Phycobilisome's Exciton Transfer Efficiency Relies on an Energetic Funnel Driven by Chromophore-Linker Protein Interactions

Abstract

The phycobilisome is the primary light-harvesting antenna in cyanobacterial and red algal oxygenic photosynthesis. It maintains near-unity efficiency of energy transfer to reaction centers despite relying on slow exciton hopping along a relatively sparse network of highly fluorescent phycobilin chromophores. How the complex maintains this high efficiency remains unexplained. Using a two-dimensional electronic spectroscopy polarization scheme that enhances energy transfer features, we directly watch energy flow in the phycobilisome complex of Synechocystis sp. PCC 6803 from the outer phycocyanin rods to the allophycocyanin core. The observed downhill flow of energy, previously hidden within congested spectra, is faster than timescales predicted by Förster hopping along single rod chromophores. We attribute the fast, 8 ps energy transfer to interactions between rod-core linker proteins and terminal rod chromophores, which facilitate unidirectionally downhill energy flow to the core. This mechanism drives the high energy transfer efficiency in the phycobilisome and suggests that linker protein-chromophore interactions have likely evolved to shape its energetic landscape.

Figure 1

(a) Cryo-electron microscopy structure of the Synechocystis sp. PCC 6803 phycobilisome rendered using coordinates from Kerfeld and co-workers: six hexameric C-phycocyanin rods (blue) are assembled on the three lateral allophycocyanin cores (red). (b) Arrangement of phycocyanobilin chromophores in the phycobilisome (blue) with terminal rod chromophores (gold) strongly associated with the CpcG rod-core linker protein, and core allophycocyanin chromophores (red). (c) Absorption (black) and fluorescence (red) spectra overlaid with the laser spectrum (gray) used in our broadband two-dimensional spectroscopy experiments.

Figure 2

(a) Purely absorptive real-valued two-dimensional electronic spectra of the phycobilisome at 0.2, 2, 20, and 200 ps in the {0, 0, 0, 0°}, or all-parallel pulse sequence. Spectra are frame-normalized. The positive features represent ground-state bleach and stimulated emission, and negative features represent photoinduced absorption. (b) Cross section of the two-dimensional spectra at various waiting times at λ_ex = 630 nm. (c) Multiexponential fit for the off-diagonal point, λ_ex = 635 nm, λ_det = 660 nm.

Figure 3

(a) Diagonal suppressed two-dimensional electronic spectra ({90, 60, 120, 0°} pulse sequence) of the phycobilisome at 0.2, 2, 20, and 200 ps. Spectra are normalized to the maximum of the entire data cube. (b) Cross section of the two-dimensional spectra at various waiting times at λ_ex = 630 nm. (c) Multiexponential fit for the off-diagonal point, λ_ex = 635 nm, λ_det = 660 nm. Representative two-dimensional spectra for the {60, 120, 0, 0°} are shown in Supporting Figure 4.

Figure 4

Phycocyanobilin rod chromophores in a single phycobilisome rod (blue) and the nearest allophycocyanin core chromophores (red). Chromophores shown in bold transfer energy directly to the core-proximal chromophores (gold). CpcG, the rod-core linker protein is shown in gray, and its aromatic and charged residues (shown in dark gray) surround the core-proximal rod chromophores (gold). All structures are rendered using coordinates from Kerfeld and co-workers. Chromophore–residue interactions lower the energy of the core-proximal chromophores by ∼250 cm^–1 to facilitate unidirectional flow from the rod to the core.