Amino acids can be classified by the -

Options

A. number of peptide bonds
B. Number of disulfide bridges they can form
C. Number of carbon-carbon double
D. Characteristics of their side chains

Answer

### Step-by-Step Solution:

1. **Understanding Amino Acids**: Amino acids are the building blocks of proteins. They contain an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group) attached to a central carbon atom (alpha carbon).

2. **Classification Criteria**: Amino acids can be classified based on the properties of their R groups. The R group determines the characteristics and functions of the amino acids.

3. **Types of R Groups**:
- **Non-polar Aliphatic Group**: These amino acids have R groups that are hydrophobic (water-repelling). An example is Glycine (G), which is the simplest amino acid.
- **Aromatic R Group**: These amino acids contain an aromatic ring (like a benzene ring) in their R group. An example is Tyrosine (Y).
- **Polar Uncharged R Group**: These amino acids have R groups that are polar but do not carry a charge. An example is Serine (S).
- **Negatively Charged R Group**: These amino acids have R groups that carry a net negative charge. An example is Aspartate (D).
- **Positively Charged R Group**: These amino acids have R groups that carry a net positive charge. An example is Lysine (K).

4. **Summary of Classification**:
- Non-polar Aliphatic (e.g., Glycine)
- Aromatic (e.g., Tyrosine)
- Polar Uncharged (e.g., Serine)
- Negatively Charged (e.g., Aspartate)
- Positively Charged (e.g., Lysine)