27.13 Twenty-four tetrapeptide combinations are possible for the four amino acids alanine (A), glycine (G), phenylalanine (F), and valine (V). Remember that the order is important; AG is not the same peptide as GA. Using the one-letter abbreviations for each amino acid the possibilities are AGFV AGVF AFGV AFVG AVGF AVFG GAFV GAVF GFAV GFVA GVFA GVAF FAGV FAVG FVAG FVGA FGAF FGFA VAGF VAFG VGAF VGFA VFAG VFGA 27.14 Chymotrypsin cleaves a peptide selectively at the carboxyl group of amino acids that have aromatic side chains. The side chain of phenylalanine is a benzyl group, C 6 H 5 CH 2 — . If the dipeptide isolated after treatment with chymotrypsin contains valine (V) and phenylalanine (F), its sequence must be VF. The possible sequences for the unknown tetrapeptide are VFAG and VFGA. 27.15 The Edman degradation removes the N-terminal amino acid, which is identi f ed as a phenylthiohy-dantoin derivative. The f rst Edman degradation of Val-Phe-Gly-Ala gives the phenylthiohydantoin
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