The role of exciton delocalization in the major photosynthetic light-harvesting antenna of plants

Abstract

In the major peripheral plant light-harvesting complex LHCII, excitation energy is transferred between chlorophylls along an energetic cascade before it is transmitted further into the photosynthetic assembly to be converted into chemical energy. The efficiency of these energy transfer processes involves a complicated interplay of pigment-protein structural reorganization and protein dynamic disorder, and the system must stay robust within the fluctuating protein environment. The final, lowest energy site has been proposed to exist within a trimeric excitonically coupled chlorophyll (Chl) cluster, comprising Chls a610-a611-a612. We studied an LHCII monomer with a site-specific mutation resulting in the loss of Chls a611and a612, and find that this mutant exhibits two predominant overlapping fluorescence bands. From a combination of bulk measurements, single-molecule fluorescence characterization, and modeling, we propose the two fluorescence bands originate from differing conditions of exciton delocalization and localization realized in the mutant. Disruption of the excitonically coupled terminal emitter Chl trimer results in an increased sensitivity of the excited state energy landscape to the disorder induced by the protein conformations. Consequently, the mutant demonstrates a loss of energy transfer efficiency. On the contrary, in the wild-type complex, the strong resonance coupling and correspondingly high degree of excitation delocalization within the Chls a610-a611-a612 cluster dampens the influence of the environment and ensures optimal communication with neighboring pigments. These results indicate that the terminal emitter trimer is thus an essential design principle for maintaining the efficient light-harvesting function of LHCII in the presence of protein disorder.

Figure 1

Room temperature steady-state absorption spectra of LHCII-WT, LHCII-A2, and the calculated difference. To see this figure in color, go online.

Figure 2

Temperature dependent steady-state emission of LHCII-A2, λ_ex = 600 nm. To see this figure in color, go online.

Figure 3

77 K emission spectra of LHCII-A2 at various excitation wavelengths exhibit differing lineshapes. Dashed lines indicate peaks at 674 and 684 nm. To see this figure in color, go online.

Figure 4

Fluorescence spectral time traces of single LHCII-A2 complexes at 278 K with continuous illumination at λ_ex = 633 nm exhibits four discernible profiles: (a) narrow emission peaking at 678 nm, (b) broad emission at up to 684 nm, (c) switching between the two conditions seen in (a) and (b), and (d) 700 nm emission analogous to that observed in LHCII-WT. Top profiles correspond to the selected 1 s time bins indicated. To see this figure in color, go online.

Figure 5

FWHM as derived from single Gaussian fits to SMS spectra versus fluorescence peak position for LHCII-WT (black) and LHCII-A2 (red). Ellipses represent the 68% confidence regions. To see this figure in color, go online.

Figure 6

Modeling (solid line) as compared with experimental results (dotted line) of LHCII-A2 absorption and emission at RT and 77K. Fluorescence traces shown are for λ_ex = 650 nm. To see this figure in color, go online.

Figure 7

Illustration of the change in the excitonic manifold between (left) LHCII-WT and (right) LHCII-A2. Excitonic contributions to the modeled fluorescence spectra are shown for the lowest energy levels, as well as a depiction of the pigment connectivity. The thickness of the black connecting line demonstrates the tendency of the excitation to be delocalized over these two pigments: $\mathit{lin}{\mathit{e}}_{\mathit{nm}}\propto {\sum }_{\mathit{i}}{\left|{\mathit{c}}_{\mathit{in}}\right|}^2{\left|{\mathit{c}}_{\mathit{im}}\right|}^2$, the size of the circle depicts the contribution of that pigment to the emission spectrum: $\mathit{circl}{\mathit{e}}_{\mathit{n}}\propto {\sum }_{\mathit{i}}{\left|{\mathit{c}}_{\mathit{in}}\right|}^2{\left|{\mu }_{\mathit{i}0}\right|}^2{\mathit{P}}_{\mathit{i}}$, and the semi-transparent ovals are to distinguish the tendency of the contained pigments to be in an excitonically coupled dimer or trimer. For notation see the section on modeling in the Materials and Methods. To see this figure in color, go online.