## **Core Concept**
The core concept being tested here involves understanding the chemical properties of amino acids and peptides, specifically the ionization state of their functional groups at physiological pH (around 7.4). The carboxy-terminal (C-terminus) of a peptide refers to the end of the peptide chain where the carboxyl group (-COOH) is not involved in a peptide bond.

## **Why the Correct Answer is Right**
At physiological pH, the carboxy-terminal of a peptide is deprotonated, forming a carboxylate ion (-COO-). This is because the pKa of the carboxyl group is typically around 2-3, which is much lower than physiological pH. Therefore, at pH 7.4, the carboxyl group donates a proton (H+), resulting in a negatively charged carboxylate ion. This is represented by option .

## **Why Each Wrong Option is Incorrect**
- **Option A:** - This option suggests the carboxy-terminal remains protonated at physiological pH, which is incorrect given the pKa of the carboxyl group is much lower than 7.4.
- **Option B:** - This option might suggest an intermediate or zwitterionic form but does not accurately represent the predominant species at physiological pH.
- **Option D:** - Similar to option A, this does not accurately reflect the ionization state of the carboxy-terminal at physiological pH.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the charge on amino acids and peptides at physiological pH depends on the pKa of their ionizable groups and the pH of the solution. For the carboxy-terminal, this means it will be negatively charged at physiological pH, which is crucial for understanding protein structure, function, and interactions.

## **Correct Answer:** .