The triplet state of the FMO complex of the green sulfur bacterium Prosthecochloris aestuarii studied with single-crystal EPR

Triplet-electron paramagnetic resonance (EPR) spectra were obtained of single crystals of the FMO complex of the green sulfur bacterium Prostecochloris aestuarii. The experiments support the results presented in a previous paper (Louwe et al., J. Phys. Chem. 101 (1997) 11280), which showed that the experimental optical spectra of this pigment-protein complex are best reproduced by assuming that one bacteriochlorophyll (BChl 3) is energetically isolated and that this BChl is the triplet-carrying BChl of the FMO complex at cryogenic temperatures for low excitation density. When comparing the experimental and simulated data sets of the triplet-EPR spectra in single crystals, the best fit is obtained for two triplet states, one localized at BChl 3 and the other at BChl 1. The existence of two different triplet states is traced to the relatively high excitation power necessary to observe the small triplet-EPR signal of the FMO single crystals.