Amino acid analysis of peptides

FIGURE 19.3 Amino acid analysis of an equimolar mixture of 17 amino acids. 

Show the structure of the product you would expect to obtain by Sn2 reaction of a cysteine residue with iodoacetic acid. 

Show the structures of the products obtained on reaction of valine with ninhydrin. 

With the identities and relative amounts of amino acids known, the peptide is then sequenced to find out in what order the amino acids are linked together. Much peptide sequencing is now done by mass spectrometry, using either electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) linked to a time-of-flight (TOP) mass analyzer, as described in Section 10.4. Also in common use is a chemical method of peptide sequencing called the Edman degradation. 

The general idea of peptide sequencing by Edman degradation is to cleave one amino acid at a time from an end of the peptide chain. That terminal amino acid is then separated and identified, and the cleavage reactions are repeated on the chain-shortened peptide until the entire peptide sequence is known. Automated protein sequencers are available that allow as many as 50 repetitive sequencing cycles to be carried out before a buildup of unwanted by-products interferes with the results. So efficient are these instruments that sequence information can be obtained from as little as 1 to 5 picomoles of sample—less than 0.1 /ug. 

---

Automated amino acid analysis of peptides containing asparagine (Asn) and glutamine (Gin) residues gives a peak corresponding to ammonia. Why ... 

Peterson, J. A., Lorenz, L. J., Risley, D. S., and Sandmann, B. J., Amino acid analysis of peptides using HPLC with evaporative light scattering detection, /. Liq. Chromatogr. Related Technol., 22, 1009, 1999. 

Light, A., and E. L, Smith Amino acid analysis of peptides. and proteins. In, The Ih-oteins Compo.sition, Structure. Function". Vol. 1, 2—44. Ed. II. Neurath, Academic Press, 2nd Ed. 1963. 

As already mentioned the EP wants to replace old TEC tests with separation methods of higher efficiency for example, the purity of amino acids is currently evaluated by a TEC test for ninhydrin-positive substances that is only able to find and limit amino acids to 0.5%. However, this test is only valid in the case the amino acids are produced by the cleavage of peptides/proteins and purification. The ninhydrin method is also used in the amino acid analysis of peptides, utilizing a cation-exchange chromatography with a post-column derivatization and a subsequent UVA is detection. This method is often used in industries for purity evaluation of amino acids. 

Source From Amino acid analysis of peptides using HPLC with evaporative light scattering detection, in J. Liq. Chromatogr. Relat. Technol. ... 

---