Efficient Intrinsic Photoprotection in Strongly Coupled (Bacterio) Chloropyll Complexes

Abstract

Chlorophyll (Chl) molecules in photosynthetic proteins are known to produce highly toxic singlet oxygen under illumination as a result of energy transfer from their triplet excited states to oxygen. To prevent the formation of singlet oxygen, carotenoids (Car) are typically positioned close to Chl to ensure rapid triplet-triplet energy transfer from Chl to Car, which can then safely dissipate the energy of the excited states. Our recent studies revealed a new, unconventional, but very efficient photoprotection mechanism in strongly coupled natural and artificial light-harvesting complexes that does not rely on the presence of carotenoids. Experimental studies on carotenoid-free chlorosomes and artificial bacteriochlorophyll (BChl) aggregates show that these structures are at least three orders of magnitude more stable to photodamage than monomeric forms of BChl. It was proposed that this photoprotection in strongly coupled arrays of pigments is due to triplet exciton formation, which lowers the energy of the triplet exciton substantially below that of singlet oxygen.In this report we present the results of our ongoing comprehensive study of the properties of triplet excited states of monomeric BChls, Chl aggregates and chlorosomes by means of EPR, time-resolved optical pump-probe spectroscopy and steady-state IR phosphorescence spectroscopy.