Spin systems peptide, identification

Correlated spectroscopy (COSY) was among the first two-dimensional (2D) NMR experiment realized [447, 448] and it is still among the most useful NMR experiments. COSY generates cross peaks in the 2D spectrum at the intersection of resonances of coupled spins (Fig. 14.48). In proteins cross peaks are observed for gem-inal, i.e. over two bonds, and vicinal, i.e. over three bonds, protons and in small peptides also couplings over four bonds may be detected. Thus the COSY spectrum allows the identification of spin systems for the assignment. However, apart from peptides, the overlap and degeneracy in chemical shifts is likely to prevent one from obtaining entire spin systems exclusively from the COSY spectrum additional experiments are required. 

The traditional methods of nmr spectroscopy have considerable value in structure determination and were widely used for corroboration of the identity of intermediates in peptide synthesis. A new era was opened up, however, with the development of two dimensional nmr spectroscopy. It is able to make signal assignment in peptides more productive. Thus determination of connectivity between spin systems within the same amino acid allows the identification of individual amino acid residues while homonuclear and heteronuclear couplings across the peptide bond... 

---