Final answer:
The correct answer is c. Alanine (Ala) contributes to the CD signal in the far UV region but not the near UV region when part of a fully folded protein, unlike aromatic amino acids which absorb in the near UV region.
Step-by-step explanation:
Out of the amino acids listed, Alanine (Ala) will contribute to the circular dichroism (CD) signal in the far ultraviolet (UV) region but not the near UV region when part of a fully folded protein. The aromatic amino acids such as Tryptophan (Trp), Phenylalanine (Phe), and Tyrosine (Tyr) absorb in the near UV region due to their aromatic rings, which are not present in alanine.
Amino acids like alanine, which lack aromatic side chains, show CD signals in the far UV region (190-240 nm) due to transitions associated with the peptide bond itself, more specifically to the n->π* and π->π* transitions of the amide bond in the polypeptide backbone. These signals are sensitive to the secondary structure of proteins such as alpha-helices and beta-sheets.
In contrast, the aromatic amino acids like Tryptophan, Phenylalanine, and Tyrosine have distinctive absorbance in the near UV region (250-300 nm) due to the presence of their aromatic rings, which leads to electronic transitions specific to those side chains.