Proteins can have a maximum of four levels of structure: primary, secondary, tertiary, and quaternary. Although the proteins can spontaneously fold to a functional conformation, there are a variety of denaturing agents that can be used to disrupt the folding strategies of proteins. Mercaptoethanol is an example of a protein denaturing agent; its mechanism for dismantling proteins is to disrupt the disulfide bonds found in the protein. When urea is introduced to a protein, the hydrogen bonds holding the protein together are disrupted. Heat can also be considered a denaturing agent, which has the potential to disrupt all intermolecular interactions in a protein. Which of the following levels of structure in a protein would not be disrupted by the introduction of mercaptoethanol? Choose one from among the possible answers and explain.

a. Tertiary structure
b. All of the given levels will be affected
c. Secondary structure
d. Quaternary structure