In the presence of water, which structure appears in the CD spectra?

In circular dichroism (CD) spectroscopy, different secondary structures of proteins produce distinct spectral patterns due to their unique arrangements of peptide bonds and overall chiral properties. The random coil structure is characterized by a lack of stable secondary structure and unfolds in a way that does not conform to the periodic backbone hydrogen bonding of alpha helices or beta sheets.

In the presence of water, proteins may adopt a random coil configuration, especially when they are disordered or in a non-stable state. This structural form results in a CD spectrum that is distinct from that of more ordered structures. Random coils typically exhibit a CD spectrum with a negative band around 198 nm and a positive band around 220 nm, indicating the absence of defined secondary structural elements like helices or sheets.

In contrast, alpha helices and beta sheets would produce characteristic peaks in the CD spectrum that reflect their organized structures. The globular structure, while it can contain helical and sheet regions, is also well-defined and would not solely correspond to the random coil spectrum. Therefore, the reason the random coil appears in the CD spectra when in the presence of water is due to its inherent lack of stable secondary structural features.