Coupling constants amides

Regions of protein secondary structure are identified via characteristic short-range (i.e. < 5 residues apart) NOEs, coupling constants, amide proton exchange and chemical shift data (Table 3). Longer-range NOEs then define the tertiary structure, which can be refined further against all available data. 

Fig. 8. Amide region of XH NMR spectra for XAO peptide. Each NH peak in the spectrum of XAO is cleanly resolved and allows accurate determination of the coupling constants. The values of 3/hn for A2, A3, A4, and A5 can be determined from unlabeled sample or 15N-labeled sample (with 15N decoupling in the pulse sequence), as seen from the lower panel those for A6 and A7 can be obtained from the sample XA026 (15N-labeled at 2 and 6 positions) without 15N decoupling in the pulse sequence, as seen from upper panel. From Shi et al. (2002). Proc. Natl Acad. Sci. USA 99, 9190-9195, 2002 National Academy of Sciences, USA.

Gridnev et al. showed in their study of the asymmetric hydrogenation of en-amides by Rh-catalysts another useful application of coupling constant patterns. By selectively labeling certain atoms, for example with 13C or 2D, additional couplings appear (as compared to the non-labeled product) and this will provide information about the exact structure [22]. 

The most abundant of the isolated amides was fagaramide (JL). The structure of fagaramide has long been known,but the geometry of its side chain double bond has not been clearly established. This has now been confirmed as trans based on the large coupling constant (16 Hz) in the 400 MHz H-NMR spectrum. 

Correlation of spectral studies for these classes of compounds involve K-band shifts in the UV spectra of 6-substituted 2,4-diamino-pyrimidines,165 NMR chemical shifts of the two amide protons and the 6-proton in 5-substituted uracils170 and of protons in pyrazines,171 MeO 13C—H coupling constants in methoxy-s-triazines,172 and 3iCl NQR in chloropyrimidines.173... 

---