Amino Acids

Key Concepts

20 amino acids are commonly found in proteins.
The 11 amino acids that the human body makes are called non-essential amino acids.
The 9 amino acids that the human body needs but cannot make are called essential amino acids. These must be eaten.
Amino acids contain both an amine (-NH₂) and a carboxylic acid (-COOH) functional group.
-amino acids, or 2-amino acids, are amino acids in which both functional groups are attached to the same carbon atom.
The general formula for a 2-amino acid (-amino acid) is:

H₂N - R
|
C
|
H - COOH

Where R is a carbon containing side chain or branch.
This carbon side chain may also contain sulfur, nitrogen or oxygen.
Amine groups are basic (proton acceptor), carboxylic acid groups are acidic (proton donator).
Amino acids are both amphiprotic (can accept or donate protons) and amphoteric (can react with acids or bases).
A zwitterion is the representation of the dipolar amino acid.
For each amino acid there is a particular pH, the isoelectric point, at which the amino acid exists as the neutral zwitter ion.
Below this pH the amino acid exists as a positively charged ion (cation).
Above this pH the amino acid exists as a negatively charged ion (anion).

The simplest

-amino acid is Glycine (symbol gly) in which R = hydrogen.

H₂N - H
|
C
|
H - COOH

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

CH₃

H₂N -
|
C - COOH
|
H

Serine (Ser) is an amino acid in which R contains an alcohol 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

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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