  The Fenna-Matthews-Olson (FMO) protein-pigment complex acts as a molecular
wire between the outer antenna system and the reaction center (RC); it is an
important model system to study the excitonic energy transfer. Recent
crystallographic studies report the existence of an additional (eighth)
bacteriochlorophyll a (BChl a). To understand the functionality of this eighth
BChl, we simulated the two-dimensional electronic spectra of both the 7-site
(apo form) and the 8-site (holo form) variant of the FMO complex from green
sulfur bacteria, Prosthecochloris aestuarii. By comparing the difference
between the spectrum, it was found that the eighth BChl can affect two
different excitonic energy transfer pathways, these being: (1) directly involve
in the first pathway 6 $\rightarrow$ 3 $\rightarrow$ 1 of the apo form model by
passing the excitonic energy to exciton 6; and (2) increase the excitonic wave
function overlap between excitons 4 and 5 in the second pathway (7
$\rightarrow$ 4,5 $\rightarrow$ 2 $\rightarrow$ 1) and thus increase the
possible downward sampling routes across the BChls.
