At pH 7, what charge would the modified Trp residues likely carry?

• A. Neutral
• B. Positive
• C. Negative
• D. Variable depending on environment

Answer: (C)

At pH 7, the modified tryptophan (Trp) residues are likely to carry a negative charge due to the characteristics of their ionizable side chains. While the side chain of tryptophan itself is generally neutral at physiological pH, modifications such as phosphorylation can introduce negatively charged groups. For instance, if tryptophan is modified by the addition of a phosphate group, which is common in many biological contexts, the resulting phosphate group carries a negative charge at neutral pH.

The influence of pH on the ionization states of amino acid side chains is a key concept in biochemistry, as it affects protein structure and function. At pH 7, the environment significantly influences the overall charge of the residues based on their specific modifications. In the situation of unmodified tryptophan, it remains neutral; however, given a common modification like phosphorylation, the residue indeed becomes negatively charged.

Thus, if the question specifically implies a common modification seen in biological systems, the modified Trp residues would predominantly exhibit a negative charge at physiological pH. Understanding how modifications impact the charge of amino acid side chains is crucial in predicting their behavior and interactions in biological systems.