Following extraction from photosynthetic organisms, chlorophylls are prone to reactions including demetalation, dephytylation and specific oxidations of the exocyclic ring E, termed allomerizations. Allomerization of chlorophylls has been well-characterized in methanol and to a lesser extent in aqueous solution. Here we detail novel allomerization-like reactions of chlorophyll a and chlorophyll b. In the presence of heme, detergent-solubilized chlorophyll a is hydroxylated at its C 132 position in ring E and, surprisingly, the C 18 position in ring D. Two major oxidation products are synthesized — a C 132- OH and a C 132- OH , C 18- OH derivative of chlorophyll a. We track the origin of the oxygen atoms added in these hydroxylated chlorophylls using 18 O 2 labeling and demonstrate that the additional oxygen atoms are derived from molecular oxygen. A similar heme-catalyzed reaction is also observed using chlorophyll b as a substrate. These results highlight the need for care when dealing with extracted chlorophylls and demonstrate an unusual hydroxylation of the C 18 position of chlorophylls in the presence of heme.
Molecular oxygen generation from the reaction of water cations with oxygen atoms