Which classification of amino acids applies to the Trp residues after photochemical modification by CCl3CO2H?

To determine the classification of amino acids that applies to tryptophan (Trp) residues after photochemical modification by CCl3CO2H, it's important to first understand the characteristics of tryptophan itself and the impact of the modification.

Tryptophan is an amino acid with a large, aromatic side chain, which typically makes it hydrophobic. However, when modified by a reagent such as CCl3CO2H (trichloroacetic acid), the side chain's properties can change. Specifically, trichloroacetic acid can introduce electron-withdrawing groups, leading to altered acidity and functional characteristics of the side chain.

After this modification, tryptophan may acquire characteristics similar to those of acidic amino acids. Acidic amino acids are defined by the presence of a carboxyl group in their side chain, which can donate protons (H+) in a solution, effectively increasing the acidity. As the modification replaces parts of the original tryptophan side chain with carboxyl groups or similar structures, it can behave in a manner akin to acidic amino acids.

Thus, the correct classification for tryptophan residues that have undergone this specific modification is "acidic," as the properties