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Magazine Chapter 4: Q3CC (page 56)
Suppose you had an organic molecule such as cysteine (see Figure 4.9, sulfhydryl group example), and you chemically removed the 鈥擭H2 group and replaced it with 鈥擟OOH. 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.
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