Effect of initial coherence in the excitation energy transfer in Fenna-Mathews-Olson complex: Distinguishing in vitro and in vivo dynamics

Long-lasting coherence among different excitons in a 7-site monomer exists in an in vitro Fenna-Mathews-Olson (FMO) complex. In the exisiting models to explain such coherence, however, all the features of the excitation energy transfer (EET) through such monomer have not been considered together, namely, non-Markovianlty, relative dipole orientations of the chromophores and their site-specific coupling to the local bath modes, phonon contribution, initial states, the presence of the eighth chromphore, and the sink-model of the reaction center. In this work, all these included, we numerically solve the non-Markovian master equation to study the dynamics of EET in the singly-excited subspace of eight chromophores and the sink, incoherently coupled to the other sites, in a suitably structured phonon bath. We show that a laser pulse creates an initial excitonic coherence (thanks to non-orthogonal dipole orientations among the sites), that leads to the EET much faster than it happens for an initial mixed state. It enhances the transfer efficiency as well, near to unity. Our detailed analysis challenges the existing two-channel model of EET and also exhibits a clear distinction between the dynamics of an in vitro and an in vivo FMO complex.