Organic Chemistry

964 CHAPTER 23 Amino Acids, Peptides, and Proteins

23.2 Configuration of Amino Acids

The of all the naturally occurring amino acids except glycine is an asymmet-

ric carbon. Therefore, 19 of the 20 amino acids listed in Table 23.1 can exist as enan-

tiomers. The Dand Lnotation used for monosaccharides (Section 22.2) is also used for

amino acids. The Dand Lisomers of monosaccharides and amino acids are defined the

same way. Thus, an amino acid drawn in a Fischer projection with the carboxyl group

on the top and the R group on the bottom of the vertical axis is a D-amino acidif the

amino group is on the right and an L-amino acidif the amino group is on the left. Un-

like monosaccharides, where the Disomer is the one found in nature, most amino acids

found in nature have the Lconfiguration. To date,D-amino acid residues have been

found only in a few peptide antibiotics and in some small peptides attached to the cell

walls of bacteria.

Why D-sugars and L-amino acids? While it makes no difference which isomer nature

“selected”to be synthesized, it is important that the same isomer be synthesized by all

organisms. For example, if mammals ended up having L-amino acids, then L-amino

acids would need to be the isomers synthesized by the organisms upon which mammals

depend for food.

CH

CH 2 OH

O

H OH

CO−

R

O

NH 3

+

H

CH

CH 2 OH

O

HO H

CO−

R

O

H 3 N H

+

D-glyceraldehyde

D-amino acid

L-glyceraldehyde

L-amino acid

a-carbon

AMINO ACIDS AND DISEASE

The Chamorro people of Guam have a high inci-

dence of a syndrome that resembles amyotrophic

lateral sclerosis (ALS) with elements of Parkinson’s disease

and dementia. This syndrome developed during World War II

when, as a result of food shortages, the tribe ate large quantities

of Cycas circinalisseeds. These seeds contain -methylamino-

L-alanine, an amino acid that binds to glutamate receptors.

When monkeys are given -methylamino-L-alanine, they de-

velop some of the features of this syndrome. There is hope that,

by studying the mechanism of action of -methylamino-

L-alanine, we may gain an understanding of how ALS and

Parkinson’s disease arise.

b

b

b

alanine

an amino acid

PROBLEM 2

a. Which isomer—(R)-alanine or (S)-alanine—is D-alanine?

b. Which isomer—(R)-aspartate or (S)-aspartate—is D-aspartate?

c. Can a general statement be made relating Rand Sto Dand L?

PROBLEM 3

Which amino acids in Table 23.1 have more than one asymmetric carbon?

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