ACCORD-HMQC experiment,

In Check it 5.5.2.2 the ACCORD-HMQC experiment is simulated [5.176]. This experiment is characterized by a low sensitivity of resulting signals to varying X, coupling constants, but an unwanted modulation in the f 1 dimension that is a function of the incrementation and the underlying heteronuclear coupling constant. The ACCORD-HSQC experiment [5.177] has been suggested to overcome this modulation in the fl dimension. 

Using the configuration file ch5522.cfg run the simulation of the H ACCORD-HMQC experiment for a spin system of four f ciH groups with different nj(i3c, H) values 1,2.5, 7.5 and 14 Hz. For comparison simulate the common gradient selected 13C, 1H HMBC experiment. 

In addition to the importance of parameter selection on F skew, the ACCORD-HMBC experiment has also been shown to exhibit strong coupling artifact responses similar to those observed in heteronuclear 2D J-resolved spectra. - Finally, the triplet character of responses in the second frequency domain superimposed over the Fi skew was mathematically accounted for by Zangger and Armitage in their development of the ACCORD-HMQC experiment. "... 

The spectral assignment process, complex as it is, is only a prelude to the use of NMR data to assist in the final determination of the three-dimensional structure of the protein. Here, the most important experiment is NOESY, often in conjunction with HMQC or other methods for spreading out the 2D NOESY peaks in three or four dimensions. For larger proteins 4D experiments are essential to spread the NOESY peaks according to both 13C and 15N chemical shifts. We... 

The HMQC sequence does not necessarily have to be based on the coherence transfer step shown in the sequence scheme above. The coherence transfer can also be generated by a DEPT element which has the advantage in a phase sensitive experiment of labelling the signal phase according to the multiplicity of the IH spin groups. 

NMR spectroscopy has been extensively used for determining the carbon framework of Amaryllidaceae alkaloids, and the major contributions are due to Crain et al (203), Zetta and Gatti (210), and Frahm et al. (143). The assignments are made on the basis of chemical shifts and multiplicities of the signals (by DEPT experiment). The use of 2D NMR tecniques such as HMQC and HMBC allow the assignments to be corroborated. Table 5 shows a compilation of the different NMR spectra classified according to the different types. 

Three dimensional techniques should only be applied in case of overlap in optimally recorded 2D spectra. The basic principle of these experiments is simply to merge two 2D experiments. The addition of a third frequency domain increases the spectral resolution, and gives additional information. The 3D HMQC-COSY spectrum, for example, consists of a set of 2D H- H COSY maps separated according to the 13C chemical shift along the third frequency dimension. Thus moving across l3C, H and H, H planes makes unequivocal assignments possible [66]. 

Synthesis of Reference Compounds. Synthesis of THpCs were according to the methods described before (27, 22), and assignment of NMR signals was confirmed by GROESY, H-H COSY, HMQC, and HMBC experiments. 

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