Which of the following is true of amino acids?

• A. All standard amino acids contain both an amino group (\(-\mathrm{NH}_2\)) and a carboxyl group (\(-\mathrm{COOH}\)) attached to the same carbon atom.
• B. Amino acids do not have chiral centers.
• C. The side chain (R group) is always a methyl group (\(-\mathrm{CH}_3\)).
• D. All amino acids are nonpolar and hydrophobic.

Answer: (A)

Verified step by step guidance

1. Step 1: Begin by understanding the structure of standard amino acids. Standard amino acids are the building blocks of proteins and share a common structure: a central carbon atom (called the alpha carbon) bonded to four groups: an amino group (\(-\mathrm{NH}_2\)), a carboxyl group (\(-\mathrm{COOH}\)), a hydrogen atom, and a variable side chain (R group).
2. Step 2: Evaluate the statement 'All standard amino acids contain both an amino group (\(-\mathrm{NH}_2\)) and a carboxyl group (\(-\mathrm{COOH}\)) attached to the same carbon atom.' This is true because the alpha carbon is the central point where these groups are attached in all standard amino acids.
3. Step 3: Analyze the statement 'Amino acids do not have chiral centers.' This is false because most amino acids (except glycine) have a chiral alpha carbon, meaning the alpha carbon is bonded to four different groups, making it asymmetric.
4. Step 4: Examine the statement 'The side chain (R group) is always a methyl group (\(-\mathrm{CH}_3\)).' This is false because the R group varies among amino acids and determines their unique properties. For example, alanine has a methyl group (\(-\mathrm{CH}_3\)) as its R group, but other amino acids have different side chains, such as hydroxyl (\(-\mathrm{OH}\)) in serine or benzyl in phenylalanine.
5. Step 5: Assess the statement 'All amino acids are nonpolar and hydrophobic.' This is false because amino acids can be polar, nonpolar, charged, hydrophilic, or hydrophobic depending on the nature of their R group. For instance, lysine is positively charged and hydrophilic, while leucine is nonpolar and hydrophobic.