Intrinsic Photoprotective Mechanisms in Chlorophylls

Abstract

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,d amage the photosynthetic apparatus.M any sophisticated photoprotective mechanisms have evolved to secure aharmless flow of excitation energy through the photosynthetic complexes.Time-resolved laser-induced optoacoustic spectroscopyw as used to compare the properties of the T 1 states of pheophytin aa nd its metallocomplexes.T he lowest quantum yield of the T 1 state is always observed in the Mg complex, which also shows the least efficient energy transfer to O 2 .Axial coordination to the central Mg further lowers the yield of both T 1 and singlet oxygen. These results reveal the existence of intrinsic photoprotective mechanisms in chlorophylls,e mbedded in their molecular design, which substantially suppress the formation of triplet states and the efficiency of energy transfer to O 2 ,e achb y2 0-25 %. Such intrinsic photoprotective effects must have created al arge evolutionary advantage for the Mg complexes during their evolution as the principal photoactive cofactors of photosynthetic proteins.

Side Methyl Groups Control the Conformation and Contribute to Symmetry Breaking of Isoprenoid Chromophores

“…Retinal (vitamin A) is the principal chromophore of the visual photoreceptor in the human eye.C rts participate in solarenergy conversion in the photosynthetic organelles of phototrophic organisms,i nw hich they act as accessory antenna pigments, [1] but their major function is to protect the photosynthetic apparatus against the harmful triplet states of chlorophylls. [2] The"isoprene rule" of isoprenoid biosynthesis was discovered by nature very early,b ut its structural rationales remain unexplained since its formulation by Ruzicka. [3] Conventionally,t he isoprenoid chromophores are treated as linear polyenes,a nd on this basis their electronic structure,c onformation, and photophysical properties are interpreted.…”