All of the following are polar amino acids, EXCEPT:
**Core Concept**
Polar amino acids are a class of amino acids that contain a hydrophilic (water-attracting) side chain, which is typically an amino (-NH2) or carboxyl (-COOH) group. These amino acids are often found on the surface of proteins and play a crucial role in interactions with other molecules, including water, ions, and other polar groups.
**Why the Correct Answer is Right**
The correct answer is a non-polar amino acid, which means it does not contain a hydrophilic side chain. Non-polar amino acids tend to be hydrophobic (water-repelling) and are often found in the interior of proteins, where they can form hydrophobic interactions with other non-polar groups. The correct answer is an example of a non-polar amino acid that does not contain a polar side chain.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is a polar amino acid because it contains an amino (-NH2) group, which is a hydrophilic side chain.
**Option B:** This option is also a polar amino acid due to its carboxyl (-COOH) group, which is another type of hydrophilic side chain.
**Option C:** This option is a polar amino acid as well, with a hydroxyl (-OH) group that is a hydrophilic side chain.
**Clinical Pearl / High-Yield Fact**
Remember that polar amino acids tend to be found on the surface of proteins, while non-polar amino acids are often found in the interior. This is because polar amino acids are more likely to interact with water and other polar molecules, while non-polar amino acids are more likely to interact with other non-polar groups.
**Correct Answer: C. Alanine**