Amino Acids, Peptides and Proteins | Organic Chemistry 3
Structure and Properties of Amino Acids
Amino acids: carboxylic acids bearing an amine group
α-amino acids (or 2-amino acids): amine group on the α-carbon. These are the most common amino acids in nature and the α-carbon has the S configuration:
Properties of amino acids:
Amine (pKa ~ 9-10) and carboxylic acid (pKa ~ 2-3) groups ⇒ amino acids are both basic and acidic. The structure of an amino acid depends on the pH:
Isoelectric pH (or isoelectric point pI): pH with the maximum number of amino acids with net zero charge
Synthesis of Amino Acids
Hell-Volhard-Zelinsky Synthesis:
Mechanism:
Gabriel Synthesis:
Mechanism:
Strecker synthesis:
All these methods produce amino acids in racemic form. Optically pure amino acids can be obtained by:
- resolution of a racemic mixture (resolution of their diastereomeric salts)
- enantioselective formation of the C2 stereocenter
Peptides and Proteins
Peptides and proteins are polypeptides (polymers of α-amino acids). Polypeptides are formed by repeated reaction between COOH and NH2 to form amide bonds called peptide bonds:
Polypeptides are characterized by their sequence of amino acid from the amino end (left) to the carboxy end (right). They are organized in 4 different structures:
- primary structure ⇒ sequence of amino acids
- secondary structure (pleated sheet + α helix) ⇒ folding of the primary structure
- tertiary structure (pleated sheet + α helix) ⇒ further folding and coiling
- quaternary structure ⇒ aggregation of several polypeptides
Amino Acids Sequencing
Amino acids sequencing is a technique for determining the primary structure of polypeptides
- Purification of the polypeptide:
gel-filtration chromatography, ion-exchange chromatography, electrophoresis ...
- Determination of which the amino acids are present
- complete hydrolysis
- amino acid analysis (amino acid analyzer)
- Sequencing from the amino end to the carboxy end
Repeated Edman degradation ⇒ access to the sequence of short polypeptides
For long polypeptides: cleavage with enzymes then Edman degradation
Edman degradation:
Synthesis of Polypeptides
The synthesis of polypeptides consists in a coupling between an amino-protected amino acid and a carboxy-protected amino acid:
Mechanism:
- Protection of the carboxy group: esterification
- Protection and deprotection of the amino group:
Merrifield Solid-Phase Peptide Synthesis
The synthesis of polypeptides has been automated: solid-phase synthesis using a solid support of polystyrene ⇒ a carboxy-anchored peptide chain is built up from amino protected monomers
Mechanism:
- Cycles of coupling and deprotection:
Attachment of protected amino acid:
Deprotection of amino terminus:
- At the end of the synthesis, disconnection from polymer: