12. Polylysine is a random coil when the pH is less than 11, but it forms an a-helix if the pH is raised above 12. This happens because (A) The ε-NH2 group is protonated into ε-NH3+ at pH 12 and the charges stabilize the a- helix structure. (B) The pKa of ε-NH2 group is 12 and the partially protonated amino groups stabilize the a- helix structure. (C) The high concentration of OH- at pH 12 reduces the effective positive charges on the ε-NH3+ groups and results in less electrostatic repulsion between R groups. (D) The ε-NH3+ groups are titrated to produce neutral NH2 groups, which eliminate electrostatic repulsions between R groups. (E) The ε-NH2 becomes negatively charged ε-NH- which stabilize thea-helix structure.