Theoretical Study on the Excitation Energy Transfer Dynamics in the Phycoerythrin PE555 Light-Harvesting Complex

Abstract

Photosynthesis in nature begins with light harvesting. The special pigment-protein complex converts sunlight into electron excitation that is transmitted to the reaction center, which triggers charge separation. Evidence shows that quantum coherence between electron excited states is important in the excitation energy transfer process. In this work, we investigate the quantum coherence of the PE555 complex in exciton dynamics and its performance and significance in photosynthetic light harvesting. To elucidate the energy transfer mechanism of the PE555 complex, an exciton model is adopted with the full Hamiltonian obtained from structure-based calculations. We used quantum dissipation theory to investigate the excitation dynamic process. The results indicate the existence of long-lived quantum coherence phenomena. We then discuss the pathway of the excitation energy transfer process, which is when the PEB chromophore molecules absorb energy and then transfer the excited energy to the DBV50/61B molecule. To further discuss the effect of the initial coherent superposition of dimeric states on the excitation energy transfer process to the DBV50/61B chromophore molecule, the results indicate that the coherent superposition of initially excited states indeed promotes the transmission of excitation energy to the acceptor state. Furthermore, we investigate the optimization behavior of individual pigment molecules, and these results show that the local protein environment among chromophore molecules can affect the throughput of the system in a controllable manner.

Figure 1

(a) PE555 complex structure with its protein scaffold, embedding eight chromophore molecules. (b) Eight chromophore molecules as depicted in the structural model of the PE555 complex.

Figure 2

Population dynamics of the eight-pigment model at T = 77 K (a) and T = 294 K (b), with initial excitation on the PEB20A chromophore molecule.

Figure 3

Population dynamics of the PE555 complex at T = 77 K, with initial excitation on PEB158B, PEB82B, PEB82D, and DBV50/61D chromophores.

Figure 4

Comparison of populations of other chromophores when the dimer state is replaced with the single mean state in order to initially remove initial coherence. Solid curves show the chromophore populations observed with the coherently coupled dimer initially states; dashed curves give populations obtained when the mean state is excited.

Figure 5

Population dynamics of various pigment states over time following initial excitation at the PEB20A site. Panels (a–d) each depict the outcomes for different system and bath coupling strengths for the PEB20A, PEB82B, DBV50/61D, and PEB20C chromophores within the network.