At physiological pH, the carboxy-terminal of a peptide is –
**Core Concept**
The carboxy-terminal of a peptide is determined by its amino acid composition and the pH of the environment. At physiological pH (around 7.4), the carboxy-terminal amino acid's carboxyl group is in its ionized form, which affects its properties and interactions.
**Why the Correct Answer is Right**
The carboxy-terminal of a peptide is typically the carboxyl-terminated amino acid, which has a carboxyl (-COOH) group. At physiological pH, the carboxyl group of the carboxy-terminal amino acid is ionized to a carboxylate (-COO-) group due to the presence of hydrogen ions (H+). This ionization affects the peptide's charge, solubility, and interactions with other molecules.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the carboxy-terminal of a peptide at physiological pH.
* **Option B:** This is incorrect because the amino-terminal of a peptide is the end with the free amino group, not the carboxy-terminal.
* **Option C:** This option is not accurate because the carboxy-terminal amino acid's carboxyl group is ionized at physiological pH.
**Clinical Pearl / High-Yield Fact**
At physiological pH, the ionization of the carboxyl group at the carboxy-terminal of a peptide affects its interactions with other molecules, such as enzymes, receptors, and other peptides. This is an important consideration in understanding peptide function, transport, and metabolism.
**Correct Answer: D. Carboxylate (-COO-)**