DEPT-HMQC

DEPT-HMQC (Kessler et al. 1989b) combines the editing capabilities of the DEPT experiment (Doddrell et al. 1982) with the high sensitivity inherent to inverse-detected heteronuclear correlation experiments. When dealing with the often complex and congested proton NMR spectra of alkaloids, the editing capability of DEPT-HMQC may be advantageous. As the DEPT-HMQC experiment is discussed in more detail in the review of Martin and Crouch (1991), we will not consider it in depth here. 

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The second Chilocorini species studied, Chilocorus cacti, contained several alkaloids closely related to the former. The structure of the heptacyclic chilo-corine A (19) was deduced from a series of NMR experiment (DEPT, HMQC, DQ-COSY, TOCSY) and from a comparison of its spectral properties with those of exochomine (18) from which it differ by having two C-C linkages between the two tricyclic partners. However, the configuration at the asymmetric center in the azaacenaphthylene ring was not established [33]. 

Identify compound C)0H18O from its H, 13C/DEPT, HMQC, HMBC, and INADEQUATE spectra. 

Spectroscopic methods (DEPT-HMQC and DEPT-SQC) have been developed which permit resolution of ISjsjH and groups in proteins (14,15). 

Multiplicity editing in 2D NMR is possible via INEPT- or DEPT-HMQC [121] experiments. Choosing the editing pulse or delay, as in ID experiments, can yield spectra with CH and CH3 contours positive and CH2 contours negative [2,74]. Useful and potentially cleaner spectra can be obtained this way however, these sequences have been little utilized in lignin work to date. They are experiencing renewed interest with the adoption of the latest shaped-pulse variants. 

Kessler, H., Schmieder, R, and Kurz, M. (1989) Implementation of the DEPT sequence in inverse shift correlation The DEPT-HMQC. J. Magn. Reson. 85(2), 400-405. 

Figure 6.17. The DEPT-HMQC sequence for multiplicity editing within the 2D correlation experiment. The conventional DEPT sequence precedes HMQC and provides editing through the proton 6 tip angle. Setting 0 = 180" inverts XH2 responses relative to those of XH and XH3.

Only one major compound (SF-X) was present in the estrogenically active fractions, and this was isolated using semipreparative HPLC. The principles of separation are the same as for qualitative HPLC (see Section 25.4.1.2), with the difference that higher amounts of material can be loaded onto the ODS-column (Alltech, Econosil, Cig lOp, 250x22mm). For the identification of SF-x, a combination of spectroscopic techniques was used. Electrospray ionization in the mass spectrometer (HPllOO LC/MSD, Hewlett-Packard) with positive ionization mode gave a pseudo-molecular ion with m/z=439. H-NMR, C-NMR, DEPT, HMQC, and COSY spectra were recorded on a Varian-300 (300MHz) spectrometer. Analysis of the COSY spectram showed the presence of a lavandulyl (5-methyl-2-isopropenyl-hex-4-enyl) side chain. From the HMQC spectrum and a DEPT experiment, it appeared that SF-x possesses a disubstituted flavanone skeleton. 

Heteronuclear chemical shift correlation methods establish the direct link between protons and the respective, directly attached carbons (or nitrogens). In the case of methylenes with inequivalent (anisochronous) protons, the "multiplicity of the carbon in question is irrefutably obvious. For isotropic methylenes and other resonances, the multiplicity of the resonance (CH, CH2 or CH3) in question may be less obvious. Early work by Kessler and co-workers addressed this issue via the development of the DEPT-HMQC experiment. [120] Multiplicity editing is also available for experiments such as GHSQC. An extra pair of delays and pulses, with the flip angle of the proton pulse being adjustable, allow the acquisition of data in... 

At present, several experiments are available for inverse-detected one-bond heteronuclear shift correlation. The HMQC experiment described by Bax and Subramanian (1986) has probably been most widely employed. Alternatives, however, are available in the form of DEPT-HMQC (Kessler et al. 1989b) and the HSQC or so-called Overbodenhausen experiment (Boden-hausen and Ruben 1980). For alkaloids with highly congested proton spectra, DEPT-HMQC may be a useful alternative to HMQC, because it allows the acquisition of edited correlation spectra. For investigators interested in correlation of protons to alkaloidal nitrogen atoms via one or two bonds, HSQC or a doubly refocused variant may be the preferred choice. 

Fig. 4. The DEPT-HMQC (Kessler et al. 1989b) spectrum of the aliphatic region of ajmaline (5). The adjustable pulse (see Martin and Crouch 1991) was set p = 180° to afford a spectrum in which the methylene resonances have positive intensity and the methine and methyl resonances are negative. Positive methylene responses are presented as solid black contours negative methine and methyl responses are shown as open contours. Practically, the DEPT-HMQC spectrum shown accomplishes the same task as the recently reported GEM-COSY experiment (Domke et al. 1991) but, in our opinion, is more convenient than the GEM-COSY experiment...

The edited HMQC spectra of a simple alkaloid recorded using the DEPT-HMQC experiment are presented in Fig. 4. The spectra were recorded using the condition P = 180° to afford a spectrum with CH2 signals with positive intensity and inverted CH and CH3 responses. Some further interesting modifications of the DEPT-HMQC experiment are possible and will be discussed in the section below describing HMQC with isotropic mixing. 

Fig. 14. Edited DEPT-HMQC-TOCSY spectra of a ajmaline (4). The spectrum was acquired using the pulse sequence shown in Fig. 13 (Seebach et al. 1991) modified to incorporate a 180°/90° pulse pair located as shown by the pulse sequence in Fig.

DEPT-HMQC DEPT-edited HMQC, which allows for the distinction of CH, CHj and CH3 groups. Exclusive selection of these multiplicities is possible with the related HDQC, HTQC and HQQC techniques... 

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