Alanine and leucine are examples of ______ amino acids.

• A. polar
• B. nonpolar
• C. acidic
• D. basic

Answer: (B)

Verified step by step guidance

1. Step 1: Begin by understanding the classification of amino acids based on their side chain properties. Amino acids can be categorized as polar, nonpolar, acidic, or basic depending on the chemical nature of their R-group (side chain).
2. Step 2: Examine the structures of alanine and leucine. Alanine has a methyl group (-CH3) as its side chain, and leucine has an isobutyl group (-CH2-CH-(CH3)2) as its side chain. Both of these side chains consist entirely of hydrocarbons, which are hydrophobic and lack polar functional groups.
3. Step 3: Recall that nonpolar amino acids have side chains that are hydrophobic and do not interact favorably with water. Since alanine and leucine have hydrocarbon side chains, they fall into the category of nonpolar amino acids.
4. Step 4: Eliminate the other options. Polar amino acids have side chains with groups that can form hydrogen bonds (e.g., hydroxyl, amide, or sulfhydryl groups). Acidic amino acids have negatively charged side chains at physiological pH (e.g., aspartate and glutamate), and basic amino acids have positively charged side chains (e.g., lysine, arginine, and histidine). Alanine and leucine do not fit these descriptions.
5. Step 5: Conclude that alanine and leucine are examples of nonpolar amino acids based on their hydrophobic side chains.