Role In Light Harvesting Research Articles

(Invited) Ultrafast Studies of Self-Assembled Biomimetic Materials, Metal-Organic-Frameworks, and Plasmonic Metamaterials for Solar Light Harvesting

Ultrafast photoprocesses can play important roles in light harvesting, such as picosecond excitonic hopping between chromophores, or femtosecond decays of hot (nonthermal) carriers in plasmonic metamaterials following photoexcitation. In this talk, I focus on ultrafast photoprocesses in nanostructured materials of interest for light harvesting. In particular, ultrafast light harvesting processes in three types of materials are described. I describe the design of new biomimetic nanostructures for light harvesting involving the use of chromophore-doped peptide amphiphiles to create varied and potentially self-healing structures for transporting energy. Through environmental controls, unique opportunities for tuning the structural shape of the self-assembled materials are possible, thereby altering the resultant energy transport properties. The dependence of excitonic transport on chromophore concentration is described. The second topic is that of exploring light harvesting in metal-organic-frameworks using porphyrin building blocks. Efforts to characterize the degree of anisotropy (directionality) in exciton flow as well as distance of exciton migration are described. Finally, I describe the study of plasmonic metamaterials, where nanostructuring and the creation of “gap-plasmons” dramatically increases the number of hot carriers produced under illumination. Our efforts to spectroscopically characterize the production and decay of hot carriers with variations in size, shape, and composition are shown. Physical insights into how to extract the hot carriers before loss of energy to thermalization is given. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

光伏组件盖板玻璃的结构优化

Cover glass used in photovoltaic panels plays an important role in light harvesting and in turn panel performance. In this paper, the effects on panel performance of surface antireflection (AR) coating and of micropatterns on cover glass were systematically studied by ray tracing simulation. It was found that inner surface patterns of glass have negligible effects on the optical and electrical properties of panels. Both AR film and front surface patterns benefit panel performances. A 45° pyramidal pattern is able to suppress surface reflection and avoid an induced second reflection at the front surface as well as total internal reflection at the glass-air interface. The dependence of the incident angles of light on performance was also examined for the different structured panels. It was found that pyramidal microstructure leads to better performance when the light is irradiated at angles from 0-10° and 60-80°, while AR film is superior at 20-50°. A combination of pyramidal micropatterns and AR film can increase daily generated electricity by 5.6%, in comparison to a panel with bare flat glass.

Transparent, Flexible Silicon Nanostructured Wire Networks with Seamless Junctions for High-Performance Photodetector Applications.

Optically transparent photodetectors are crucial in next-generation optoelectronic applications including smart windows and transparent image sensors. Designing photodetectors with high transparency, photoresponsivity, and robust mechanical flexibility remains a significant challenge, as is managing the inevitable trade-off between high transparency and strong photoresponse. Here we report a scalable method to produce flexible crystalline Si nanostructured wire (NW) networks fabricated from silicon-on-insulator (SOI) with seamless junctions and highly responsive porous Si segments that combine to deliver exceptional performance. These networks show high transparency (∼92% at 550 nm), broadband photodetection (350 to 950 nm) with excellent responsivity (25 A/W), optical response time (0.58 ms), and mechanical flexibility (1000 cycles). Temperature-dependent photocurrent measurements indicate the presence of localized electronic states in the porous Si segments, which play a crucial role in light harvesting and photocarrier generation. The scalable low-cost approach based on SOI has the potential to deliver new classes of flexible optoelectronic devices, including next-generation photodetectors and solar cells.

Mutations in the MIT3 gene encoding a caroteniod isomerase lead to increased tiller number in rice

Carotenoids not only play important roles in light harvesting and photoprotection against excess light, but also serve as precursors for apocaroteniod hormones such as abscisic acid (ABA) and strigolactones (SLs). Although light- and ABA-associated phenotypes of the carotenoid biosynthesis mutants such as albino, leaf variegation and preharvest sprouting have been studied extensively, the SLs-related branching phenotype is rarely explored. Here we characterized four allelic rice mutants named mit3, which exhibited moderately increased tiller number, semi-dwarfism and leaf variegation. Map-based cloning revealed that MIT3 encodes a carotenoid isomerase (CRTISO), the key enzyme catalyzing the conversion from prolycopene to all-trans-lycopene in carotenoid biosynthesis. Prolycopene was accumulated while all-trans-lycopene was barely detectable in the dark-grown mit3 seedlings. Accordingly, content of lutein and β-carotene, the two most abundant carotenoids, was significantly reduced. Furthermore, content of epi-5DS, a native SL, was significantly reduced in mit3. Exogenously applied GR24, a synthetic SL, could rescue the tillering phenotype of mit3. Double mutant analysis of mit3 with the SLs biosynthesis mutant d17 revealed that MIT3 controls tiller development upstream of the SLs biosynthesis pathway. Our results reveal that the tillering phenotype of mit3 is due to SL deficiency and directly link carotenoid deficiency with SL-regulated rice tillering.

Proteomic characterization of hierarchical megacomplex formation in Arabidopsis thylakoid membrane.

Conversion of solar energy into chemical energy in plant chloroplasts concomitantly modifies the thylakoid architecture and hierarchical interactions between pigment-protein complexes. Here, the thylakoids were isolated from light-acclimated Arabidopsis leaves and investigated with respect to the composition of the thylakoid protein complexes and their association into higher molecular mass complexes, the largest one comprising both photosystems (PSII and PSI) and light-harvesting chlorophyll a/b-binding complexes (LHCII). Because the majority of plant light-harvesting capacity is accommodated in LHCII complexes, their structural interaction with photosystem core complexes is extremely important for efficient light harvesting. Specific differences in the strength of LHCII binding to PSII core complexes and the formation of PSII supercomplexes are well characterized. Yet, the role of loosely bound L-LHCII that disconnects to a large extent during the isolation of thylakoid protein complexes remains elusive. Because L-LHCII apparently has a flexible role in light harvesting and energy dissipation, depending on environmental conditions, its close interaction with photosystems is a prerequisite for successful light harvesting invivo. Here, to reveal the labile and fragile light-dependent protein interactions in the thylakoid network, isolated membranes were subjected to sequential solubilization using detergents with differential solubilization capacity and applying strict quality control. Optimized 3D-lpBN-lpBN-sodium dodecyl sulfate-polyacrylamide gel electrophoresis system demonstrated that PSII-LHCII supercomplexes, together with PSI complexes, hierarchically form larger megacomplexes via interactions with L-LHCII trimers. The polypeptide composition of LHCII trimers and the phosphorylation of Lhcb1 and Lhcb2 were examined to determine the light-dependent supramolecular organization of the photosystems into megacomplexes.

Protein binding pigment by Bacillus pumilus SF214

Carotenoids are the most widespread group of naturally occurring pigments. These yellow, orange and red colored molecules are found in both eukaryotes and prokaryotes. In photosynthetic organisms, carotenoids play an essential role in light harvesting, while in non-photosynthetic organisms their principal function is to provide protection against photo-oxidative damages.In order to prepare spores of Bacillus pumilus SF214, sporulation was induced by exhaustion method. Electrophoresis of proteins on polyacrylamide gels (SDS-PAGE) was done using extracted of proteins coating spores, for an aliquot of spores treated with 0.1 M NaOH and to determine the sequence of protein band we used Edman degradation technique and the database (BLASTp) software. Protein extracts of red and white spores showed several differences for what concerns the intensity of some protein bands. Clear increasing in pigment production with the progression of the stationary phase of growth. The analysis of protein band transferred on nitrocellulose membrane using Edman degradation technique showed that the sequence alignment obtained is the homologous N-terminal protein of Bacillus subtilis Tasa. This study suggests that pigmentation in spores represents an additional and maybe alternative protection strategy against oxidative stress. For the first time, this study was performed to describe the correlation between the pigment, protein and spore formation, Which support the proposed application in the field of food or medical production as probiotics, this study was performed to describe the correlation between the pigment, protein and spore formation, Which support the proposed application in the field of food or medical production as probiotics.

A General Strategy to Enhance the Performance of Dye-Sensitized Solar Cells by Incorporating a Light-Harvesting Dye with a Hydrophobic Polydiacetylene Electrolyte-Blocking Layer.

A unique strategy to suppress charge recombination effectively and enhance light harvesting in dye-sensitized solar cells (DSSCs) is demonstrated by the design of a new dipolar organic dye functionalized with a diacetylene unit, which is capable of undergoing a photoinduced crosslinking reaction to generate a hydrophobic polydiacetylene layer. The polydiacetylene layer serves as an electrolyte-blocking layer that effectively blocks the approach of the oxidized redox mediator and suppresses the dark current, and also plays a role in light harvesting owing to efficient energy transfer to the dipolar dyes. A 15 % efficiency improvement was achieved on going from the monomer dye (JSC =13.5 mA cm-2 , Voc =0.728 V, fill factor=0.73, η=7.17 %) to the crosslinked dye (JSC =14.9 mA cm-2 , Voc =0.750 V, fill factor=0.74, η=8.27 %) under AM 1.5 conditions.

Identification of Plastoglobules as a Site of Carotenoid Cleavage.

Carotenoids play an essential role in light harvesting and protection from excess light. During chloroplast senescence carotenoids are released from their binding proteins and are eventually metabolized. Carotenoid cleavage dioxygenase 4 (CCD4) is involved in carotenoid breakdown in senescing leaf and desiccating seed, and is part of the proteome of plastoglobules (PG), which are thylakoid-associated lipid droplets. Here, we demonstrate that CCD4 is functionally active in PG. Leaves of Arabidopsis thaliana ccd4 mutants constitutively expressing CCD4 fused to yellow fluorescent protein showed strong fluorescence in PG and reduced carotenoid levels upon dark-induced senescence. Lipidome-wide analysis indicated that β-carotene, lutein, and violaxanthin were the principle substrates of CCD4 in vivo and were cleaved in senescing chloroplasts. Moreover, carotenoids were shown to accumulate in PG of ccd4 mutant plants during senescence, indicating translocation of carotenoids to PG prior to degradation.

Observation of Electronic Excitation Transfer Through Light Harvesting Complex II Using Two-Dimensional Electronic-Vibrational Spectroscopy.

Light-harvesting complex II (LHCII) serves a central role in light harvesting for oxygenic photosynthesis and is arguably the most important photosynthetic antenna complex. In this work, we present two-dimensional electronic-vibrational (2DEV) spectra of LHCII isolated from spinach, demonstrating the possibility of using this technique to track the transfer of electronic excitation energy between specific pigments within the complex. We assign the spectral bands via comparison with the 2DEV spectra of the isolated chromophores, chlorophyll a and b, and present evidence that excitation energy between the pigments of the complex are observed in these spectra. Finally, we analyze the essential components of the 2DEV spectra using singular value decomposition, which makes it possible to reveal the relaxation pathways within this complex.

Biochemical and Spectroscopic Characterization of Highly Stable Photosystem II Supercomplexes from Arabidopsis

Photosystem II (PSII) is a large membrane supercomplex involved in the first step of oxygenic photosynthesis. It is organized as a dimer, with each monomer consisting of more than 20 subunits as well as several cofactors, including chlorophyll and carotenoid pigments, lipids, and ions. The isolation of stable and homogeneous PSII supercomplexes from plants has been a hindrance for their deep structural and functional characterization. In recent years, purification of complexes with different antenna sizes was achieved with mild detergent solubilization of photosynthetic membranes and fractionation on a sucrose gradient, but these preparations were only stable in the cold for a few hours. In this work, we present an improved protocol to obtain plant PSII supercomplexes that are stable for several hours/days at a wide range of temperatures and can be concentrated without degradation. Biochemical and spectroscopic properties of the purified PSII are presented, as well as a study of the complex solubility in the presence of salts. We also tested the impact of a large panel of detergents on PSII stability and found that very few are able to maintain the integrity of PSII. Such new PSII preparation opens the possibility of performing experiments that require room temperature conditions and/or high protein concentrations, and thus it will allow more detailed investigations into the structure and molecular mechanisms that underlie plant PSII function.

QM/MM dynamics of a Peridinin model in triplet state in three prototypical solvents

Peridinin (Per) is a carbonyl-containing carotenoid playing a key role in light harvesting and photoprotection in dinoflagellates. This carotenoid plays its photoprotective role by quenching the potentially dangerous 3Chl-a triplet state through the formation of the non-reactive 3Per triplet state through Dexter energy transfer mechanism. We have investigated by means of Quantum Mechanics/Molecular Mechanics dynamics simulations at room temperature the structural and dynamical properties of a Peridinin model system (PMS) in triplet state in three different solvents: cyclohexane, apolar/aprotic; acetonitrile, polar/aprotic; and methanol (MeOH), polar/protic. Our results of 3PMS in MeOH show that the lactonic carbonyl has a stronger tendency to accept hydrogen bonds compared to the corresponding singlet ground state (1PMS). This effect may play some so far overlooked role in Per-containing proteins (notably the water soluble Peridinin-Chlorophyll-Proteins – PCPs).The vibrational properties of the 3PMS dynamics in the three solvents have been analyzed by means of decomposition of the vibrational density of states in effective normal modes. The results show that the solute-solvent interactions can influence some vibrational bands of 3PMS; in particular, they are able to modulate the position of the lactonic CO stretching band. The situation is particularly evident in the case of MeOH, where the dynamics of the MeOH⋯OC hydrogen bond interactions can strongly influence the band position and shape. As vibrational spectroscopy (notably step-scan FTIR difference spectroscopy) has been largely used to investigate 3Per in PCPs, especially using the lactonic carbonyl stretching as a marker band to investigate the different photophysical role of each Per in the protein complex, this study represents an important step to understand the experimental spectra and to identify the Per(s) molecule(s) bearing the triplet in PCPs.

Photosynthetic pigments of oceanic Chlorophyta belonging to prasinophytes clade VII.

The ecological importance and diversity of pico/nanoplanktonic algae remains poorly studied in marine waters, in part because many are tiny and without distinctive morphological features. Amongst green algae, Mamiellophyceae such as Micromonas or Bathycoccus are dominant in coastal waters while prasinophytes clade VII, yet not formerly described, appear to be major players in open oceanic waters. The pigment composition of 14 strains representative of different subclades of clade VII was analyzed using a method that improves the separation of loroxanthin and neoxanthin. All the prasinophytes clade VII analyzed here showed a pigment composition similar to that previously reported for RCC287 corresponding to pigment group prasino-2A. However, we detected in addition astaxanthin for which it is the first report in prasinophytes. Among the strains analyzed, the pigment signature is qualitatively similar within subclades A and B. By contrast, RCC3402 from subclade C (Picocystis) lacks loroxanthin, astaxanthin, and antheraxanthin but contains alloxanthin, diatoxanthin, and monadoxanthin that are usually found in diatoms or cryptophytes. For subclades A and B, loroxanthin was lowest at highest light irradiance suggesting a light-harvesting role of this pigment in clade VII as in Tetraselmis.

Identification and functional analysis of the geranylgeranyl pyrophosphate synthase gene (crtE) and phytoene synthase gene (crtB) for carotenoid biosynthesis in Euglena gracilis

BackgroundEuglena gracilis, a unicellular phytoflagellate within Euglenida, has attracted much attention as a potential feedstock for renewable energy production. In outdoor open-pond cultivation for biofuel production, excess direct sunlight can inhibit photosynthesis in this alga and decrease its productivity. Carotenoids play important roles in light harvesting during photosynthesis and offer photoprotection for certain non-photosynthetic and photosynthetic organisms including cyanobacteria, algae, and higher plants. Although, Euglenida contains β-carotene and xanthophylls (such as zeaxanthin, diatoxanthin, diadinoxanthin and 9′-cis neoxanthin), the pathway of carotenoid biosynthesis has not been elucidated.ResultsTo clarify the carotenoid biosynthetic pathway in E. gracilis, we searched for the putative E. gracilis geranylgeranyl pyrophosphate (GGPP) synthase gene (crtE) and phytoene synthase gene (crtB) by tblastn searches from RNA-seq data and obtained their cDNAs. Complementation experiments in Escherichia coli with carotenoid biosynthetic genes of Pantoea ananatis showed that E. gracilis crtE (EgcrtE) and EgcrtB cDNAs encode GGPP synthase and phytoene synthase, respectively. Phylogenetic analyses indicated that the predicted proteins of EgcrtE and EgcrtB belong to a clade distinct from a group of GGPP synthase and phytoene synthase proteins, respectively, of algae and higher plants.In addition, we investigated the effects of light stress on the expression of crtE and crtB in E. gracilis. Continuous illumination at 460 or 920 μmol m−2 s−1 at 25 °C decreased the E. gracilis cell concentration by 28–40 % and 13–91 %, respectively, relative to the control light intensity (55 μmol m−2 s−1). When grown under continuous light at 920 μmol m−2 s−1, the algal cells turned reddish-orange and showed a 1.3-fold increase in the crtB expression. In contrast, EgcrtE expression was not significantly affected by the light-stress treatments examined.ConclusionsWe identified genes encoding CrtE and CrtB in E. gracilis and found that their protein products catalyze the early steps of carotenoid biosynthesis. Further, we found that the response of the carotenoid biosynthetic pathway to light stress in E. gracilis is controlled, at least in part, by the level of crtB transcription. This is the first functional analysis of crtE and crtB in Euglena.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0698-8) contains supplementary material, which is available to authorized users.

Carotenoids in Microalgae.

Carotenoids are a class of isoprenoids synthesized by all photosynthetic organisms as well as by some non-photosynthetic bacteria and fungi with broad applications in food, feed and cosmetics, and also in the nutraceutical and pharmaceutical industries. Microalgae represent an important source of high-value products, which include carotenoids, among others. Carotenoids play key roles in light harvesting and energy transfer during photosynthesis and in the protection of the photosynthetic apparatus against photooxidative damage. Carotenoids are generally divided into carotenes and xanthophyls, but accumulation in microalgae can also be classified as primary (essential for survival) and secondary (by exposure to specific stimuli).In this chapter, we outline the high value carotenoids produced by commercially important microalgae, their production pathways, the improved production rates that can be achieved by genetic engineering as well as their biotechnological applications.

Organic-Semiconductor Quantum Dot Hybrids: Controlling Photoinduced Energy and Electron Transfer

Self-assembled monolayes of photoactive compounds on semiconductor surfaces have been a subject of active research for a few decades already [1]. Apart from fundamental interest in electronic interactions at organic-semiconductor interfaces, the driving force for this activity is wide range of promising applications such as photo-voltaics, molecular senor, and nano-electronics. Although many semiconductor were tested in designing such organic-semiconductor hybrids, the properties of bulk semiconductors cannot be tuned easily and most of the research efforts were focused at the organic counterpart by synthesis of organic compounds with varying electronic properties, and providing different orientation and distance form the semiconductor surface [2]. Recent invention of semiconductor quantum dots (QDs) has increased tunability of organic-semiconductor hybrids drastically by allowing to very the energetic properties of the semiconductor counterpart [3]. The latter is controlled by the size of QDs which limits the spatial confinement of electrons in QDs and has two effects. Firstly, the energies of valence and conduction bands can be tuned in a rather wide range which affect the electron transfer at the organic-semiconductor interface. Secondly, the optical properties, absorption and emission of QDs, also depend on the size. Thus QDs can play an active role in light harvesting by organic-QD hybrids. The main objectives of this study is to gain information on energy and electron transfer in organic-QD hybrids when both parts can absorb light, and find bases to control the direction of electron transfer between organic and semiconductor counterparts. The QDs used in this study are CdSe core and core/shell QDs of varying size. Organic compounds are phthalocyanine, porphyrin and fullerene derivatives. The study was carried out using a range of spectroscopy techniques including steady state and ultra-fast time resolved methods. The steady state emission quenching and reduction of the emission lifetime of both QDs and organic compounds were used as primary tools to ensure hybrid formation and to evaluate the degree of electronic interactions. Femtosecond to nanosecond transient absorption spectroscopy [4] was used then for selected systems to identify the intermediate states formed and to obtain quantitative information on the reaction rate constants. In the visible part of the spectrum the responses were dominated by the QDs showing some bleaching and reshaping of the QD ground state absorption. However, in the red and near infrared (NIR) part of the spectrum the responses from organic chromophores had comparable or, in some cases, stronger contribution than that from the QDs. Therefore the NIR part of the spectrum was more informative for identification of the intermediate states and establishing the reaction mechanisms. The results show that the photoinduced electron transfer from CdSe QDs to fullerene C60derivative takes place for QDs of different size. The photoinduced electron transfer to the QDs was observed for QD-phthalocyanine hybrids, in which case only large QDs (with emission in the red part of the spectrum) operated as electron acceptors, whereas for hybrids with smaller QDs only energy transfer was observed. This can be rationalized considering the dependence of the energy of QD conduction band lower edge on the size. The larger QDs have lower energy and are better electron acceptor. This property of QDs can be effectively used to control photophyiscal reactions in QD-organic hybrids.

Data for iTRAQ-based quantitative proteomics analysis of Brassica napus leaves in response to chlorophyll deficiency.

The essential pigment chlorophyll (Chl) plays important roles in light harvesting and energy transfer during photosynthesis. Here we present the data from a comparative proteomic analysis of chlorophyll-deficient Brassica napus mutant cde1 and its corresponding wild-type using the iTRAQ approach (Pu Chu et al., 2014 [1]). The distribution of length and number of peptides, mass and sequence coverage of proteins identified was calculated, and the repeatability of the replicates was analyzed. A total of 443 differentially expressed proteins were identified in B. napus leaves, including 228 down-accumulated proteins mainly involved in photosynthesis, porphyrin and chlorophyll metabolism, biosynthesis of secondary metabolites, carbon fixation and 215 up-accumulated proteins that enriched in the spliceosome, mRNA surveillance and RNA degradation.

ITRAQ-based quantitative proteomics analysis of Brassica napus leaves reveals pathways associated with chlorophyll deficiency.

Photosynthesis, the primary source of plant biomass, is important for plant growth and crop yield. Chlorophyll is highly abundant in plant leaves and plays essential roles in photosynthesis. We recently isolated a chlorophyll-deficient mutant (cde1) from ethyl methanesulfonate (EMS) mutagenized Brassica napus. Herein, quantitative proteomics analysis using the iTRAQ approach was conducted to investigate cde1-induced changes in the proteome. We identified 5069 proteins from B. napus leaves, of which 443 showed differential accumulations between the cde1 mutant and its corresponding wild-type. The differentially accumulated proteins were found to be involved in photosynthesis, porphyrin and chlorophyll metabolism, biosynthesis of secondary metabolites, carbon fixation, spliceosome, mRNA surveillance and RNA degradation. Our results suggest that decreased abundance of chlorophyll biosynthetic enzymes and photosynthetic proteins, impaired carbon fixation efficiency and disturbed redox homeostasis might account for the reduced chlorophyll contents, impaired photosynthetic capacity and increased lipid peroxidation in this mutant. Epigenetics was implicated in the regulation of gene expression in cde1, as proteins involved in DNA/RNA/histone methylation and methylation-dependent chromatin silencing were up-accumulated in the mutant. Biological significance Photosynthesis produces more than 90% of plant biomass and is an important factor influencing potential crop yield. The pigment chlorophyll plays essential roles in light harvesting and energy transfer during photosynthesis. Mutants deficient in chlorophyll synthesis have been used extensively to investigate the chlorophyll metabolism, development and photosynthesis. However, limited information is available with regard to the changes of protein profiles upon chlorophyll deficiency. Here, a combined physiological, histological, proteomics and molecular analysis revealed several important pathways associated with chlorophyll deficiency. This work provides new insights into the regulation of chlorophyll biosynthesis and photosynthesis in higher plants and these findings may be applied to genetic engineering for high photosynthetic efficiency in crops.

Multicarrier Interactions in Semiconductor Nanocrystals in Relation to the Phenomena of Auger Recombination and Carrier Multiplication

Chemically synthesized semiconductor nanocrystals (NCs) have been extensively studied as a test bed for exploring the physics of strong quantum confinement and as a highly flexible materials platform for the realization of a new generation of solution-processed optical, electronic, and optoelectronic devices. Because of readily tunable, size-dependent emission and absorption spectra, colloidal NCs are especially attractive for applications in light-emitting diodes, solid-state lighting, lasing, and solar cells. It is universally recognized that the realization of these and other prospective applications of NCs requires a detailed understanding of carrier-carrier interactions in these structures, as they have a strong effect on both recombination and photogeneration dynamics of charge carriers. For example, nonradiative Auger recombination is one of the key factors limiting the performance of NC-based lasers and light-emitting diodes. The inverse of this process, carrier multiplication, plays a beneficial role in light harvesting and can be used to boost the efficiency of photovoltaics through increased photocurrent. This article reviews recent progress in the understanding of multicarrier processes in NCs of various complexities, including zero-dimensional spherical quantum dots, quasi-one-dimensional nanorods, and various types of core-shell heterostructures. This review’s specific focus is on recent efforts toward controlling multicarrier interactions using traditional approaches, such as size and shape control, as well as newly developed methods involving interface engineering for suppression of Auger decay and engineering of intraband cooling rates for enhancement of carrier multiplication.

On the question of the light-harvesting role of β-carotene in photosystem II and photosystem I core complexes

β-Carotene is the only carotenoid present in the core complexes of Photosystems I and II. Its proximity to chlorophyll a molecules enables intermolecular electronic interactions, including β-carotene to chlorophyll a electronic excitation transfers. However, it has been well documented that, compared to chlorophylls and to phycobilins, the light harvesting efficiency of β-carotenes for photosynthetic O2 evolution is poor. This is more evident in cyanobacteria than in plants and algae because they lack accessory light harvesting pigments with absorptions that overlap the β-carotene absorption. In the present work we investigated the light harvesting role of β-carotenes in the cyanobacterium Synechococcus sp. PCC 7942 using selective β-carotene excitation and selective Photosystem detection of photo-induced electron transport to and from the intersystem plastoquinones (the plastoquinone pool). We report that, although selectively excited β-carotenes transfer electronic excitation to the chlorophyll a of both photosystems, they enable only the oxidation of the plastoquinone pool by Photosystem I but not its reduction by Photosystem II. This may suggest a light harvesting role for the β-carotenes of the Photosystem I core complex but not for those of the Photosystem II core complex. According to the present investigation, performed with whole cyanobacterial cells, the lower photosynthesis yields measured with β-Car-absorbed light can be attributed to the different excitation trapping efficiencies in the reaction centers of PSI and PSII.

Making proteins green; biosynthesis of chlorophyll-binding proteins in cyanobacteria

Chlorophyll (Chl) is an essential component of the photosynthetic apparatus. Embedded into Chl-binding proteins, Chl molecules play a central role in light harvesting and charge separation within the photosystems. It is critical for the photosynthetic cell to not only ensure the synthesis of a sufficient amount of new Chl-binding proteins but also avoids any misbalance between apoprotein synthesis and the formation of potentially phototoxic Chl molecules. According to the available data, Chl-binding proteins are translated on membrane bound ribosomes and their integration into the membrane is provided by the SecYEG/Alb3 translocon machinery. It appears that the insertion of Chl molecules into growing polypeptide is a prerequisite for the correct folding and finishing of Chl-binding protein synthesis. Although the Chl biosynthetic pathway is fairly well-described on the level of enzymatic steps, a link between Chl biosynthesis and the synthesis of apoproteins remains elusive. In this review, I summarize the current knowledge about this issue putting emphasis on protein-protein interactions. I present a model of the Chl biosynthetic pathway organized into a multi-enzymatic complex and physically attached to the SecYEG/Alb3 translocon. Localization of this hypothetical large biosynthetic centre in the cyanobacterial cell is also discussed as well as regulatory mechanisms coordinating the rate of Chl and apoprotein synthesis.