Diagonal signals

The structure of the dimer can be derived simply by evaluation of the cross signals in the HH COSY plot. The cycloalkene protons form two AB systems with such small shift differences that the cross signals lie within the contours of the diagonal signals. 

Locahzed motion can also lead to local variations in correlation times. Folded peptides with unfolded C- or N-terminal residues, for example, will have varying correlation times for the rigid and flexible parts of the molecule, resulting in different cross-relaxation rates. Such effects can usually be distinguished by the Unewidths and intensities of the corresponding diagonal signals, since the autorelaxation rates also depend on the correlation time. 

The DQMAS-type experiments have the distinct advantage over the SQ-SQ spectra in that they are not dominated by the diagonal signals representing the non-exchanged magnetization. The first demonstrations of such spectra reported by... 

The NOESY spectrum has a diagonal set of peaks (open circles) as well as peaks which are off the diagonal (filled circles). The off-diagonal peaks occur at positions where a proton on the Fi axis is close in space to a one on the F2 axis. In the schematic spectrum on the right, the off-diagonal signals show that Ha must be located near Hd and Hb must be located near He. 

Finally, for an example of the effect of a single distance restraint on the final model, take another look at Fig. 10.6 and Plate 14 c. In Fig. 10.6, one of the NOESY off-diagonal signals is labeled F89N, . This notation means that the... 

In the present context, the COSY experiment is best performed with a pulse sequence (i), which enhances small coupling and reduces diagonal signals [84]. 

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