758 AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS 27.11(b) When amino acid residues in a dipeptide are indicated without a prefix, it is assumed that the configuration at the a carbon atom is L. For all amino acids except cysteine, the L configura tion corresponds to S. The stereochemistry of Ala-Phe may therefore be indicated for the zigzag conformation as shown. H H C The L configuration corresponds to S for each of the stereogenic centers in Ala-Phe. (c) Similarly, Phe-Ala has its substituent at the N-terminal amino acid directed away from us, whereas the C-terminal side chain is pointing toward us, and the L configuration corresponds to s for each stereogenic center. OH,C H H-N C6HsCH, (d) There is only one stereogenic center in Gly-Glu. It has the L (or S) configuration. CH,CH,CO H (e) In order for the N-terminal amino acid in Lys-Gly to have the L (or S)configuration, its side chain must be directed away from us in the conformation indicated H-N H,NCH, CH, CH, CH, H (f) The configuration at both a-carbon atoms in D-Ala-D-Ala is exactly the reverse of the config- uration of the stereogenic centers in parts(a)through(e). Both stereogenic centers have the D H CH 27.12 Figure 27.7 in the text gives the structure of leucine enkephalin Methionine enkephalin differs from it only with respect to the C-terminal amino acid. The amino acid sequences of the two pentapep- Tyr-Gly-Gly-Phe-Leu Tyr-Gly-Gly-Phe-Met Leucine enkephalin Methionine enkephalin The peptide sequence of a polypeptide can also be expressed using the one-letter abbreviations listed in text Table 27.1. Methionine enkephalin becomes YGGFM Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website27.11 (b) When amino acid residues in a dipeptide are indicated without a prefix, it is assumed that the configuration at the carbon atom is L. For all amino acids except cysteine, the L configura￾tion corresponds to S. The stereochemistry of Ala-Phe may therefore be indicated for the zigzag conformation as shown. The L configuration corresponds to S for each of the stereogenic centers in Ala-Phe. (c) Similarly, Phe-Ala has its substituent at the N-terminal amino acid directed away from us, whereas the C-terminal side chain is pointing toward us, and the L configuration corresponds to S for each stereogenic center. (d) There is only one stereogenic center in Gly-Glu. It has the L (or S) configuration. (e) In order for the N-terminal amino acid in Lys-Gly to have the L (or S) configuration, its side chain must be directed away from us in the conformation indicated. ( f ) The configuration at both -carbon atoms in D-Ala-D-Ala is exactly the reverse of the config￾uration of the stereogenic centers in parts (a) through (e). Both stereogenic centers have the D (or R) configuration. 27.12 Figure 27.7 in the text gives the structure of leucine enkephalin. Methionine enkephalin differs from it only with respect to the C-terminal amino acid. The amino acid sequences of the two pentapep￾tides are The peptide sequence of a polypeptide can also be expressed using the one-letter abbreviations listed in text Table 27.1. Methionine enkephalin becomes YGGFM. Tyr-Gly-Gly-Phe-Leu Leucine enkephalin Tyr-Gly-Gly-Phe-Met Methionine enkephalin H3N H O N H H CH3 CH3 CO2 H3N H3NCH2CH2CH2CH2 O N H H CO2 H3N CH2CH2CO2 O N H H CO2 H3N C6H5CH2 OH3C N H H H CO2 H3N H3C O N H H H CO2 CH2C6H5 758 AMINO ACIDS, PEPTIDES, AND PROTEINS. NUCLEIC ACIDS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website