Question

An amino acid A, isolated from the acid-catalyzed hydrolysis of a peptide antibiotic, gave a positive ninhydrin test and had a specific optical rotation (HCl solution) of 37.5 mL/(g·dm). Compound A was not identical to any of the 20 essential amino acids. The isoelectric point of compound A was found to be 9.4. Compound A can be prepared by the reaction of L-glutamine with Br2 in NaOH followed by acidification. Suggest a structure for A.

Answer 1

The answer could be 23

Answer 2

A possible structure for compound A is γ-Bromoglutamic acid (2-amino-4-bromobutanedioic acid).

What is this structure?

Ninhydrin test: A positive ninhydrin test indicates the presence of a primary or secondary amine. Glutamine has a primary amine, which would explain the positive test.

Specific optical rotation: The specific rotation (37.5 mL/(g·dm)) suggests chirality, which glutamine also possesses.

Not one of the 20 essential amino acids: This eliminates most common amino acids.

Isoelectric point (pI) of 9.4: This is consistent with γ-bromoglutamic acid, as the additional bromine group introduces a new acidic side chain (pKa ≈ 4.5) that lowers the pI compared to glutamine (pI ≈ 5.6).

Synthesis from L-glutamine and Br2/NaOH: This reaction is known to selectively brominate at the γ-carbon of glutamine, making it a plausible route for obtaining γ-bromoglutamic acid.