Observing Spin Systems using COSY

COSY (Correlated Spectroscopy) is a 2D homonuclear (1H, 1H)-correlated NMR experiment that allows coupling proton partners to be identified. For many molecules, proton coupling is directly related to the carbon skeleton connectivity and therefore COSY is one of the most useful 2D NMR experiments.

COSY dipeptide

Typically, COSY correlations are detected for coupling over 2 or 3 bonds, meaning the protons have a geminal or vicinal relationship. Functional groups that do not have a proton available for coupling, for example carbonyls, cause a ‘road-block’ in the coupling. This breaks the molecule up into fragments that are known as spin systems. Spin systems often form ‘puzzle pieces’ in structural elucidation that can be pieced together using other 2D NMR experiments.

Spin systems observed in COSY spectra can also be used to identify the amino acids in simple peptides. Amino acids have a characteristic fingerprint in COSY NMR spectra. Peptide bonds act as ‘road-blocks’ and do not allow coupling between amino acid moieties. This means the amino acids in a peptide chain retain their COSY fingerprint, which allows simple identification of the amino acids present. We have demonstrated the ability to identify amino acid moieties in the dipeptide glycyl-L-phenylalanine using the Spinsolve in Observing Spin Systems using COSY.