Animated Solution for Chemistry - Biomolecules: The total number of distinct naturally occurring amino acids obtained by complete acidic hydrolysis of the peptide shown below is

A Journey Through Acidic Hydrolysis

Imagine you are a molecular detective, and you've just been handed a highly complex, heavily modified peptide structure. Your mission? To break it down completely using acidic hydrolysis and identify the standard, naturally occurring building blocks hidden within. This problem tests not just your ability to recognize functional groups, but your structural intuition in organic chemistry.

Before we dive into the molecule, let's establish our primary tool: complete acidic hydrolysis. When a peptide is subjected to strong aqueous acid and heat, the robust amide bonds (the peptide linkages, $-CO-NH-$) are systematically cleaved.

Water molecules, catalyzed by the acid, attack the carbonyl carbons, ultimately breaking the chain and restoring the original amino and carboxyl groups. Our goal is to mentally perform this reaction on every single amide bond in the provided structure.

Let's start our analysis with the central linear portion of the molecule. As we trace the zig-zag backbone, we identify repeating units connected by amide bonds.

If we cleave these bonds, we isolate the central amino acid residues. Looking closely at their side chains, we see a central carbon bonded to three methyl groups—a tert-butyl group. This means the amino acid is tert-Leucine ($H_{2}N-CH(C(C{H}_3{)}_3)-COOH$).

While this is a perfectly valid amino acid chemically, we must ask: is it naturally occurring? The answer is no. The standard 20 amino acids encoded by our DNA include Leucine (with an isobutyl group) and Isoleucine (with a sec-butyl group), but tert-Leucine is a synthetic creation. So, our tally of naturally occurring amino acids remains at zero.

Now, let's turn our attention to the intimidating cyclic structures at both ends of the main chain. These six-membered rings containing two nitrogen atoms and two carbonyl groups are classic examples of diketopiperazines.

Diketopiperazines are formed when two amino acids condense head-to-tail, creating a cyclic dipeptide. To find out which amino acids formed them, we simply reverse the process by hydrolyzing the two amide bonds within the ring.

Analyzing the left ring, we trace the backbone and find two distinct segments: 1. A $-NH-C{H}_2-CO-$ segment. Upon hydrolysis, this becomes $H_{2}N-C{H}_2-COOH$, which is Glycine. 2. A $-NH-CH(R)-CO-$ segment, where the R group is a cyclopropylmethyl group. This yields Cyclopropylalanine.

Glycine is the simplest of all standard, naturally occurring amino acids. We've found our first one! Cyclopropylalanine, however, with its strained three-membered ring, is synthetic.

The right ring follows a very similar structural motif. Hydrolyzing its amide bonds also yields Glycine and a cyclopropyl-containing synthetic amino acid derivative.

We have completely dismantled the complex peptide. Let's review our inventory of distinct amino acids: - tert-Leucine (Synthetic) - Cyclopropylalanine (Synthetic) - Glycine (Naturally occurring)

The question specifically asks for the total number of distinct naturally occurring amino acids. Out of all the fragments generated, only Glycine fits this strict criterion.

Therefore, the final answer is elegantly simple: 1.