Homonuclear helicate structures

A protein subject to NMR analysis may have 100-200 amino acid residues, which provide a 1H NMR spectrum of many hundreds of lines. Because the amino acid sequence can be assumed to have been determined previously by non-NMR methods, the first step in the NMR study is to assign each line in the spectrum to a specific moiety (NH, ot-CH, side chain CH3, etc.) of a specific amino acid residue. Without the 2D methods that we have discussed, it would be virtually impossible to make such assignments. For relatively small proteins ( 50—100 residues) it is often possible to use conventional homonuclear 2D methods, such as COSY and HOHAHA, to define some bonding paths and to supplement these results with NOE data for residues that are very close in space as a result of secondary structural elements such as a helices. However, for proteins of moderate size such techniques are insufficient, and special methods had to be developed and now constitute the standard method of making sequential assignments. 

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