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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.
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
C6H5CH2 (d ) H CO2 There is only one stereogenic center in Gly-Glu. It has the L (or S ) conﬁguration.
H3N (e) N
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.
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
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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