Most of the `alpha`-amino acids exist as .............

Explanation

**Step-by-Step Solution:**

1. **Understanding Alpha Amino Acids**:
- Alpha amino acids are characterized by having an amino group (-NH2) and a carboxylic acid group (-COOH) attached to the same carbon atom, known as the alpha carbon.

2. **Structure of Alpha Amino Acids**:
- The general structure of an alpha amino acid includes:
- A carboxylic acid group (-COOH)
...