AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 757 27.10 (b) Alanine is the N-terminal amino acid in Ala-Phe Its carboxyl group is joined to the nitrogen of phenylalanine by a peptide bo H3NCHC--NHCHCO Alanine (c) The positions of the amino acids are reversed in Phe-Ala. Phenylalanine is the N terminus and alanine is the C terr HANCHC-NHCHCO, C6HSCH Alanine (d) The carboxyl group of glycine is joined by a peptide bond to the amino group of glutamic acid H3NCHC-NHCHCO CHCH CO The dipeptide is written in its anionic form because the carboxyl group of the side chain is ionized at pH 7. Alternatively, it could have been written as a neutral zwitterion with a CH,CH, CO, H side chain (e) The peptide bond in Lys-Gly is between the carboxyl group of lysine and the amino group of H3NCHC--NHCH, CO, KG H,NCH,CH, CH,CH Glycine The amino group of the lysine side chain is protonated at pH 7, and so the dipeptide is written here in its cationic form it could have also been written as a neutral zwitterion chain H,NCH,CH,Ch,CH (f) Both amino acids are alanine in D-Ala-D-Ala. The fact that they have the d configuration has no effect on the constitution of the dipeptide. H-NCHC-NHCHCO D-A-D-A H3 Alanine Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website27.10 (b) Alanine is the N-terminal amino acid in Ala-Phe. Its carboxyl group is joined to the nitrogen of phenylalanine by a peptide bond. (c) The positions of the amino acids are reversed in Phe-Ala. Phenylalanine is the N terminus and alanine is the C terminus. (d) The carboxyl group of glycine is joined by a peptide bond to the amino group of glutamic acid. The dipeptide is written in its anionic form because the carboxyl group of the side chain is ionized at pH 7. Alternatively, it could have been written as a neutral zwitterion with a CH2CH2CO2H side chain. (e) The peptide bond in Lys-Gly is between the carboxyl group of lysine and the amino group of glycine. The amino group of the lysine side chain is protonated at pH 7, and so the dipeptide is written here in its cationic form. It could have also been written as a neutral zwitterion with the side chain H2NCH2CH2CH2CH2. ( f ) Both amino acids are alanine in D-Ala-D-Ala. The fact that they have the D configuration has no effect on the constitution of the dipeptide. NHCHCO2 d-A-d-A CH3 H3NCHC CH3 O Alanine Alanine H3NCHC KG H3NCH2CH2CH2CH2 O Lysine NHCH2CO2 Glycine H3NCH2C NHCHCO2 GE CH2CH2CO2 O Glycine Glutamic acid H3NCHC FA C6H5CH2 O Phenylalanine NHCHCO2 CH3 Alanine H AF 3NCHC CH3 O Alanine NHCHCO2 CH2C6H5 Phenylalanine AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 757 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website