Final answer:
Alanine (Ala) is the amino acid that contributes to the CD signal in the far UV region but not in the near UV region when part of a fully folded protein, due to its non-aromatic side chain.
Step-by-step explanation:
In the context of protein structure and circular dichroism (CD) spectroscopy, the far UV region (around 190-250 nm) primarily probes the secondary structure of proteins through absorption by peptide bonds and certain amino acids, notably those with non-aromatic side chains. The near UV region (250-350 nm), however, involves the absorption by the aromatic amino acids: tyrosine (Tyr), tryptophan (Trp), and phenylalanine (Phe), which tend to have larger CD signals due to their aromatic rings.
Looking at the options provided, alanine (Ala) does not have an aromatic side chain and thus would only contribute to the CD signal in the far UV region corresponding with the secondary structure of the protein it is part of. Whereas, Trp, Phe, and Tyr all have aromatic side chains and contribute to the CD signal in both the far and near UV regions.
Therefore, the correct answer is C. Ala.
In the context of far UV versus near UV CD spectroscopy, alanine will contribute to the far UV signal due to its involvement in the backbone conformation of a fully folded protein, but not to the near UV which is dominated by the presence of aromatic side chains.