Question

Suppose you had an organic molecule such as cysteine (see Figure 4.9, sulfhydryl group example), and you chemically removed the —NH₂ group and replaced it with —COOH. Draw this structure. How would this change the chemical properties of the molecule? Is the central carbon asymmetric before the change? After?

Short answer

If any basic molecule is replaced with an acidic molecule, the structure will have enhanced acidic properties.

The molecule's conformation would also change, changing the chemical properties and interaction with different molecules.

The original cysteine molecule has an asymmetric carbon in the center. But when the carboxyl group is placed in its position, the structure will have symmetry.

Step-by-step solution

Structure of Cysteine

Cysteine is a sulfur-containing amino acid. It contains a sulfhydryl group (-SH) attached to the central carbon atom, a carboxyl group at one end (-COOH), and an a-amino group at the other end (-NH₂).

Change in the structure of cysteine

When the amino group (-NH₂) is replaced by a carboxyl group (-COOH), the structure looks like the figure given below:

The molecule's shape changes, and thus, it will show a difference in the interactions with other molecules.

Chemical properties of the molecules

An amine group is a chemical group that acts as a base. It is replaced by a carboxyl group that acts as an acid.Thus, the molecule becomes more acidic.

Nature of the central Carbon atom

Before the change, the structure of cysteine has an asymmetrical central carbon atom due to the attachment of three different chemical groups.

But after the change, as it has two ends with the same functional group, i.e., the carboxyl group, it is no longer asymmetrical.