Intrinsic Photoprotective Mechanisms in Chlorophylls.

Photosynthetic energy conversion competes with the formation of chlorophyll triplet states and the generation of reactive oxygen species. These may, especially under high light stress, damage the photosynthetic apparatus. Many sophisticated photoprotective mechanisms have evolved to secure a harmless flow of excitation energy through the photosynthetic complexes. Time-resolved laser-induced optoacoustic spectroscopy was used to compare the properties of the T1 states of pheophytin a and its metallocomplexes. The lowest quantum yield of the T1 state is always observed in the Mg complex, which also shows the least efficient energy transfer to O2 . Axial coordination to the central Mg further lowers the yield of both T1 and singlet oxygen. These results reveal the existence of intrinsic photoprotective mechanisms in chlorophylls, embedded in their molecular design, which substantially suppress the formation of triplet states and the efficiency of energy transfer to O2 , each by 20-25 %. Such intrinsic photoprotective effects must have created a large evolutionary advantage for the Mg complexes during their evolution as the principal photoactive cofactors of photosynthetic proteins.