EPR Spin-Trapping for Monitoring Temporal Dynamics of Singlet Oxygen during Photoprotection in Photosynthesis.

A central goal of photoprotective energy dissipation processes is the regulation of singlet oxygen (1O2*) and reactive oxygen species in the photosynthetic apparatus. Despite the involvement of 1O2* in photodamage and cell signaling, few studies directly correlate 1O2* formation to nonphotochemical quenching (NPQ) or lack thereof. Here, we combine spin-trapping electron paramagnetic resonance (EPR) and time-resolved fluorescence spectroscopies to track in real time the involvement of 1O2* during photoprotection in plant thylakoid membranes. The EPR spin-trapping method for detection of 1O2* was first optimized for photosensitization in dye-based chemical systems and then used to establish methods for monitoring the temporal dynamics of 1O2* in chlorophyll-containing photosynthetic membranes. We find that the apparent 1O2* concentration in membranes changes throughout a 1 h period of continuous illumination. During an initial response to high light intensity, the concentration of 1O2* decreased in parallel with a decrease in the chlorophyll fluorescence lifetime via NPQ. Treatment of membranes with nigericin, an uncoupler of the transmembrane proton gradient, delayed the activation of NPQ and the associated quenching of 1O2* during high light. Upon saturation of NPQ, the concentration of 1O2* increased in both untreated and nigericin-treated membranes, reflecting the utility of excess energy dissipation in mitigating photooxidative stress in the short term (i.e., the initial ∼10 min of high light).

EPR studies of ferredoxin in spinach and cyanobacterial thylakoids related to photosystem I-driven NADP+ reduction.

Photosynthetic light-dependent reactions occur in thylakoid membranes where embedded proteins capture light energy and convert it to chemical energy in the form of ATP and NADPH for use in carbon fixation. One of these integral membrane proteins is Photosystem I (PSI). PSI catalyzes light-driven transmembrane electron transfer from plastocyanin (Pc) to oxidized ferredoxin (Fd). Electrons from reduced Fd are used by the enzyme ferredoxin-NADP+ reductase (FNR) for the reduction of NADP+ to NADPH. Fd and Pc are both small soluble proteins whereas the larger FNR enzyme is associated with the membrane. To investigate electron shuttling between these diffusible and embedded proteins, thylakoid photoreduction of NADP+ was studied. As isolated, both spinach and cyanobacterial thylakoids generate NADPH upon illumination without extraneous addition of Fd. These findings indicate that isolated thylakoids either (i) retain a "pool" of Fd which diffuses between PSI and membrane bound FNR or (ii) that a fraction of PSI is associated with Fd, with the membrane environment facilitating PSI-Fd-FNR interactions which enable multiple turnovers of the complex with a single Fd. To explore the functional association of Fd with PSI in thylakoids, electron paramagnetic resonance (EPR) spectroscopic methodologies were developed to distinguish the signals for the reduced Fe-S clusters of PSI and Fd. Temperature-dependent EPR studies show that the EPR signals of the terminal [4Fe-4S] cluster of PSI can be distinguished from the [2Fe-2S] cluster of Fd at > 30 K. At 50 K, the cw X-band EPR spectra of cyanobacterial and spinach thylakoids reduced with dithionite exhibit EPR signals of a [2Fe-2S] cluster with g-values gx = 2.05, gy = 1.96, and gz = 1.89, confirming that Fd is present in thylakoid preparations capable of NADP+ photoreduction. Quantitation of the EPR signals of P700+ and dithionite reduced Fd reveal that Fd is present at a ratio of ~ 1 Fd per PSI monomer in both spinach and cyanobacterial thylakoids. Light-driven electron transfer from PSI to Fd in thylakoids confirms Fd is functionally associated (< 0.4 Fd/PSI) with the acceptor end of PSI in isolated cyanobacterial thylakoids. These EPR experiments provide a benchmark for future spectroscopic characterization of Fd interactions involved in multistep relay of electrons following PSI charge separation in the context of photosynthetic thylakoid microenvironments.

Coherences of Photoinduced Electron Spin Qubit Pair States in Photosystem I.

This publication presents the first comprehensive experimental study of electron spin coherences in photosynthetic reaction center proteins, specifically focusing on photosystem I (PSI). The ultrafast electron transfer in PSI generates spin-correlated radical pairs (SCRPs), which are entangled spin pairs formed in well-defined spin states (Bell states). Since their discovery in our group in the 1980s, SCRPs have been extensively used to enhance our understanding of structure-function relationships in photosynthetic proteins. More recently, SCRPs have been utilized as tools for quantum sensing. Electron spin decoherence poses a significant challenge in realizing practical applications of electron spin qubits, particularly the creation of quantum entanglement between multiple electron spins. This work is focused on the systematic characterization of decoherence in SCRPs of PSI. These decoherence times were measured as electron spin echo decay times, termed phase memory times (TM), at various temperatures. Decoherence was recorded on both transient SCRP states P700+A1- and thermalized states. Our study reveals that TM exhibits minimal dependence on the biological species, biochemical treatment, and paramagnetic species. The analysis indicates that nuclear spin diffusion and instantaneous diffusion mechanisms alone cannot explain the observed decoherence. As a plausible explanation we discuss the assumption that the low-temperature dynamics of methyl groups in the protein surrounding the unpaired electron spin centers is the main factor governing the loss of the spin coherence in PSI.

Solar water splitting Pt-nanoparticle photosystem I thylakoid systems: Catalyst identification, location and oligomeric structure.

Photosynthetic conversion of light energy into chemical energy occurs in sheet-like membrane-bound compartments called thylakoids and is mediated by large integral membrane protein-pigment complexes called reaction centers (RCs). Oxygenic photosynthesis of higher plants, cyanobacteria and algae requires the symbiotic linking of two RCs, photosystem II (PSII) and photosystem I (PSI), to split water and assimilate carbon dioxide. Worldwide there is a large research investment in developing RC-based hybrids that utilize the highly evolved solar energy conversion capabilities of RCs to power catalytic reactions for solar fuel generation. Of particular interest is the solar-powered production of H2, a clean and renewable energy source that can replace carbon-based fossil fuels and help provide for ever-increasing global energy demands. Recently, we developed thylakoid membrane hybrids with abiotic catalysts and demonstrated that photosynthetic Z-scheme electron flow from the light-driven water oxidation at PSII can drive H2 production from PSI. One of these hybrid systems was created by self-assembling Pt-nanoparticles (PtNPs) with the stromal subunits of PSI that extend beyond the membrane plane in both spinach and cyanobacterial thylakoids. Using PtNPs as site-specific probe molecules, we report the electron microscopic (EM) imaging of oligomeric structure, location and organization of PSI in thylakoid membranes and provide the first direct visualization of photosynthetic Z-scheme solar water-splitting biohybrids for clean H2 production.

Biohybrid photosynthetic charge accumulation detected by flavin semiquinone formation in ferredoxin-NADP+ reductase.

Flavin chemistry is ubiquitous in biological systems with flavoproteins engaged in important redox reactions. In photosynthesis, flavin cofactors are used as electron donors/acceptors to facilitate charge transfer and accumulation for ultimate use in carbon fixation. Following light-induced charge separation in the photosynthetic transmembrane reaction center photosystem I (PSI), an electron is transferred to one of two small soluble shuttle proteins, a ferredoxin (Fd) or a flavodoxin (Fld) (the latter in the condition of Fe-deficiency), followed by electron transfer to the ferredoxin-NADP+ reductase (FNR) enzyme. FNR accepts two of these sequential one electron transfers, with its flavin adenine dinucleotide (FAD) cofactor becoming doubly reduced, forming a hydride which is then passed onto the substrate NADP+ to form NADPH. The two one-electron potentials (oxidized/semiquinone and semiquinone/hydroquinone) are similar to each other with the FNR protein stabilizing the hydroquinone, making spectroscopic detection of the intermediate semiquinone state difficult. We employed a new biohybrid-based strategy that involved truncating the native three-protein electron transfer cascade PSI → Fd → FNR to a two-protein cascade by replacing PSI with a molecular Ru(ii) photosensitizer (RuPS) which is covalently bound to Fd and Fld to form biohybrid complexes that successfully mimic PSI in light-driven NADPH formation. RuFd → FNR and RuFld → FNR electron transfer experiments revealed a notable distinction in photosynthetic charge accumulation that we attribute to the different protein cofactors [2Fe2S] and flavin. After freeze quenching the two-protein systems under illumination, an intermediate semiquinone state of FNR was readily observed with cw X-band EPR spectroscopy. The increased spectral resolution from selective deuteration allowed EPR detection of inter-flavoprotein electron transfer. This work establishes a biohybrid experimental approach for further studies of photosynthetic light-driven electron transfer chain that culminates at FNR and highlights nature's mechanisms that couple single electron transfer chemistry to charge accumulation, providing important insight for the development of photon-to-fuel schemes.

Quantum Sensing of Electron Transfer Pathways in Natural Photosynthesis Using Time-Resolved High-Field Electron Paramagnetic Resonance/Electron-Nuclear Double Resonance Spectroscopy.

Photosynthetic integral membrane reaction center (RC) proteins capture and convert sunlight into chemical energy via efficient charge separation achieved through a series of rapid, photoinitiated electron transfer steps. These fast electron transfers create an entangled spin qubit (radical) pair that contains detailed information about the weak magnetic interactions, structure, and dynamics of localized protein environments involved in charge separation events. Herein, we investigate how these entangled electron spin qubits interact with nuclear spins of the protein environment using the high spectral resolution of 130 GHz electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). Spectroscopic interrogation enabled the observation and probing of protons located in the electron transfer pathway between the membrane-spanning electron pair P+QA- (where P+ is the primary donor, a special pair of bacteriochlorophylls, and QA is the primary quinone acceptor) in the bacterial RC. Spectroscopic analysis reveals hydrogen-bonding interactions involved in regulating the route that light-induced electrons travel through the RC protein during charge separation.

Interprotein electron transfer biohybrid system for photocatalytic H2 production.

Worldwide there is a large research investment in developing solar fuel systems as clean and sustainable sources of energy. The fundamental mechanisms of natural photosynthesis can provide a source of inspiration for these studies. Photosynthetic reaction center (RC) proteins capture and convert light energy into chemical energy that is ultimately used to drive oxygenic water-splitting and carbon fixation. For the light energy to be used, the RC communicates with other donor/acceptor components via a sophisticated electron transfer scheme that includes electron transfer reactions between soluble and membrane bound proteins. Herein, we reengineer an inherent interprotein electron transfer pathway in a natural photosynthetic system to make it photocatalytic for aqueous H2 production. The native electron shuttle protein ferredoxin (Fd) is used as a scaffold for binding of a ruthenium photosensitizer and H2 catalytic function is imparted to its partner protein, ferredoxin-NADP+-reductase (FNR), by attachment of cobaloxime molecules. We find that this 2-protein biohybrid system produces H2 in aqueous solutions via light-induced interprotein electron transfer reactions (TON > 2500 H2/FNR), providing insight about using native protein-protein interactions as a method for fuel generation.

Spin-Correlated Radical Pairs as Quantum Sensors of Bidirectional ET Mechanisms in Photosystem I.

Following light-generated electron transfer reactions in photosynthetic reaction center proteins, an entangled spin qubit (radical) pair is created. The exceptional sensitivity of entangled quantum spin states to weak magnetic interactions, structure, and local environments was used to monitor the directionality of electron transfer in Photosystem I (PSI). Electron paramagnetic resonance (EPR) spectra of radical pairs formed via each symmetric branch of cofactors, A or B, exhibit distinctive line shapes. By photochemical reduction and biochemical modification of PSI we created samples where the radical pair(s) from (1) only A branch, (2) only B branch, or (3) both A and B branches are detectable. These PSI samples were used to analyze the asymmetry of electron transfer as a function of temperature, freezing condition, and temperature cycling. The temperature dependency agrees with a dynamic model in which the conformational states of the protein regulate the directionality of electron transfer. High spectral resolution afforded by high-frequency (130 GHz) EPR, combined with extra resolution afforded by deuterated proteins, provides new mechanistic insight via structural and environmental sensitivity of the entangled electron spins of photogenerated radical pairs.

Z-scheme solar water splitting via self-assembly of photosystem I-catalyst hybrids in thylakoid membranes.

Nature's solar energy converters, the Photosystem I (PSI) and Photosystem II (PSII) reaction center proteins, flawlessly manage photon capture and conversion processes in plants, algae, and cyanobacteria to drive oxygenic water-splitting and carbon fixation. Herein, we utilize the native photosynthetic Z-scheme electron transport chain to drive hydrogen production from thylakoid membranes by directional electron transport to abiotic catalysts bound at the stromal end of PSI. Pt-nanoparticles readily self-assemble with PSI in spinach and cyanobacterial membranes as evidenced by light-driven H2 production in the presence of a mediating electron shuttle protein and the sacrificial electron donor sodium ascorbate. EPR characterization confirms placement of the Pt-nanoparticles on the acceptor end of PSI. In the absence of sacrificial reductant, H2 production at PSI occurs via coupling to light-induced PSII O2 evolution as confirmed by correlation of catalytic activity to the presence or absence of the PSII inhibitor DCMU. To create a more sustainable system, first-row transition metal molecular cobaloxime and nickel diphosphine catalysts were found to perform photocatalysis when bound in situ to cyanobacterial thylakoid membranes. Thus, the self-assembly of abiotic catalysts with photosynthetic membranes demonstrates a tenable method for accomplishing solar overall water splitting to generate H2, a renewable and clean fuel. This work benchmarks a significant advance toward improving photosynthetic efficiency for solar fuel production.

Resolution of Electronic and Structural Factors Underlying Oxygen-Evolving Performance in Amorphous Cobalt Oxide Catalysts.

Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, whereas CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4 p-3 d hybridization through Co-O-Co bonding was detected by RXES and the intersite dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure, whereas CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance oxygen evolving catalysis performance.

Structural and Functional Characterization of a Short-Chain Flavodoxin Associated with a Noncanonical 1,2-Propanediol Utilization Bacterial Microcompartment.

Bacterial microcompartments (BMCs) are proteinaceous organelles that encapsulate enzymes involved in CO2 fixation (carboxysomes) or carbon catabolism (metabolosomes). Metabolosomes share a common core of enzymes and a distinct signature enzyme for substrate degradation that defines the function of the BMC (e.g., propanediol or ethanolamine utilization BMCs, or glycyl-radical enzyme microcompartments). Loci encoding metabolosomes also typically contain genes for proteins that support organelle function, such as regulation, transport of substrate, and cofactor (e.g., vitamin B12) synthesis and recycling. Flavoproteins are frequently among these ancillary gene products, suggesting that these redox active proteins play an undetermined function in many metabolosomes. Here, we report the first characterization of a BMC-associated flavodoxin (Fld1C), a small flavoprotein, derived from the noncanonical 1,2-propanediol utilization BMC locus (PDU1C) of Lactobacillus reuteri. The 2.0 Å X-ray structure of Fld1C displays the α/ß flavodoxin fold, which noncovalently binds a single flavin mononucleotide molecule. Fld1C is a short-chain flavodoxin with redox potentials of -240 ± 3 mV oxidized/semiquinone and -344 ± 1 mV semiquinone/hydroquinone versus the standard hydrogen electrode at pH 7.5. It can participate in an electron transfer reaction with a photoreductant to form a stable semiquinone species. Collectively, our structural and functional results suggest that PDU1C BMCs encapsulate Fld1C to store and transfer electrons for the reactivation and/or recycling of the B12 cofactor utilized by the signature enzyme.

Modular Homogeneous Chromophore-Catalyst Assemblies.

Photosynthetic reaction center (RC) proteins convert incident solar energy to chemical energy through a network of molecular cofactors which have been evolutionarily tuned to couple efficient light-harvesting, directional electron transfer, and long-lived charge separation with secondary reaction sequences. These molecular cofactors are embedded within a complex protein environment which precisely positions each cofactor in optimal geometries along efficient electron transfer pathways with localized protein environments facilitating sequential and accumulative charge transfer. By contrast, it is difficult to approach a similar level of structural complexity in synthetic architectures for solar energy conversion. However, by using appropriate self-assembly strategies, we anticipate that molecular modules, which are independently synthesized and optimized for either light-harvesting or redox catalysis, can be organized into spatial arrangements that functionally mimic natural photosynthesis. In this Account, we describe a modular approach to new structural designs for artificial photosynthesis which is largely inspired by photosynthetic RC proteins. We focus on recent work from our lab which uses molecular modules for light-harvesting or proton reduction catalysis in different coordination geometries and different platforms, spanning from discrete supramolecular assemblies to molecule-nanoparticle hybrids to protein-based biohybrids. Molecular modules are particularly amenable to high-resolution characterization of the ground and excited state of each module using a variety of physical techniques; such spectroscopic interrogation helps our understanding of primary artificial photosynthetic mechanisms. In particular, we discuss the use of transient optical spectroscopy, EPR, and X-ray scattering techniques to elucidate dynamic structural behavior and light-induced kinetics and the impact on photocatalytic mechanism. Two different coordination geometries of supramolecular photocatalyst based on the [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine) light-harvesting module with cobaloxime-based catalyst module are compared, with progress in stabilizing photoinduced charge separation identified. These same modules embedded in the small electron transfer protein ferredoxin exhibit much longer charge-separation, enabled by stepwise electron transfer through the native [2Fe-2S] cofactor. We anticipate that the use of interchangeable, molecular modules which can interact in different coordination geometries or within entirely different structural platforms will provide important fundamental insights into the effect of environment on parameters such as electron transfer and charge separation, and ultimately drive more efficient designs for artificial photosynthesis.

Ru-protein-Co biohybrids designed for solar hydrogen production: understanding electron transfer pathways related to photocatalytic function.

A series of Ru-protein-Co biohybrids have been prepared using the electron transfer proteins ferredoxin (Fd) and flavodoxin (Fld) as scaffolds for photocatalytic hydrogen production. The light-generated charge separation within these hybrids has been monitored by transient optical and electron paramagnetic resonance spectroscopies. Two distinct electron transfer pathways are observed. The Ru-Fd-Co biohybrid produces up to 650 turnovers of H2 utilizing an oxidative quenching mechanism for Ru(ii)* and a sequential electron transfer pathway via the native [2Fe-2S] cluster to generate a Ru(iii)-Fd-Co(i) charge separated state that lasts for ∼6 ms. In contrast, a direct electron transfer pathway occurs for the Ru-ApoFld-Co biohybrid, which lacks an internal electron relay, generating Ru(i)-ApoFld-Co(i) charge separated state that persists for ∼800 µs and produces 85 turnovers of H2 by a reductive quenching mechanism for Ru(ii)*. This work demonstrates the utility of protein architectures for linking donor and catalytic function via direct or sequential electron transfer pathways to enable stabilized charge separation which facilitates photocatalysis for solar fuel production.

Directionality of Electron Transfer in Type I Reaction Center Proteins: High-Frequency EPR Study of PS I with Removed Iron-Sulfur Centers.

A key step of photosynthetic solar energy conversion involves rapid light-induced sequential electron-transfer steps that result in the formation of a stabilized charge-separated state. These primary reactions take place in large integral membrane reaction center (RC) proteins, wherein a series of donor/acceptor cofactors are specifically positioned for efficient electron transfer. RCs can be divided in two classes, Type I and Type II and examples of both types, photosystem I (PS I) and photosystem II (PS II), are involved in the oxygenic photosynthesis of higher plants, cyanobacteria, and algae. High-resolution X-ray crystal structures reveal that PS I and PS II contain two nearly symmetric branches of redox cofactors, termed the A and B branches. While unidirectional ET along the A branch in Type II RCs is well established, there is still a debate of whether primary photochemistry in Type I RCs is unidirectional along the A branch or bidirectional proceeding down both of the A and B branches. Light-induced electron transfer through the B branch has been observed in genetically modified PS I and in native PS I pretreated with strong reducing conditions to reduce three [4Fe-4S] clusters, the terminal electron acceptors of PS I; however, the extent of asymmetry of ET along both cofactor branches remains an open question. To prove that bidirectional ET in PS I is not simply an artifact of a reducing environment or genetic modification and to determine the degree of PS I ET asymmetry, we have examined biochemically modified Synechococcus leopoliensis PS I RCs, wherein the [4Fe-4S] clusters FX, FA, and FB have been removed to prevent secondary ET from phylloquinones (A1A/A1B) to FX. For these Fe-removed proteins, we observe that ET along both the A and B branches occurs with a ratio close to 1. Together with previously reported data, the concomitant structural and kinetic information obtained with HF EPR unambiguously proves the bidirectional nature of ET in PS I over a broad temperature range.

Light-driven hydrogen production from Photosystem I-catalyst hybrids.

Solar energy conversion of water into environmentally clean fuels, such as hydrogen, offers one of the best long-term solutions for meeting future global energy needs. In photosynthesis, high quantum yield charge separation is achieved by a series of rapid, photoinitiated electron transfer steps that take place in proteins called reaction centers (RCs). Of current interest are new strategies that couple RC photochemistry to the direct synthesis of energy-rich molecules, offering opportunities to more directly tune the products of photosynthesis and potentially to increase solar energy conversion capacity. Innovative designs link RC photochemistry with synthetic molecular catalysts to create earth abundant biohybrid complexes that use light to rapidly produce hydrogen from water.

Protein delivery of a Ni catalyst to photosystem I for light-driven hydrogen production.

The direct conversion of sunlight into fuel is a promising means for the production of storable renewable energy. Herein, we use Nature's specialized photosynthetic machinery found in the Photosystem I (PSI) protein to drive solar fuel production from a nickel diphosphine molecular catalyst. Upon exposure to visible light, a self-assembled PSI-[Ni(P2(Ph)N2(Ph))2](BF4)2 hybrid generates H2 at a rate 2 orders of magnitude greater than rates reported for photosensitizer/[Ni(P2(Ph)N2(Ph))2](BF4)2 systems. The protein environment enables photocatalysis at pH 6.3 in completely aqueous conditions. In addition, we have developed a strategy for incorporating the Ni molecular catalyst with the native acceptor protein of PSI, flavodoxin. Photocatalysis experiments with this modified flavodoxin demonstrate a new mechanism for biohybrid creation that involves protein-directed delivery of a molecular catalyst to the reducing side of Photosystem I for light-driven catalysis. This work further establishes strategies for constructing functional, inexpensive, earth-abundant solar fuel-producing PSI hybrids that use light to rapidly produce hydrogen directly from water.

Nature-driven photochemistry for catalytic solar hydrogen production: a Photosystem I-transition metal catalyst hybrid.

Solar energy conversion of water into the environmentally clean fuel hydrogen offers one of the best long-term solutions for meeting future energy demands. Nature provides highly evolved, finely tuned molecular machinery for solar energy conversion that exquisitely manages photon capture and conversion processes to drive oxygenic water-splitting and carbon fixation. Herein, we use one of Nature's specialized energy-converters, the Photosystem I (PSI) protein, to drive hydrogen production from a synthetic molecular catalyst comprised of inexpensive, earth-abundant materials. PSI and a cobaloxime catalyst self-assemble, and the resultant complex rapidly produces hydrogen in aqueous solution upon exposure to visible light. This work establishes a strategy for enhancing photosynthetic efficiency for solar fuel production by augmenting natural photosynthetic systems with synthetically tunable abiotic catalysts.

Metal nanoparticle plasmon-enhanced light-harvesting in a photosystem I thin film.

Silver metal nanoparticle (NP) enhanced fluorescence is investigated in thin films of cyanobacterial Photosystem I trimer complexes (PSI) by correlating confocal laser scanning microscopy, dark-field imaging, and fluorescence lifetime measurements. PSI represents an interesting light-harvesting complex with a 20 nm diameter that is not uniformly contained within the surface-localized plasmon field of the NPs. With weak far-field illumination, 5- to 20-fold fluorescence enhancement is observed for PSI complexes adjacent to NPs, arising from efficient nanoparticle light collection and subsequent localized, surface plasmon excitation of PSI. Enhanced PSI fluorescence is detected most prominently near "rafts" of aggregated NPs that more completely fill the confocal field of view. These results demonstrate opportunities to probe energy transfer within photosynthetic complexes using plasmonic excitation and to design nanostructures for optimizing artificial light-harvesting systems.

Light-induced alteration of low-temperature interprotein electron transfer between photosystem I and flavodoxin.

Electron paramagnetic resonance (EPR) was used to study light-induced electron transfer in Photosystem I-flavodoxin complexes. Deuteration of flavodoxin enables the signals of the reduced flavin acceptor and oxidized primary donor, P(700)(+), to be well-resolved at X- and D-band EPR. In dark-adapted samples, photoinitiated interprotein electron transfer does not occur at 5 K. However, for samples prepared in dim light, significant interprotein electron transfer occurs at 5 K and a concomitant loss of the spin-correlated radical pair P(+)A(1A)(-) signal is observed. These results indicate a light-induced reorientation of flavodoxin in the PSI docking site that allows a high quantum yield efficiency for the interprotein electron transfer reaction.