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 Pros tecochloris aestuarii. The experiments support the results presented i n a previous paper (Louwe et al., J. Phys. Chem. 101 (1997) 11280), wh ich showed that the experimental optical spectra of this pigment-prote in complex are best reproduced by assuming that one bacteriochlorophyl l (BChl 3) is energetically isolated and that this BChl is the triplet -carrying BChl of the FMO complex at cryogenic temperatures for low ex citation density. When comparing the experimental and simulated data s ets of the triplet-EPR spectra in single crystals, the best fit is obt ained 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 th e relatively high excitation power necessary to observe the small trip let-EPR signal of the FMO single crystals. (C) 1998 Elsevier Science B .V. All rights reserved.