5,10-Methenyltetrahydrofolate is a tetrahydrofolate derivative involved in photoreactivation. Photolyases are enzymes that use the energy of light to break covalent bonds between adjacent pyrimidines on a DNA strand. 5,10-Methenyltetrahydrofolate functions like the photochemical reaction center, translating light energy to facilitate the transfer of an electron to the pyrimidine dimer and breaking the pyrimidine-pyrimidine bonds by a free radical mechanism, as shown in Figure 25.10.