The Fate of the Triplet Excitations in the Fenna-Matthews-Olson Complex

The fate of triplet excited states in the Fenna-Matthew-Olson (FMO) pigment-protein complex is studied by means of time-resolved nanosecond spectroscopy and exciton model simulations. Experiments reveal microsecond triplet excited-state energy transfer between the bacteriochlorophyll (BChl) pigments, but show no evidence of triplet energy transfer to molecular oxygen, which is known to produce highly reactive singlet oxygen and is the leading cause of photo damage in photosynthetic proteins. The FMO complex is exceptionally photo stable despite the fact it contains no carotenoids, which could effectively quench triplet excited states of (bacterio)chlorophylls and are usually found within pigment-protein complexes. It is inferred that the triplet excitation is transferred to the lowest energy pigment, BChl 3, within the FMO complex, whose triplet state energy is shifted by pigment-protein interactions below that of the singlet oxygen excitation. Thus, the energy transfer to molecular oxygen is blocked and the FMO does not need carotenoids for photo protection.