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The water-splitting capability of Photosystem II (PSII) of plants and green algae requires the system to balance efficient light harvesting along with effective photoprotection against excitation in excess of the photosynthetic capacity, particularly under the naturally fluctuating sunlight intensity. The comparatively flat energy landscape of the multicomponent structure, inferred from the spectra of the individual pigment-protein complexes and the rather narrow and featureless absorption spectrum, is well known. However, how the combination of the required functions emerges from the interactions among the multiple components of the PSII supercomplex (PSII-SC) cannot be inferred from the individual pigment-protein complexes. In this work, we investigate the energy transfer dynamics of the C2S2-type PSII-SC with a combined spectroscopic and modeling approach. Specifically, two-dimensional electronic-vibrational (2DEV) spectroscopy provides enhanced spectral resolution and the ability to map energy evolution in real space, while the quantum dynamical simulation allows complete kinetic modeling of the 210 chromophores. We demonstrate that additional pathways emerge within the supercomplex. In particular, we show that excitation energy can leave the vicinity of the charge separation components, the reaction center (RC), faster than it can transfer to it. This enables activatable quenching centers in the periphery of the PSII-SC to be effective in removing excessive energy in cases of overexcitation. Overall, we provide a quantitative description of how the seemingly contradictory functions of PSII-SC arise from the combination of its individual components. This provides a fundamental understanding that will allow further improvement of artificial solar energy devices and bioengineering processes for increasing crop yield.

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Photosynthesis is generally assumed to be initiated by a single photon1-3 from the Sun, which, as a weak light source, delivers at most a few tens of photons per nanometre squared per second within a chlorophyll absorption band1. Yet much experimental and theoretical work over the past 40 years has explored the events during photosynthesis subsequent to absorption of light from intense, ultrashort laser pulses2-15. Here, we use single photons to excite under ambient conditions the light-harvesting 2 (LH2) complex of the purple bacterium Rhodobacter sphaeroides, comprising B800 and B850 rings that contain 9 and 18 bacteriochlorophyll molecules, respectively. Excitation of the B800 ring leads to electronic energy transfer to the B850 ring in approximately 0.7 ps, followed by rapid B850-to-B850 energy transfer on an approximately 100-fs timescale and light emission at 850-875 nm (refs. 16-19). Using a heralded single-photon source20,21 along with coincidence counting, we establish time correlation functions for B800 excitation and B850 fluorescence emission and demonstrate that both events involve single photons. We also find that the probability distribution of the number of heralds per detected fluorescence photon supports the view that a single photon can upon absorption drive the subsequent energy transfer and fluorescence emission and hence, by extension, the primary charge separation of photosynthesis. An analytical stochastic model and a Monte Carlo numerical model capture the data, further confirming that absorption of single photons is correlated with emission of single photons in a natural light-harvesting complex.

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Aquatic photosynthetic organisms evolved to use a variety of light frequencies to perform photosynthesis. Phycobiliprotein phycocyanin 645 (PC645) is a light-harvesting complex in cryptophyte algae able to transfer the absorbed green solar light to other antennas with over 99% efficiency. The infrared signatures of the phycobilin pigments embedded in PC645 are difficult to access and could provide useful information to understand the mechanism behind the high efficiency of energy transfer in PC645. We use visible-pump IR-probe and two-dimensional electronic vibrational spectroscopy to study the dynamical evolution and assign the fingerprint mid-infrared signatures to each pigment in PC645. Here, we report the pigment-specific vibrational markers that enable us to track the spatial flow of excitation energy between the phycobilin pigment pairs. We speculate that two high-frequency modes (1588 and 1596 cm-1) are involved in the vibronic coupling leading to fast (

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The United States is a principal producer of tree nuts (almonds, pistachios, and walnuts), resulting in the generation of excess of tree-nutshell by-products each year, with few market outlets. A nutshell is an essential, lignocellulosic layer that protects a kernel (seed) from the environment during cultivation. The objective of this study was to develop nutshell by-products as herbicide delivery systems, which would not only enable sustainable weed control in fields but also increases nutshell value and reduce the cost of waste disposal. We recently identified a natural salicylaldehyde (SA) that emits volatiles with both herbicidal and antifungal properties. In this study, walnut shell particles saturated with 0.8 to 1.6 M SA were developed as delivery vehicles for SA to soil, which allowed for the controlled release of an SA fumigant for weed control. The pre- and post-emergent herbicidal efficacy of SA was investigated using model monocot (Lolium arundinaceum (Schreb.) Darbysh; turfgrass) and dicot (Brassica rapa var. pekinensis; Chinese cabbage) plants. We compared (1) the effects of different types of solvents for dissolving SA (dimethyl sulfoxide (DMSO) and ethanol (60%, v/v)), and (2) the effect of covering soil with plastic layers (i.e., soil pasteurization) or not covering soil during SA fumigation using nutshells. Results: In the pre-emergent herbicidal testing with the soil covered, the dicot plants exhibited levels of higher susceptibility to SA in DMSO emitted from nutshells when compared to the monocot plants. The seed germination frequencies in the dicots were 15% and 1% with 0.8 and 1.6 M SA, respectively, while those in the monocots were 32% and 18%, respectively, under the same test conditions. In the post-emergent herbicidal testing with the soil covered, the growth of both the monocot and dicot plants was completely prevented after 5 to 7 days of SA fumigation, resulting in the deaths of entire plants. It was noteworthy that in the post-emergent herbicidal testing, SA dissolved in ethanol (60%, v/v) completely disrupted the growth of the monocot and dicot plants as early as 3 days after SA emission from the nutshells, even without the soil being covered. Tree-nutshell particles could serve as effective SA delivery vehicles with controlled release capabilities for SA. The SA exhibited pre- and post-emergent herbicidal activities against the monocot and dicot plants at most growth stages. SA (0.8 and 1.6 M) dissolved in ethanol (60%, v/v) might exert a synergism for higher herbicidal activity after emission from nutshells. Since tree nuts capture/store a substantial amount of carbon over their life-cycles, the new and sustainable utility of using nutshells not only reduces carbon emissions but also valorizes tree-nut by-products, thus benefitting the tree-nut industry.

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The photosystem II core complex (PSII-CC) is the smallest subunit of the oxygenic photosynthetic apparatus that contains core antennas and a reaction center, which together allow for rapid energy transfer and charge separation, ultimately leading to efficient solar energy conversion. However, there is a lack of consensus on the interplay between the energy transfer and charge separation dynamics of the core complex. Here, we report the application of two-dimensional electronic-vibrational (2DEV) spectroscopy to the spinach PSII-CC at 77 K. The simultaneous temporal and spectral resolution afforded by 2DEV spectroscopy facilitates the separation and direct assignment of coexisting dynamical processes. Our results show that the dominant dynamics of the PSII-CC are distinct in different excitation energy regions. By separating the excitation regions, we are able to distinguish the intraprotein dynamics and interprotein energy transfer. Additionally, with the improved resolution, we are able to identify the key pigments involved in the pathways, allowing for a direct connection between dynamical and structural information. Specifically, we show that C505 in CP43 and the peripheral chlorophyll ChlzD1 in the reaction center are most likely responsible for energy transfer from CP43 to the reaction center.

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Photosystem II is crucial for life on Earth as it provides oxygen as a result of photoinduced electron transfer and water splitting reactions. The excited state dynamics of the photosystem II-reaction center (PSII-RC) has been a matter of vivid debate because the absorption spectra of the embedded chromophores significantly overlap and hence it is extremely difficult to distinguish transients. Here, we report the two-dimensional electronic-vibrational spectroscopic study of the PSII-RC. The simultaneous resolution along both the visible excitation and infrared detection axis is crucial in allowing for the character of the excitonic states and interplay between them to be clearly distinguished. In particular, this work demonstrates that the mixed exciton-charge transfer state, previously proposed to be responsible for the far-red light operation of photosynthesis, is characterized by the ChlD1+Phe radical pair and can be directly prepared upon photoexcitation. Further, we find that the initial electron acceptor in the PSII-RC is Phe, rather than PD1, regardless of excitation wavelength.

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