Proteins are high molecular mass complex biomolecules of amino acids. The important proteins required for our body are enzymes, hormones, antibodies, transport proteins, structural proteins, contractile proteins etc. Except for glycine, all `alpha`-amino acids have chiral carbon atom and have L-configuration. The amino acids exists as dipolar ion called zwitter ion, in which a proton goes from the carboxyl group to the amino group. A large number of `alpha`-amino acids are joined by peptide bonds forming polypeptides. The peptides having very large molecular mass (more than 10,000) are called proteins. The structure of proteins is described as primary structure giving sequence of linking of amino acids, secondary structure giving manner in which polypeptide chains are arranged and folded, tertiary structure giving folding, coiling or bonding polypeptide chains producing three dimensional structures and quaternary structure giving arrangement of sub-units in an aggregate protein meolecule.
Which of the following `alpha`-amino acid does not form optical isomers?

Options

A. `N^(+)H_(3)CH(CH_(3))COO^(-)`
B. `N^(+)H_(3)CH_(2)COO^(-)`
C. `N^(+)H_(3)CH(CH_(2)Ph)COO^(-)`
D. `N^(+)H_(3)CH[CH(CH_(3))_(2)]COO^(-)`

Answer

To determine which α-amino acid does not form optical isomers, we need to understand the concept of chirality and optical isomers.

### Step 1: Understand Chirality
- Optical isomers (enantiomers) are formed when a molecule has a chiral center, which is typically a carbon atom bonded to four different groups.
- For an amino acid to be chiral, it must have a carbon atom that is attached to four different substituents.

**Hint:** Look for the presence of a chiral carbon in the amino acid structure.

### Step 2: Analyze the Given Amino Acids
- **Alanine:** This amino acid has a chiral center because its central carbon is attached to an amino group, a carboxyl group, a hydrogen atom, and a methyl group (–CH3). Therefore, alanine can form optical isomers.

- **Glycine:** Glycine is unique because its side chain is a hydrogen atom. The central carbon of glycine is attached to two hydrogen atoms, an amino group, and a carboxyl group. Since it does not have four different substituents, it does not have a chiral center and thus cannot form optical isomers.

**Hint:** Check the side chains of the amino acids to identify if they provide different substituents.

- **Phenylalanine:** This amino acid has a chiral center due to its side chain (a phenyl group), which is different from the other groups attached to the central carbon.

- **Valine:** Valine also has a chiral center because its side chain (–CH(CH3)2) provides different substituents.

**Hint:** Compare the side chains of the amino acids to determine if they have a chiral center.

### Step 3: Conclusion
Based on the analysis:
- The only α-amino acid among the options that does not form optical isomers due to the absence of a chiral center is **glycine**.

**Final Answer:** Glycine does not form optical isomers.