pulses to improve the Fi resolution in proton-detected heteronuclear long-range correlation experiments (4). Supportive ancillary work has addressed the problem of extracting coupling constants from complex cross-peak multiplets observed in proton-detected heteronuclear correlation spectra (5, 6). The importance of measuring small heteronuclear coupling constants has not waned, as evidenced by recently reported heteronuclear two( 7, 8) and three-dimensional (9) studies designed to determine and utilize small heteronuclear coupling constants of biologically important molecules. In a departure from existing multidimensional methods, we report a simple, one-dimensional pulse sequence designed to allow the measurement of long-range heteronuclear coupling constants and to provide the means for establishing long-range heteronuclear connectivities. The earliest reports of one-dimensional analogs of two-dimensional NMR experiments were those of Berger ( IO, II), which were followed by the work of Kessler and colleagues (12, 13). In an extension of these efforts, we describe a one-dimensional analogue of the HMBC experiment ( 14)) for which we propose the acronym SIMBA (selective inverse multiple bond analysis). With this new experiment, each proton with a resolved long-range coupling to the selected carbon simultaneously appears in the resultant spectrum with an apparent

13C NMR Chemical Shift Assignments in Dibucaine and Dibucaine Hydrochloride Determined by Two-Dimensional NMR Spectroscopy

Abstract 13C NMR chemical shift assignments were obtained for dibucaine in CDCl3 and dibucaine hydrochloride in D2O solution using one-bond and long-range two-dimensional heteronuclear correlation techniques. The 13C chemical shift assignments, together with the fully coupled 13C spectrum, allowed the measurement of one-bond, and long-range 13C1H coupling constants.

Long-range heteronuclear correlation 2D NMR by MLEV-16 isotropic mixing

Coherence transfer by MLEV-16 isotropic mixing is shown to be an efficient way of obtaining phase-sensitive heteronuclear chemical shift correlation 2D spectra. Both long- and short-range correlations are possible by appropriate choice of mixing time. At long mixing times, the contribution of coherence transfer pathways through multiple couplings extends the range of correlation. Long mixing time experiments may therefore be useful for structure determination in total unknowns. Results are presented for a 13C-detected isotropic mixing experiment on menthol.

Improved resolution in proton-detected heteronuclear long-range correlation

Heteronuclear long-range correlations can provide valuable information in many circumstances of spectroscopic work. They can provide correlations to quaternary carbons, which can be useful, e.g., either in the assignment of aromatic resonances or during sequencing of a peptide

Conformation-independent sequential NMR connections in isotope-enriched polypeptides by 1H 13C 15N triple-resonance experiments

Sequence-specific resonance assignments provide the basis for the structural interpretation of polypeptide and protein NMR data. A key step in making these assignments involves identifying connections between spin systems of sequential amino acid residues. So far, this has mainly been achieved using conformation-dependent nuclear Overhauser effects (l-5). Recently, alternative methods of establishing the sequential order of spin systems have been developed including long-range heteronuclear correlation between backbone protons and the carbon carbonyl or amide nitrogen of the peptide bond (6-9)) and direct correlation of “N amide and 13C carbonyl chemical shifts of peptide bonds ( IO, 1 I). The former technique relies on observation of antiphase cross peaks where the active couplings depend on the dihedral angles 4 and #, respectively. The latter method requires isotope enrichment with both “N and 13C. In this Communication we describe a family of new ‘H- 13C- 15N triple-resonance experiments which provide sequential connectivity information in polypeptides enriched with either “N or 13C. These relayed coherence transfer experiments use only one- and two-bond heteronuclear couplings, all of which are greater than 7 Hz and are essentially independent of the polypeptide backbone conformation. All cross peaks are in-phase and therefore do not suffer from intrapeak cancellation. Aside from differential relaxation, all sequential cross peaks are equally intense. Since there are only intraresidue and sequential cross peaks in these spectra, they are simpler and more amenable to automated analysis than NOESY or ROESY spectra. They also provide 13Ca and lSN backbone amide assignments in the course of determining sequence-specific ‘H assignments. The pulse sequences we have developed are shown in Fig. 1. The first sequence (Fig. IA) we call a 2D H”-C*(wl)-N-HN(wz) heteronuclear relayed coherence transfer experiment ( HETERO-RELAY) . It begins with transfer of a-proton polarization to directly bound 13C nuclei using refocused INEPT (12-15), tuned for a onebond ‘H- 13C coupling constant of 140 Hz. During tl , this 13C in-phase magnetization

A New Bisbenzylisoquinoline Alkaloid from Phaenthus vietnamensis and Its Antibacterial Activity

A new bisbenzylisoquinoline alkaloid (−)-(1S,1'R)-O,O'-dimethylgrisabine was isolated from the leaves of Phaeanthus vietnamensis Ban. Its structure was determined on the basis of the extensive 2-D and 1-D nmr long-range heteronuclear correlations. Its antibacterial activity is also described

The determination of long-range (C,H) coupling constants by one- and two-dimensional NMR experiments

A comparative study is presented between the NMR techniques of two-dimensional heteronuclear correlation (HETCOR) with optimal delay times, HETCOR with the inclusion of a BIRD pulse sequence in the middle of t 1, and of one-dimensional selective population inversion (SPI), all of which are utilized in establishing connectivities between carbon and proton nuclei over more than one bond for molecular-structure determinations. Some of the connectivities observed in salicylaldehyde with the SPI experiment could not be detected with long-range HETCOR. Various aspects of the most appropriate values to be used for the delay times in long-range HETCOR are discussed, and the absence of any long-range (C,H) connectivities is reasoned. SPI is the preferred technique for the estimation of the magnitudes of >1 J(C,H).

Response-intensity modulation in long-range heteronuclear two-dimensional NMR chemical-shift correlation

Two-dimensional NMR spectroscopy has had a very significant impact on the way in which structure elucidation and spectral assignment studies are performed. Investigators have begun to exploit long-range heteronuclear coupling constants by utilizing delays in the conventional 2D NMR heteronuclear chemical-shift correlation experiment optimized for long-range magnetization transfer through two, three, and/or four bonds (‘Jcu, 3JcH, and “J,,) (1-16). Alternatively, several new experiments specifically for the observation of long-range heteronuclear spin-coupling information (14, 15, 2 7-22) have been reported. Despite the now numerous applications of longrange heteronuclear chemical-shift correlation contained in the literature, factors affecting the magnetization transfer step in these experiments have not been carefully investigated. Most long-range experiments are optimized for a 10 Hz magnetization transfer based upon the work of Reynolds and co-workers (2) or that of Wemly and Lauterwein (9), although other optimizations have also been used with success (I, 4, 10). As a consequence of the absence of responses in some of our own work despite optimization of the experiment at or near the magnitude of the coupling in question we now wish to communicate the results of our preliminary investigation into factors affecting response intensity in long-range heteronuclear chemical-shift correlation. Schenker and von Philipsborn (23) have reported a systematic investigation of the optimization of the INEPT and DEPT experiments in weakly coupled spin-f systems. They have shown that when magnetization is transferred via small heteronuclear couplings, that the transfer may be modulated by other homoand/or heteronuclear spin couplings which are experienced by those nuclei involved in the polarization transfer. Population transfer involved in the long-range heteronuclear chemical-shift correlation experiments may also be modulated in similar fashion, as recently suggested by Freeman (18) and Bax (21).

A unified product operator formalism. Application to uniform excitation in heteronuclear correlation 2D NMR

A product operator formalism using the basis set 1 2 E , I z , I +, I −∼ for spin- 1 2 nuclei is developed for description of 2D NMR experiments. This formalism provides a powerful means for dealing with different types of coherences, phase relationships, and mixing by radiofrequency pulses. The formalism is demonstrated by development of an improved experiment for long-range heteronuclear correlation 2D NMR, i.e., using n J, n > 1.