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

Organic Chemistry 3 - Quiz - Amino Acids, Peptides, Proteins - 1


Which of the following statements about amino acids is incorrect?

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


What is the dominant form of an amino acid in a basic aqueous solution (pH > 13)?

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:


What is the isoelectric pH of glycine (pKaCOOH = 2.3; pKaNH2 = 9.7)?

Isoelectric pH (or isoelectric point pI):

pI=pKa COOH + pKa NH22=2.3 + 9.72=6.0 


At pH > pI, the population of amino acids with neutral R groups in solution will have:


Which of the following structures describes the overall 3D folding of a polypeptide?

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)


Which of the following statements about polypeptides is incorrect?

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


Which of the following reactions results in the formation of an α-amino nitrile from an aldehyde?

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


What is the final step in the Merrifield synthesis?

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.


What is the role of dicyclohexylcarbodiimide (DCC) in peptide bond formation?

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.


How are protected amino acids coupled during peptide synthesis?

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.