Resonance assignment problems

This chapter contains (relatively) simple NMR resonance assignment problems. The structure of the molecule is given, and it is up to the reader to assign the and resonances to the hydrogen and carbon sites in the molecular structure. 

The NMR spectra of this typical vinyl polymer are complicated due to the influence of stereochemistiy. Mirau and Bovey presented a scheme to solve the resolution and resonance assignment problem through the stepwise application of a series of 2D-NMR experiments. The 500-MHz H (top) and 125-MHz C (bottom) spectra of PVC at 65 °C are displayed in Figure 51. The resonance assignments from 2D-NMR data are labeled on these spectra. The CH regions in both spectra are split into three parts due to rr, mr, and mm triads. The CHj regions of both spectra are considerably more complicated. There is more overlap in the NMR and it is hard to interpret. 

More recently, reverse detection HETCOR (25) and long range, constant time HETCOR (26) methods have been successfully applied to the P assignment problem. Because of these methods, assignment of the P resonances of modest-size oligonucleotide duplexes is now quite feasible (19, 27). Thus the assignment problems that initially held back the use of P NMR have now been solved. 

Isotopic labeling plays a very important role in molecular structure determination in SSNMR. It not only enhances the spectral sensitivity and improves the spectral resolution, but also helps with the resonance assignments of NMR spectra and tackles the specific problem with the structure and dynamics of proteins through the designed labeling scheme. 

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