Hopkins-Cole test is performed on proteins to detect the presence of
**Core Concept**
The Hopkins-Cole test is a chemical test used to detect the presence of a specific functional group in proteins. This test relies on the ability of certain amino acids to react with specific reagents, resulting in a colorimetric or precipitative response.
**Why the Correct Answer is Right**
The Hopkins-Cole test specifically detects the presence of arginine in proteins. Arginine is a basic amino acid that contains a guanidinium group (-NH-C(=NH)-NH2). When proteins containing arginine are treated with Hopkins-Cole reagent, the guanidinium group reacts to form a purple-colored compound, indicating the presence of arginine. This reaction is based on the selective oxidation of the guanidinium group, which is a unique feature of arginine among amino acids.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Hopkins-Cole test does not detect the presence of lysine, which is another basic amino acid. Lysine does not contain a guanidinium group, and therefore does not react with the Hopkins-Cole reagent.
* **Option B:** This option is incorrect because the Hopkins-Cole test is not used to detect the presence of tryptophan, which is an aromatic amino acid. Tryptophan has a different chemical structure and does not react with the Hopkins-Cole reagent.
* **Option C:** This option is incorrect because the Hopkins-Cole test is not used to detect the presence of histidine, which is a basic amino acid. Histidine does not contain a guanidinium group, and therefore does not react with the Hopkins-Cole reagent.
**Clinical Pearl / High-Yield Fact**
The Hopkins-Cole test is a useful tool for detecting the presence of arginine in proteins, which can be an important diagnostic feature in some clinical contexts. For example, the test can be used to detect the presence of arginine in proteins isolated from biological fluids, such as urine or blood.
**Correct Answer:** D. Arginine.