HMQC spectra

H-Detected Heteronuclear Multiple-Quantum Coherence (HMQC) Spectra... 

The heteronuclear multiple-quantum coherence (HMQC) spectrum, H-NMR chemical shift assignments, and C-NMR data of podophyllo-toxin are shown. Determine the chemical shifts of various carbons and connected protons. The HMQC spectra provide information about the one-bond correlations of protons and attached carbons. These spectra are fairly straightforward to interpret The correlations are made by noting the position of each crossf)eak and identifying the corresponding 8h and 8c values. Based on this technique, interpret the following spectrum. 

Fig. 22 (a) 14N-13C SQ-HMQC spectrum of Ala Gly Gly tripeptide with natural abundance of 13C, taken at 14.1 T under 25 kHz MAS. The corresponding 15N MAS spectrum is shown in the top right comer. (Reproduced with permission from reference [258].) (b) l4N- H SQ-HMQC and DQ-HMQC spectra of partly deuterated glycine-d2 NH3+CD2COO, taken at 9.4 T under 30 kHz MAS. The 14N projections were simulated using second-order quadrupolar powder patterns with Cq = 1.18 MHz and i/q = 0.5, assuming uniform excitation of all crystallite orientations. (Reproduced with permission from [264])... 

Figure 8.5 Comparison of HMQC spectra obtained on resin-cleaved and resin-bound...

The structure of the two rhodium-acyl complexes was elucidated using COSY90 spectra, selective decoupling of the phosphorus resonances, and HMQC spectra. Spectrum 3d (Figure 6.13) shows the complete simulation of the NMR spectrum obtained at 223 K. The NMR spectrum... 

The H- and C-NMR spectra of dihydroarcyriarubin C (353) were very similar to those of arcyriarubin C (351), except for the presence of one sp methine proton at 6 4.44, which was assignable to the C-8 proton. The HMBC spectrum showed a cross peak from H-8 to C-8 (5c 48.2), and this HMBC correlation may be assigned to H-8 to C-8 (or H-8 to C-8), indicating the symmetrical structure of this alkaloid. This structure was further supported by its H- H COSY and HMQC spectra (334). Comparison of the H-NMR spectral data of synthetic cis- and frans-dihydroarcyr-iarubin A (339) indicated the frans-relative stereochemistry for the natural dihydroarcyriarubin C (353). Based on these spectral data, and comparison with arcyriarubin C (351), as well as with synthetic fraMS-dihydroarcyriarubin A, the structure 353 was assigned to dihydroarcyriarubin C (334,339) (Scheme 2.90). 

Table 6.2 Analysis of the proton-decoupled DEPT 135 and HMQC spectra of unknown A...

Of great importance in the determination of protein structures is the use of 15N- or 13C-enriched samples to obtain isotope-edited spectra. For example in HSQC or in 15N-multiple quantum cohenence (HMQC) spectra we see only NH protons in a plot of H chemical shift in one dimension versus the 15N chemical shift of the attached... 

HPLC analysis was carried out using a 250 x 4.6 mm id Spherisorb ODS-2 column at 35°C with elution using isocratic D20/phosphate buffer at pH 2.5 for 5 min, followed by a linear gradient of acetonitrile to 50% after 50 min with UV monitoring of the eluent peaks at 225 nm. The NMR measurements were carried out with solvent suppression at a H NMR frequency of 600 MHz. The assignment of the resonances were based on those of standard compounds such as A-A-A-OH and Y-Y-Y-OH using standard 1-dimensional (ID) spectroscopy and 2-dimensional (2D) heteronuclear correlation methods such as and H-15N HMQC spectra. 

The NMR data of xH-COSY and HMQC spectra reveal that at the beginning of the reaction (below a DS of about 0.3) only three possible monosubsti-tuted glucoses prevail as building blocks of the polymer chain. The values for the rate coefficients are fa = 1-2, fa = 0.5 and fa = 0.8. Beyond this DS (up to a value of about 0.8), disubstituted units are also observed but no trisubstitution occurs [220,221]. 

Identify the compound C6H10O from its H, 13C/DEPT, COSY, and HMQC spectra and show all correlations. 

Despite the very different mechanism, the HSQC sequence (//eteronuclear Single Quantum Correlation) yields results equivalent to an HMQC sequence except that HSQC offers an additional benefit—the cross-peaks do not exhibit homonuclear JH—XH couplings along the FI axis. These splittings reduce sensitivity and resolution along this axis in HMQC spectra. On the other hand, the HSQC sequence contains more pulses and is more sensitive to errors in calibrations etc. The sequence is209 ... 

A 2002 review by Reynolds and Enriquez describes the most effective pulse sequences for natural product structure elucidation.86 For natural product chemists, the review recommends HSQC over HMQC, T-ROESY (transverse rotating-frame Overhauser enhancement) in place of NOESY (nuclear Over-hauser enhancement spectroscopy) and CIGAR (constant time inverse-detected gradient accordion rescaled) or constant time HMBC over HMBC. HSQC spectra provide better line shapes than HMQC spectra, but are more demanding on spectrometer hardware. The T-ROESY or transverse ROESY provides better signal to noise for most small molecules compared with a NOESY and limits scalar coupling artefacts. In small-molecule NMR at natural abundance, the 2D HMBC or variants experiment stands out as one of the key NMR experiments for structure elucidation. HMBC spectra provide correlations over multiple bonds and, while this is desirable, it poses the problem of distinguishing between two- and three-bond correlations. 

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