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Dolbier HW Solutions 664

Dolbier HW Solutions 664 - 758 AMINO ACIDS PEPTIDES AND...

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Unformatted text preview: 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 carbon atom is L. For all amino acids except cysteine, the L configuration corresponds to S. The stereochemistry of Ala-Phe may therefore be indicated for the zigzag conformation as shown. O CH2C6H5 H H3N N CO2 H H3C H (c) The L configuration corresponds to S for each of the stereogenic centers in Ala-Phe. 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. O H3C H H3N C6H5CH2 (d ) H CO2 There is only one stereogenic center in Gly-Glu. It has the L (or S ) configuration. O H3N (e) N H CH2CH2CO2 H N CO2 H 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. O H3N H3NCH2CH2CH2CH2 H (f) N H CO2 The configuration at both -carbon atoms in D-Ala-D-Ala is exactly the reverse of the configuration of the stereogenic centers in parts (a) through (e). Both stereogenic centers have the D (or R) configuration. O H CH3 H3N N H H CH3 27.12 CO2 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 pentapeptides are 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 Website ...
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