Terminal amino acids, reversed order

The chemical scheme for C-terminal sequencing is shown in Figure 2. The first step involves treatment of the peptide or protein sample with diisopropylethylamine in order to convert the C-terminal carboxylic acid into a carboxylate salt. Derivatization of the C-terminal amino acid to a thiohydantoin is accomplished with diphenylisothiocyanatidate (liquid phase) and pyridine (gas phase). The peptide is then extensively washed with ethyl acetate and acetonitrile to remove reaction by-products. The peptide is then treated briefly with gas phase trifluoroacetic acid, followed by water vapor in case the C-terminal residue is a proline (this treatment has no effect on residues which are not proline). The derivatized amino acid is then specifically cleaved with sodium or potassium trimethylsilanolate to generate a shortened peptide or protein which is ready for continued sequencing. In the case of a C-terminal proline which was already removed by water vapor, the silanolate treatment merely converts the C-terminal carboxylic acid group on the shortened peptide to a carboxylate. The thiohydantoin amino acid is then quantitated and identified by reverse-phase HPLC. 

Moreover, those workers have also studied a series of peptides with reversed order of the terminal amino acids, H-Tyr-(Pro)n Trp OH. They found that in this case, the slopes of In fcet versus number, n, of Pro residues in the bridge were somewhat different than for the former series, and they attributed this difference to directional specificity of LRET along the peptide backbone... 

Six possible tiipeptides can form from these three amino acids. They are (1) Ser-Gly-Ala (2) Gly-Ala-Ser (3) Ala-Ser-Gly (4) Ala-Gly-Ser (5) Ser-Ala-Gly (6) Gly-Ser-Ala. Notice that bonding the amino acids in reverse order results in a different molecule because the N-tenninal and the C-terminal ends reside on different amino acids. For example, in Ser-Gly-Ala, the N-terminal amino acid is Ser (conventionally drawn on the left) and the C-terminal side is Ala. In Ala-Gly-Ser, in contrast, the N-terminal amino acid is Ala and the C-terminal one is Ser. 

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