How reduced excitonic coupling enhances light harvesting in the main photosynthetic antennae of diatoms

Abstract

Strong excitonic interactions are a key design strategy in photosynthetic light harvesting, expanding the spectral cross-section for light absorption and creating considerably faster and more robust excitation energy transfer. These molecular excitons are a direct result of exceptionally densely packed pigments in photosynthetic proteins. The main light-harvesting complexes of diatoms, known as fucoxanthin-chlorophyll proteins (FCPs), are an exception, displaying surprisingly weak excitonic coupling between their chlorophyll (Chl) 's, despite a high pigment density. Here, we show, using single-molecule spectroscopy, that the FCP complexes of switch frequently into stable, strongly emissive states shifted 4-10 nm toward the red. A few percent of isolated FCPa complexes and ∼20% of isolated FCPb complexes, on average, were observed to populate these previously unobserved states, percentages that agree with the steady-state fluorescence spectra of FCP ensembles. Thus, the complexes use their enhanced sensitivity to static disorder to increase their light-harvesting capability in a number of ways. A disordered exciton model based on the structure of the main plant light-harvesting complex explains the red-shifted emission by strong localization of the excitation energy on a single Chl pigment in the terminal emitter domain due to very specific pigment orientations. We suggest that the specific construction of FCP gives the complex a unique strategy to ensure that its light-harvesting function remains robust in the fluctuating protein environment despite limited excitonic interactions.

The robustness of the terminal emitter site in major LHCII complexes controls xanthophyll function during photoprotection

“…Due to its average nature, bulk spectroscopic measurements can't discriminate precisely between these different conformations, which is instead possible in singlemolecule studies. 68 The quenching induction could thus increase the number of states accessible, leading to a broader fluorescence peak. Generally, small spectral shifts can identify variations of the chlorophyll site energies or their excitonic coupling, since the magnitude of both effects is similar.…”

Triangular Rhodamine Triads and Their Intrinsic Photophysics Revealed from Gas‐Phase Ion Fluorescence Experiments

“…An interesting aspect to consider is exciton coupling between two dyes in the trimers, which could potentially enhance the energy‐transfer efficiency in a process often denoted “supertransfer”. This has been discussed in detail in the case of photosynthetic light‐harvesting proteins [16–18] . The coupling in our case would either involve two R575 in [2, 1] , i.e., collective photoexcitation of both R575, or two R640 in [1, 2] , i.e., collective excitation of both R640 when they receive the excitation energy from R575.…”

Unique photosynthetic electron transport tuning and excitation distribution in heterokont algae

“…The fluorescent component having a fluorescence maximum at 686 nm does not match fluorescence components that have been previously reported for isolated diatoms. This fluorescence emission may be due to highly fluorescent red-shifted emissions by fucoxanthin-chlorophyll binding proteins (FCPs) as recently described in the diatom Cyclotella meneghiniana [49].…”