NOESY artifacts

Fig. 10.11. 2D NOESY data of astemizole (5). The mixed phase correlations in the aromatic region (boxed) are examples of the artifacts described in the text.

Fig. 3.3 A contour plot of a 750 MHz two-dimensional NOESY spectrum of the mating pheromone Er-22 adapted from Ref. [6]. B Cross section along to, displaying strong artifacts at multiples of A=340 Hz corresponding to a time variation of...

Occasionally, COSY-type artifacts appear in NOESY and ROESY spectra but these are easy to identify by their anti-phase multiplet structure. 

At least three factors complicate the analysis of NOESY spectra. First, COSY signals may be present from scalar couplings and may interfere with interpretations intended to be based entirely on interproton distances. (Vicinal couplings, for example, are largest when the coupled nuclei are farthest apart, a condition that obtains in the antiperiplanar geometry.) Such unwanted spectral features are called artifacts. COSY signals may be reduced through... 

Another reason for the careful selection of RT is that NOESY spectra are susceptible to artifacts from pulsing too rapidly. Problems also occur for molecules (MW 750-2,000) in the crossover region in which NOESY cross-peak intensities approach zero for even... 

Noteworthy results in the phytochemical studies, the N-oxides of several Amaryllidaceae alkaloids have been found for the first time in Amaryllidaceae plants, accompanied by the corresponding free bases (52) ungiminor-ine A-oxide (10) (from Pancratium maritimum), and O-methyllycorenine A-oxide (329) and homolycorine A-oxide (330) (from Lapiedra maritinezii). The stereochemistry of their nitrogen atoms was deduced based on 2D NOESY experiments. Furthermore, it has been established that these A-oxide derivatives are genuine natural products, and not artifacts, by the demonstration that the free bases are not converted into A-oxides when subjected to the same extraction procedure. It is noteworthy that the A-oxides have been isolated accompanied by the corresponding free bases. 

Kazimierczuk and co-workers apphed their semi-automatic CLEAN procedure to suppress artifacts in a randomly sampled N-labeled NOESY-HSQC spectrum of ubiquitin [60]. It was demonstrated that the process does not systematically influence relative peak amplitudes, and is therefore applicable to NOESY spectra. Similar conclusions were later drawn by Stanek and Kozmihski [85], and by Werner-AUen and co-workers [84], who compared their reconstructions with conventionally sampled three-dimensional spectra of the same spectral resolution. The algorithm proposed by Kazimierczuk and co-workers was later also applied to higher-dimensional experiments [80]. 

A more challenging example was demonstrated by Stanek and Kozmihski [85], who applied their algorithm to 3D N- and C-labeled NOESY spectra of ubiquitin without suppression of diagonal peaks. The efficiency of artifact suppression was investigated by comparison of the reconstruction with the conventionally acquired reference spectrum. Less than 2% of peaks were missing, and about 1.5% false peaks were reported. The correlation coefficient between peak volumes of... 

This is a very powerful technique for imaging particles in their native state. However, unfortunately, it is not suitable for big particles and artifacts can also be observed. In contrast, 2D NOESY NMR may provide indirect evidence of a phase-separated nature within a particle based on whether different domains are in close contact or not. However, using this technique, it is not possible to distinguish which kind of anisotropic particles have been prepared. Overall, a combination of the above techniques is required as well as the development of new methods to provide convincing evidence for the formation and characterization of anisotropic particles. 

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