Describe the term D- and L-configuration used for amino acids with examples.

Explanation

### Step-by-Step Text Solution:

1. **Understanding Chiral Centers in Amino Acids**:
- Amino acids are organic compounds that serve as the building blocks of proteins. Most amino acids are classified as alpha amino acids, which means they have an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain) attached to a central carbon atom (the alpha carbon).
- The presence of four different groups attached to the alpha carbon makes it a chiral center, allowing for optical activity.

2. **Optical Activity and Exceptions**:
- Optical activity refers to the ability of a compound to rotate plane-polarized light. Since most alpha amino acids are chiral, they exist in two enantiomeric forms (D and L forms).
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