Animated Solution for Chemistry - Organic Chemistry: The number of chiral carbon(s) present in peptide, Ile-Arg-Pro, is …… .

Before we dive into the specific amino acids, let's quickly refresh our memory on what makes a carbon atom chiral.

A chiral carbon (often denoted with an asterisk, $C^{\ast }$) is an $s{p}^3$ hybridized carbon atom that is bonded to four completely different groups. This lack of symmetry is what gives the molecule its "handedness" or chirality.

When amino acids link together to form a peptide bond, the reaction occurs between the $\alpha$-amino group of one amino acid and the $\alpha$-carboxyl group of another. This process forms an amide linkage but does not alter the chirality of the $\alpha$-carbons or any chiral centers in the side chains. Therefore, to find the total number of chiral carbons in the tripeptide Ile-Arg-Pro, we simply need to count the chiral centers in each individual amino acid.

Let's start with the first amino acid in our sequence: Isoleucine.

If we look at the $\alpha$-carbon of Isoleucine, we see it is attached to an amino group ($-N{H}_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and a sec-butyl side chain. Since all four groups are different, the $\alpha$-carbon is indeed chiral.

But wait, we must always check the side chain! The side chain of Isoleucine is a sec-butyl group. Let's examine the $\beta$-carbon (the first carbon of the side chain). It is attached to a methyl group ($-C{H}_3$), an ethyl group ($-C{H}_{2}C{H}_3$), a hydrogen atom ($-H$), and the $\alpha$-carbon of the main chain. That is another set of four distinct groups!

Thus, Isoleucine is special—it contains two chiral centers.

Next up is Arginine.

Looking at its $\alpha$-carbon, it is bonded to an amino group, a carboxyl group, a hydrogen atom, and a long aliphatic side chain that terminates in a complex guanidino group. These four groups are distinct, making the $\alpha$-carbon chiral.

Now, let's inspect the side chain. The side chain consists of three methylene groups ($-C{H}_2-$) followed by the guanidino group. Each of these methylene carbons is bonded to at least two identical hydrogen atoms. Because they do not have four different groups, none of the side chain carbons are chiral.

Therefore, Arginine contributes exactly one chiral center.

Finally, we arrive at Proline, which is famous for its unique cyclic structure.

In Proline, the side chain loops back and forms a covalent bond with the $\alpha$-amino nitrogen, creating a five-membered pyrrolidine ring. Let's evaluate the $\alpha$-carbon. It is attached to a carboxyl group, a hydrogen atom, the nitrogen atom of the ring, and the $\beta$-carbon of the ring. Because the path around the ring is asymmetrical (one way leads to a nitrogen, the other to a carbon), these four attachments are considered different. Hence, the $\alpha$-carbon is chiral.

The other three carbons in the pyrrolidine ring are all methylene groups ($-C{H}_2-$), meaning they each have two identical hydrogen atoms. They are achiral.

So, Proline contributes one chiral center.

Now that we have analyzed each component of the tripeptide, we can easily find the total number of chiral carbons by summing them up:

Total Chiral Carbons = (Chiral Carbons in Ile) + (Chiral Carbons in Arg) + (Chiral Carbons in Pro)

Total Chiral Carbons = $2+1+1=4$

The tripeptide Ile-Arg-Pro contains a total of 4 chiral carbons.