Quiz - Amino Acids, Peptides, Proteins - 1 | Amino Acids, Peptides and Proteins

Amino acids are carboxylic acids bearing an amino group but they can have additional chemical functions.

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):

$\mathrm{pI}=\frac{{\mathrm{pKa}}_{ \mathrm{COOH}} + {\mathrm{pKa}}_{ \mathrm{NH}2}}{2}=\frac{2.3 + 9.7}{2}=6.0$

Polypeptides 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 (overall 3D folding)

Polypeptides are characterized by their sequence of amino acid from the amino end (left) to the carboxy end (right)

Strecker synthesis converts an aldehyde into an α-amino nitrile.

After the sequential addition of Fmoc-protected amino acids and formation of the peptide chain, the final step in the Merrifield synthesis involves cleaving the polypeptide chain from the polymer support. This is typically achieved using HF (hydrofluoric acid), which releases the peptide from the solid support.

Dicyclohexylcarbodiimide (DCC) acts as a dehydrating agent in peptide bond formation. It converts the carboxyl OH group into a better leaving group, facilitating the nucleophilic attack of the amine on the carbonyl group and promoting the formation of an amide bond between the amino acids.

Protected amino acids are coupled during peptide synthesis by using protecting groups to shield reactive functional groups, allowing selective reactions at desired sites. The protecting groups are later removed in a controlled manner to expose the reactive groups for subsequent coupling steps.