Which of the following is NOT considered a chromophore in proteins?

A chromophore is a part of a molecule responsible for its color, typically due to the absorption of light at specific wavelengths. In proteins, chromophores are often structural units that contain conjugated systems of double bonds or are part of aromatic structures.

Aromatic rings, such as those found in amino acids like tryptophan, tyrosine, and phenylalanine, absorb light due to their electron-rich systems and contribute to the color of proteins. Amide carbonyls can also absorb UV light, particularly in the region around 280 nm. Conjugated double bonds are capable of absorbing light because their extended π-electron systems allow electrons to transition between energy levels when exposed to specific wavelengths of light.

In contrast, polypeptide chains as a whole are not a chromophore. While they can contain various chromophores (like the aforementioned aromatic rings and amide groups), the polypeptide backbone itself does not absorb light in a way that creates a distinct color or chromophoric effect. The backbone lacks the necessary conjugated systems or specific functional groups that would allow it to interact with light in the same way that these other components can.