Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Synthetic Polymers and Biopolymers
20.8 Amino Acids, Monomers of Peptides and Proteins

Amino acids all have the same basic structural features, which are shown below.

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Amino acids with these features are also referred to as α-amino acids because on the α-carbon (the first carbon next to the acid functionality) is an amino functionality. There are 20 amino acids that occur naturally, and they can be classified based on the nature of the side chain. Table 20.2 shows the classification, the structure of the side chains, R and their abbreviations.

With the exception of glycine, all naturally occurring amino acids are optically active, that is, they will all rotate the plane of polarized light; they are also classified as L-configuration. The assignment of L and D for optically active amino acids is based on the system derived from the assignment of L and D for glyceraldehyde. The D and L means that the OH group that is present on the stereogenic carbon is on the right or left, respectively. The (+) and (−) means that these compounds rotate the plane of polarized light to the right or to the left, respectively. The terms dextrorotatory and levorotatory are also used to represent (+) and (−), respectively.

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For amino acids, if the amino acid is drawn in the Fischer projection with the carboxylic acid group at the top and the R group is at the bottom, then the L and D assignment is based on the position of the amino group (NH2), similar to that of the —OH group in glyceraldehyde. If the amino group is to the left, then the assignment is the L; and if the amino group is to the right, the assignment is the D.

Table 20.2 Side-chain groups of amino acids that occur naturally.

Classification

R

Amino acid

Abbreviations

Alkyl side chain

─H

Glycine

Gly

─CH3

Alanine

Ala

─CH(CH3)2

Valine

Val

─CH2CH(CH3)CHs

Leucine

Leu

─CH2CH2CH(CH3)

Isoleucine

Ile

HO side chain

─CH2CH(OH)CH3

Threonine

Thr

─CH2OH

Serine

Ser

Sulfur side chain

─CH2CH2SH

Cysteine

Cys

─CH2CH2SCH3

Methionine

Met

Aromatic side chain

p-HO-C6H4CH2

Tyrosine

Tyr

C6H5CH2

Phenylalanine

Phe

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Tryptophan

Trp

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Histidine

His

Acidic side chain

─CH2COOH

Aspartic acid

Asp

─CH2CH2COOH

Glutamic acid

Glu

Basic side chain

CH2CONH2

Asparagine

Asn

CH2CH2CONH2

Glutamine

Glu

(CH2)4NH2

Lysine

Lys

CH2CH2CH2NH(NH)NH2

Arginine

Arg

Amino group part of ring

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Proline

Pro

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It should be pointed out that the absolute R and S configurations discussed in Chapter 5 are determined by a different method and hence there is not necessarily a direct correlation between the R and S, and the L and D assignments of these compounds. Shown below is the absolute configuration of the amino acids shown above (the assumption is made that the R group is the third priority).

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Problem 20.3

Determine the absolute configuration (R or S) of the following α-amino acids.

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