Techniques of peptide chemistry

TECHNIQUES OF PEPTIDE CHEMISTRY Methods of Structural Elucidation 

The methods employed to deduce the structures of peptides have been reviewed by several authors and are not commented on here. These methods, when applied to peptides of animal origin with their limited number of amino acids, can be used with a minimum amount of peptide and the released amino acids or their reaction products can usually be identified unequivocally by chromatographic procedures. In the case of peptide antibiotics with their many uncommon constituents, the characterization of these components is an additional task. Even the amino acids themselves need a careful investigation. They may be derivatives of common amino acids, they may be secondary degradation products from unusual components, or they may have D-configuration. Therefore the components have usually to be isolated in amounts which allow a careful chemical and physicochemical characterization, and often direct comparison with synthetic compounds is necessary. 

Another difficulty often encountered in the field of peptide antibiotics is the proof of homogeneity. Most of these antibiotics are produced as a group of closely related compounds, which are difficult to separate. Here, extensive countercurrent distribution and the method of multiple dialysis are very valuable. 

The determination of the molecular weight has also often led to difficulties. Freezing-point depression, diffusion and sedimentation in the ultracentrifuge have sometimes led to discrepancies owing to association or solvents of crystallization. The partial substitution method of Battersby and Craig , the thermoelectric method of Simon and Tomlinson , mass-spectrometric measurements and x-ray analysis have been used recently to overcome such difficulties. 

Final evidence for the accuracy of the proposed structure is usually 

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