Terminal Residues and Peptides

The terminal residues of proteins differ from other residues in the chain, since they contain free amino or free carboxyl groups, and this fact may be used to identify them. Fox (1945) has reviewed the earlier literature on the study of terminal residues. At that time the position of only one amino acid in one protein was known. This was the presence of phenylalanine as an W-terminal residue in insulin. It was identified by Jensen and Evans (1935) who isolated the phenylhydantoin of phenylalanine from a hydrolyzate of insulin that had been treated with phenylisocyanate. More recently a general method has been worked out for the study of iV-terminal residues, and preliminary investigations have been carried out on three methods of stepwise degradation which promise to be of great use in the future. 

The principle of this general method (Sanger, 1945 Porter and Sanger, 1948) for the identification and estimation of the W-terminal residues of proteins may be summarized by the following formulae  

The FDNB reacts with the free amino groups of the protein under mild (slightly alkaline) conditions where the peptide bond is quite stable. On hydrolysis of the protein the JV-terminal residues are liberated in the form of DNP amino acids. These are bright yellow compounds that can be extracted with an organic solvent, fractionated chromatographically, and estimated colorimetrically. The accuracy varies somewhat with the particular amino acids involved, due to differences in the stability of the DNP derivatives. In most cases the N-terminal residues of proteins and peptides may be estimated to within 10-15%. 

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