How do the structures of photosynthetic pigment-protein complexes and membranes affect the efficiency of light harvesting and photo-protection mechanisms?

Chlorophylls that absorb light and transfer excitations to reaction centers are bound inside proteins. These proteins, in turn, form large networks of pigment-protein complexes that transfer energy both within individual complexes and between complexes and can respond to changes in light conditions.

Because of the large variation in length scales, a single perturbative regime for simulating the dynamics of energy transfer is not valid over the entire system (324 chlorophyll pigments), and the resources to compute non-perturbative treatments scale poorly with system size. Our approach is to investigate the dynamics of large light harvesting networks by describing intra-complex transfer with Redfield theory and inter-complex transfer with Forster theory, in order to build a master equation for excitation populations.

Simulations of the Photosystem II supercomplex (PSII), consisting of outer and inner core antenna complexes and reaction centers (where charge separation occurs), using this treatment demonstrate that exciton diffusion occurs on longer length scales than individual PSII units. This indicates that the membrane-level organizational structure of photosynthetic systems is relevant to the efficiency of light harvesting, and requires further modeling. These same simulations also give strong evidence that there is no single rate-limiting step for energy transfer in Photosystem II, rendering models based on fluorescence lifetime measurements too coarse grained to distinguish between different kinetic schemes.

Our treatment is extensible to membrane-level simulations that may provide insight into how the structural organization of the membrane influences the efficiency of light harvesting, and aims to connect structural information to photo-protection mechanisms.

Figure 1: Reproduced from [1]. Linearized timescale of stages of light harvesting in photosystem II and a schematic representation of transfer of energy from antenna pigment-protein complexes to PSII, the reaction center, and charge separated states.

Helpful Background Reading:

1. A structure-based model of energy transfer reveals the principles of light harvesting in photosystem II supercomplexes. D. I. G. Bennett, K. Amarnath, G. R. Fleming, Journal of the American Chemical Society, 135 (24), 9164-9173 (2013).

2. Influence of Phonons on Exciton Transfer Dynamics: Comparison of the Redfield, Forster, and Modified Redfield Equations. M. Yang and G. R. Fleming. Chem. Phys., 275, 355 (2002).