The role of quantum coherence in photochemical functions of molecular systems such as photosynthetic complexes is a broadly debated topic. Coexistence and intermixing of electronic and vibrational coherences has been proposed to be responsible for the observed long-lived coherences and high energy transfer efficiency. However, clear experimental evidence of coherences with different origins operating at the same time has been elusive. In this work, multidimensional spectra obtained from a six-porphyrin nanoring system are analyzed in detail with support from theoretical modeling. We uncover a great diversity of separable electronic, vibrational, and mixed coherences and show their cooperation in shaping the spectroscopic response. The results permit direct assignment of electronic and vibronic states and characterization of the excitation dynamics. The clear disentanglement of coherences in molecules with extended π-conjugation opens up new avenues for exploring coherent phenomena and understanding their importance for the function of complex systems.
Water-soluble chlorophyll-binding proteins (WSCPs) are interesting model systems for the study of pigment-pigment and pigment-protein interactions. While class IIa WSCP has been extensively studied by spectroscopic and theoretical methods, a comprehensive spectroscopic study of class IIb WSCP was lacking so far despite the fact that its structure was determined by X-ray crystallography. In this paper, results of two-dimensional electronic spectroscopy applied to the class IIb WSCP from Lepidium virginicum are presented. Global analysis of 2D data allowed determination of energy levels and excitation energy transfer pathways in the system. Some additional pathways, not present in class IIa WSCP, were observed. The data were interpreted in terms of a model comprising two interacting chlorophyll dimers. In addition, oscillatory signals were observed and identified as coherent beatings of vibrational origin.
We have used time-resolved absorption and fluorescence spectroscopy with nanosecond resolution to study triplet energy transfer from chlorophylls to carotenoids in a protective process that prevents the formation of reactive singlet oxygen. The light-harvesting complexes studied were isolated from Chromera velia, belonging to a group Alveolata, and Xanthonema debile and Nannochloropsis oceanica, both from Stramenopiles. All three light-harvesting complexes are related to fucoxanthin-chlorophyll protein, but contain only chlorophyll a and no chlorophyll c. In addition, they differ in the carotenoid content. This composition of the complexes allowed us to study the quenching of chlorophyll a triplet states by different carotenoids in a comparable environment. The triplet states of chlorophylls bound to the light-harvesting complexes were quenched by carotenoids with an efficiency close to 100%. Carotenoid triplet states were observed to rise with a ~5 ns lifetime and were spectrally and kinetically homogeneous. The triplet states were formed predominantly on the red-most chlorophylls and were quenched by carotenoids which were further identified or at least spectrally characterized.
Chlorophyll (Chl) triplet states generated in photosynthetic light-harvesting complexes (LHCs) can be quenched by carotenoids to prevent the formation of reactive singlet oxygen. Although this quenching occurs with an efficiency close to 100% at physiological temperatures, the Chl triplets are often observed at low temperatures. This might be due to the intrinsic temperature dependence of the Dexter mechanism of excitation energy transfer, which governs triplet quenching, or by temperature-induced conformational changes. Here, we report about the temperature dependence of Chl triplet quenching in two LHCs. We show that both the effects contribute significantly. In LHC II of higher plants, the core Chls are quenched with a high efficiency independent of temperature. A different subpopulation of Chls, which increases with lowering temperature, is not quenched at all. This is probably caused by the conformational changes which detach these Chls from the energy-transfer chain. In a membrane-intrinsic LHC of dinoflagellates, similarly two subpopulations of Chls were observed. In addition, another part of Chl triplets is quenched by carotenoids with a rate which decreases with temperature. This allowed us to study the temperature dependence of Dexter energy transfer. Finally, a part of Chls was quenched by triplet-triplet annihilation, a phenomenon which was not observed for LHCs before.
Carotenoids are together with bacteriochlorophylls important constituents of chlorosomes, the light-harvesting antennae of green photosynthetic bacteria. Majority of bacteriochlorophyll molecules form self-assembling aggregates inside the chlorosomes. Aggregates of bacteriochlorophylls with optical properties similar to those of chlorosomes can also be prepared in non-polar organic solvents or in aqueous environments when a suitable non-polar molecule is added. In this work, the ability of β-carotene to induce aggregation of bacteriochlorophyll c in aqueous buffer was studied. Excitation relaxation and energy transfer in the carotenoid-bacteriochlorophyll assemblies were measured using femtosecond and nanosecond transient absorption spectroscopy. A fast, ~100-fs energy transfer from the S 2 state of β-carotene to bacteriochlorophyll c was revealed, while no evidence for significant energy transfer from the S 1 state was found. Picosecond formation of the carotenoid triplet state (T 1 ) was observed, which was likely generated by singlet homo-fission from the S 1 state of β-carotene. *Manuscript Click here to view linked References 2 IntroductionChlorosomes of green photosynthetic bacteria are the most efficient light-harvesting complexes in Nature [1]. In addition, chlorosomes differ from other photosynthetic complexes in that they lack a protein scaffold housing pigments. Instead, bacteriochlorophyll (BChl) c, dor e molecules found in the interior of the chlorosome are present in the form of selforganized aggregates [2,3]. Proteins are located only in the chlorosome baseplate and envelope [4,5]. Other constituents of chlorosomes are lipids, quinones, and carotenoids [2,3].Lipids are probably part of the chlorosome envelope, together with proteins [6,7]. Quinones are involved in a protective redox-dependent excitation energy quenching mechanism [8], while carotenoids have both light harvesting and photoprotective functions [9][10][11][12][13][14][15]. Singlet excitation energy transfer from carotenoid to aggregated BChls with efficiency between 50 and 80% has been determined in chlorosomes [9,12,14]. The dominant energy transfer pathway proceeds via the carotenoid S 2 state with a fast rate constant of ~(100 fs) -1 [14]. In addition, it was suggested that carotenoids are important for the biogenesis of the chlorosome baseplate and its attachment to the cytoplasmic membrane [16,17].BChl aggregates are organized in curved lamellar structures in chlorosomes [18][19][20][21].Relatively polar chlorin rings of BChls assemble into lamellar layers via several specific interactions; however, the actual molecular arrangement is still a matter of debate [21][22][23].The lamellar layers held together by hydrophobic interactions between interdigitated esterifying alcohols of BChls [18,24]. The hydrophobic space between the chlorin layers is occupied by carotenoids and quinones, together with esterifying alcohols [19].Self-organized aggregates of chlorosomal BChls with spectral properties similar to those of chlorosom...
Bacteriochlorophyll (BChl) c is the main light-harvesting pigment of certain photosynthetic bacteria. It is found in the form of self-assembled aggregates in the so-called chlorosomes. Here we report the results of co-aggregation experiments of BChl c with azulene and its tailored derivatives. We have performed spectroscopic and quantum chemical characterization of the azulenes, followed by self-assembly experiments. The results show that only azulenes with sufficient hydrophobicity are able to induce aggregation of BChl c. Interestingly, only azulene derivatives possessing a conjugated phenyl ring were capable of efficient (∼50%) excitation energy transfer to BChl molecules. These aggregates represent an artificial light-harvesting complex with enhanced absorption between 220 and 350 nm compared to aggregates of pure BChl c. The results provide insight into the principles of self-assembly of BChl aggregates and suggest an important role of the π-π interactions in efficient energy transfer.
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