Which of the following may be classified as a hydrophobic amino acid at pH 7.0?
**Core Concept**
The question is testing the student's understanding of the chemical properties of amino acids, specifically their ability to be classified as hydrophobic or hydrophilic at physiological pH.
**Why the Correct Answer is Right**
At physiological pH (7.0), amino acids can be classified into three main categories: hydrophilic (water-loving), hydrophobic (water-repelling), and amphipathic (having both hydrophobic and hydrophilic regions). Hydrophobic amino acids are those that have a nonpolar side chain, which is unable to form hydrogen bonds with water. This property makes them resistant to water and often results in their accumulation in hydrophobic environments, such as cell membranes.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid choice, as it is incomplete. The student should not be penalized for not providing a valid option.
**Option B:** This option is incorrect because it is a hydrophilic amino acid, which means it has a polar side chain that can form hydrogen bonds with water.
**Option C:** This option is incorrect because it is an amphipathic amino acid, which means it has both hydrophobic and hydrophilic regions.
**Option D:** This option is incorrect because it is a hydrophilic amino acid, which means it has a polar side chain that can form hydrogen bonds with water.
**Clinical Pearl / High-Yield Fact**
One important thing to remember is that the classification of amino acids as hydrophobic or hydrophilic is not always straightforward and can depend on the specific pH of the environment. Additionally, some amino acids can have multiple properties depending on the context.
**Correct Answer: C. Leucine**