AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 759 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 ACⅤF AFGV AFVG AVGE AVG GAFV GAVF GFAV GVA GVFA GVAF FAGV FAVG FVAG FVGA FGAF FGFA VAGF VAFGⅤ GAF GFAⅤFA 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, CBH_ CH2. If the dipeptide isolated after treatment with chymotrypsin contains valine (V) and phenylalanine(F), its sequenc beⅤF H3 NCHC--NHCHC-—计 Rest of peptide chymotrypsin NCHC-NHCHCO+ Rest of peptide (CH3)2CH CHCH (CH3),CH H,C6H5 alanine Valinylphenylalanine(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 identified as a phenylthiohy danton derivative. The first Edman degradation of Val-Phe-Gly-Ala gives the phenylthiohydantoin derived from valine; the second gives the phenylthiohydantoin derived from phenylalar CaH CsH second Val-Phe-Gly-Ala Edman degradation Phe-Gly-Ala SAo Edman degradation. Gly-Ala CH(CH3)2 CH, CAH5 27.16 Lysine has two amino groups. Both amino functions are converted to amides on reaction with benzyloxycarbonyl chloride H2NCHCO2+ 2C6H_ CH,OCCI C6HS CH,OCNHCHCO,H H,NCH, CH, CHCH C6HSCHOCNHCH, CH,CH,CH We therefore need to protect the amino group of alanine and the carboxyl group of leucine leucine 27 17 The peptide bond of Ala-Leu connects the carboxyl group of alanine and the amino group of Protect the amino group of alanine as its benzyloxycarbonyl derivative H3NCHCO2 CHsCH,OCCl CAHCH,OCNHCHCO,H CH Alanine Benzyloxycarbonyl Z-Protected alanine Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website27.13 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, C6H5CH2 —. 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 identified as a phenylthiohy￾dantoin derivative. The first Edman degradation of Val-Phe-Gly-Ala gives the phenylthiohydantoin derived from valine; the second gives the phenylthiohydantoin derived from phenylalanine. 27.16 Lysine has two amino groups. Both amino functions are converted to amides on reaction with benzyloxycarbonyl chloride. 27.17 The peptide bond of Ala-Leu connects the carboxyl group of alanine and the amino group of leucine. We therefore need to protect the amino group of alanine and the carboxyl group of leucine. Protect the amino group of alanine as its benzyloxycarbonyl derivative. H3NCHCO2 CH3 Alanine C6H5CH2OCCl O Benzyloxycarbonyl chloride C6H5CH2OCNHCHCO2H CH3 O Z-Protected alanine O H2NCHCO2 2C6H5CH2OCCl O O C6H5CH2OCNHCHCO2H C6H5CH2OCNHCH2CH2CH2CH2 H2NCH2CH2CH2CH2 Val-Phe-Gly-Ala Phe-Gly-Ala first Edman degradation second Edman degradation S HN CH(CH3)2 O Gly-Ala S HN CH2C6H5 N O C6H5 N C6H5 CH2C6H5 NHCHC O H3NCHC O (CH3)2CH Valine Phenylalanine CH2C6H5 NHCHCO O H3NCHC O (CH3)2CH Valinylphenylalanine (VF) Rest of peptide Rest of peptide chymotrypsin AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 759 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website