Ultrafast Spectroscopic Investigation of Energy Transfer in Site-Directed Mutants of the Fenna–Matthews–Olson (FMO) Antenna Complex from <i>Chlorobaculum tepidum</i>

Ultrafast transient absorption (TA) and time-resolved fluorescence (TRF) spectroscopic studies were performed on several mutants of the bacteriochlorophyll (BChl) <i>a</i>-containing Fenna–Matthews–Olson (FMO) complex from the green sulfur bacterium <i>Chlorobaculum tepidum</i>. These mutants were generated to perturb a particular BChl <i>a</i> site and determine its effects on the optical spectroscopic properties of the pigment–protein complex. Measurements conducted at 77 K under both oxidizing and reducing conditions revealed changes in the dynamics of the various spectral components as compared to the data set from wild-type FMO. TRF results show that under reducing conditions all FMO samples decay with a similar lifetime in the ∼2 ns range. The oxidized samples revealed varying fluorescence lifetimes of the terminal BChl <i>a</i> emitter, considerably shorter than those recorded for the reduced samples, indicating that the quenching mechanism in wild-type FMO is still present in the mutants. Global fitting of TA data yielded similar overall results, and in addition, the lifetimes of early decaying components were determined. Target analyses of TA data for select FMO samples generated kinetic models that better simulate the TA data. A comparison of the lifetime of excitonic components for all samples reveals that the mutations affect mainly the early kinetic components, but not that of the lowest energy exciton, which reflects the flexibility of energy transfer in FMO.