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Amino Acids

Key Concepts

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Structure of an Amino Acid

The simplest α-amino acid (or 2-amino acid) is glycine (symbol Gly) in which R = H (hydrogen).

H2N - H
|
C
|
H
- COOH

If R = CH3 (methyl group) the amino acid is alanine (symbol Ala)

  CH3
H2N -
 
|
C - COOH
|
H

Serine (Ser) is an amino acid in which R contains an alcohol (hydroxyl or OH) functional group.

  CH2OH
H2N -
 
|
C - COOH
|
H

Aspartic acid (Asp) contains a carboxylic acid functional group.

  CH2-COOH
H2N -
 
|
C - COOH
|
H

Cysteine (Cys) contains sulfur.

  CH2-SH
H2N -
 
|
C - COOH
|
H

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Zwitterions

The carboxylic acid group being acidic tends to lose a proton, and the amine group being basic tends to gain a proton.

Amino acids in solution exist as an equilibrium mixture of neutral molecules and dipolar ions called zwitterions:

H2N - R
|
C
|
H
- COOH
neutral molecule
+H3N - R
|
C
|
H
- COO-
zwitterion

In acid solution the zwitterion converts to a positively charged ion:

+H3N - R
|
C
|
H
- COO-
zwitterion
+ H3O+
+H3N - R
|
C
|
H
- COOH
positive ion
+ H2O

By Le Chatelier′s Principle, the more acidic the solution, the more positive ions will be formed.

In alkaline solution the zwitterion converts to a negatively charged ion:

+H3N - R
|
C
|
H
- COO-
zwitterion
+ OH-
H2N - R
|
C
|
H
- COO-
negative ion
+ H2O

By Le Chatelier′s Principle, the more alkaline the solution, the more negative ions will be formed.

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