Antibody cross-reactivity — A monoclonal antibody specific for the aromatic hapten 2,4-dinitrophenyl (DNP) might also bind which amino acid side chains most plausibly?

Introduction:
Haptens like 2,4-dinitrophenyl (DNP) are small aromatic molecules that elicit specific antibodies when coupled to carriers. Antibody paratopes recognize shape, charge, and hydrophobic features; cross-reactivity often occurs with structurally similar epitopes. This question probes recognition of aromaticity as a key determinant.

Given Data / Assumptions:

• DNP contains a substituted benzene ring (planar, aromatic, hydrophobic).
• Some amino acids have aromatic side chains (Phe, Tyr, Trp).
• Cross-reactivity is more likely when shape/chemistry resembles the hapten.

Concept / Approach:
Pair the DNP aromatic ring with amino acids that also present aromatic ring systems. Tyrosine and phenylalanine provide phenyl rings; tyrosine adds a phenolic OH, enabling potential π-stacking and hydrophobic interactions reminiscent of DNP’s ring.

Step-by-Step Solution:

Verification / Alternative check:
Experimental binding assays often reveal weak binding of DNP-specific mAbs to other aromatic moieties compared with strong binding to DNP-conjugates, consistent with partial shape/chemistry overlap.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing hydrophobicity in general with aromatic ring recognition; not all hydrophobic residues mimic aromatic epitopes.

Final Answer:
Tyrosine or phenylalanine.