Photosynthetic Reaction Center Research Articles

Solid-State NMR Analysis of the Dynamics of Cofactors: Comparison of Heliobacterial and Purple Bacterial Reaction Centers.

Photosynthetic reaction centers (RCs) serve as natural engines converting solar energy to chemical energy. Understanding the principles of efficient charge separation and light-induced electron transfer (ET) between the chlorophyll-type pigments might guide the synthesis for artificial photosynthetic systems. We present detailed insight into the dynamics at the atomic level using solid-state NMR techniques applied to the RCs of Heliobacillus (Hb.) mobilis (HbRCs) and the purple bacterium Rhodobacter (R.) sphaeroides (PbRCs). It is assumed that heliobacteria were among the first phototrophic organisms; therefore, their RC can be regarded as ancient. They are constructed homodimerically with perfect C2 symmetry, enabling ET over both branches of cofactors. Modern RCs of R. sphaeroides wild-type (WT) have higher redox power and are functionally highly asymmetric. The dynamics of the cofactors in both RCs has been explored using nuclear hyperpolarization, induced by the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect. Based on the individual incorporation of 13C positions of the cofactors (through supplementation by 13C-δ-aminolevulinic acid), photo-CIDNP magic-angle spinning (MAS) NMR experiments provide access to the local dynamics of the cofactors along the ET path over a broad range of time scales. Theoretical analysis of the dynamic deformation of these macrocycles is also discussed in terms of function. The dynamics observed in HbRCs appears to be correlated to ET. The cofactors in PbRC are significantly less dynamic than those in the HbRC. Relevance for efficiency and redox properties are discussed.

Tuning Electron-Transfer Driving Force in Photosynthetic Special Pair Models.

Visible-light excitation of a family of bimetallic ruthenium polypyridines with the formula [RuII(tpy)(bpy)(𝜇-CN)RuII(py)4L]n+ (RuRuLn+), where L = Cl-, NCS-, DMAP and ACN, was used to prepare photoinduced mixed-valence (PI-MV) MLCT states as models of the photosynthetic reaction center. Ultrafast transient absorption spectroscopy allowed to monitor photoinduced IVCT bands between 6000 and 11000 cm-1. Mulliken spin densities resulting from DFT and (TD)DFT computations revealed the modulation of the charge density distribution depending on the ligand substitution pattern. Results are consistent with PI-MV systems ranging from non-degenerate Class II to degenerate Class III, with electronic couplings between 1000 and 3500 cm-1.These findings guide the control electron localization-delocalization in charge-transfer/charge-separated excited states, like those involved in the photosynthetic reaction center.

Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of Rhodobacter sphaeroides

The pH dependence of the free energy level of the flash-induced primary charge pair P+IA− was determined by a combination of the results from the indirect charge recombination of P+QA− and from the delayed fluorescence of the excited dimer (P*) in the reaction center of the photosynthetic bacterium Rhodobacter sphaeroides, where the native ubiquinone at the primary quinone binding site QA was replaced by low-potential anthraquinone (AQ) derivatives. The following observations were made: (1) The free energy state of P+IA− was pH independent below pH 10 (–370 ± 10 meV relative to that of the excited dimer P*) and showed a remarkable decrease (about 20 meV/pH unit) above pH 10. A part of the dielectric relaxation of the P+IA− charge pair that is not insignificant (about 120 meV) should come from protonation-related changes. (2) The single exponential decay character of the kinetics proves that the protonated/unprotonated P+IA− and P+QA− states are in equilibria and the rate constants of protonation konH +koffH are much larger than those of the charge back reaction kback ~103 s−1. (3) Highly similar pH profiles were measured to determine the free energy states of P+QA− and P+IA−, indicating that the same acidic cluster at around QB should respond to both anionic species. This was supported by model calculations based on anticooperative proton distribution in the cluster with key residues of GluL212, AspL213, AspM17, and GluH173, and the effect of the polarization of the aqueous phase on electrostatic interactions. The larger distance of IA− from the cluster (25.2 Å) compared to that of QA− (14.5 Å) is compensated by a smaller effective dielectric constant (6.5 ± 0.5 and 10.0 ± 0.5, respectively). (4) The P* → P+QA− and IA−QA → IAQA− electron transfers are enthalpy-driven reactions with the exemption of very large (>60%) or negligible entropic contributions in cases of substitution by 2,3-dimethyl-AQ or 1-chloro-AQ, respectively. The possible structural consequences are discussed.

A Structural and Functional Mimic of P680+

One or multiple chlorophyll molecules are employed in the reaction center of photosystem II's main electron donor (defined as P680). We have a poor understanding of how the reaction center facilitates water oxidation and the roles that mono‐ and/or multimeric chlorophyll groups play when P680 oxidizes a neighboring tyrosine in order to drive water oxidation at the oxygen evolving complex. We have prepared a dimeric MgII‐porphyrin complex [Mg2(BTPP)] (1, H4‐BTPP = 1,2‐bis‐(10,15,20‐triphenylporphyrin‐5‐yl)‐benzene) as a structural and functional mimic of the dimeric core of P680. 1 was oxidized by one‐electron to the corresponding π‐cation radical complex 2. The radical cation was characterized by UV‐Vis‐NIR, FT‐IR, and EPR spectroscopic techniques. 2 was shown to be reactive towards phenols to give the corresponding phenoxyl radicals, mimicking the reactivity of the P680 cation radical which oxidizes tyrosine to tyrosyl radical. Critically, the dimeric π‐cation radical showed markedly higher rates of proton coupled electron transfer oxidation (PCET) of phenols when compared to its monomeric counterpart [Mg(TPP)] (TPP = 5,10,15,20‐tetraphenylporphyrin). Our findings demonstrate that MgII‐porphyrin complexes are reliable mimics of photosynthetic PCET processes and suggest that photosynthetic reaction centers with multiple π‐conjugated complexes likely lower the barrier to PCET oxidation by π‐cation radical species.

A Structural and Functional Mimic of P680.

One or multiple chlorophyll molecules are employed in the reaction center of photosystem II's main electron donor (defined as P680). We have a poor understanding of how the reaction center facilitates water oxidation and the roles that mono- and/or multimeric chlorophyll groups play when P680 oxidizes a neighboring tyrosine in order to drive water oxidation at the oxygen evolving complex. We have prepared a dimeric MgII-porphyrin complex [Mg2(BTPP)] (1, H4-BTPP = 1,2-bis-(10,15,20-triphenylporphyrin-5-yl)-benzene) as a structural and functional mimic of the dimeric core of P680. 1 was oxidized by one-electron to the corresponding π-cation radical complex 2. The radical cation was characterized by UV-Vis-NIR, FT-IR, and EPR spectroscopic techniques. 2 was shown to be reactive towards phenols to give the corresponding phenoxyl radicals, mimicking the reactivity of the P680 cation radical which oxidizes tyrosine to tyrosyl radical. Critically, the dimeric π-cation radical showed markedly higher rates of proton coupled electron transfer oxidation (PCET) of phenols when compared to its monomeric counterpart [Mg(TPP)] (TPP = 5,10,15,20-tetraphenylporphyrin). Our findings demonstrate that MgII-porphyrin complexes are reliable mimics of photosynthetic PCET processes and suggest that photosynthetic reaction centers with multiple π-conjugated complexes likely lower the barrier to PCET oxidation by π-cation radical species.

Nuclear hyperpolarization in electron-transfer proteins: Revealing unexpected light-induced 15N signals with field-cycling magic-angle spinning NMR

The solid-state photo-CIDNP (photo-chemically induced dynamic nuclear polarization) effect allows for nuclear hyperpolarization, i.e., non-Boltzmann nuclear spin population. The effect relies on the light-induced formation of a spin-correlated radical pair (SCRP) and has been observed in various photosynthetic reaction center (RC) proteins and flavin-containing light, oxygen, voltage (LOV) proteins. Both systems exhibit strongly enhanced NMR signals originating from the electron transfer partners. Here, we present experimental data on the magnetic field dependence of the 15N solid-state photo-CIDNP effect in both phototropin LOV1 C57S from Chlamydomonas reinhardtii and the bacterial photosynthetic RC from Rhodobacter sphaeroides. Using a pneumatic field-cycling system, samples containing a frozen solution of the proteins are explored between 0.25 T and 9.4 T. Both systems yield hyperpolarized 15N NMR signals across the entire magnetic field range originating from the electron transfer moieties. Also, in both systems, hyperpolarized signals from unexpected positions are detected between 1.0 T and 2.0 T: position N-1 of the flavin in the LOV1 protein and the τ-N of the axial magnesium-coordinating histidine of the donor. A first attempt to explain the occurrence of these unexpected signals based on quantum chemical calculations is presented.

Metataxonomy and pigments analyses unravel microbial diversity and the relevance of retinal-based photoheterotrophy at different salinities in the Odiel Salterns (SW, Spain)

Salinity has a strong influence on microorganisms distribution patterns and consequently on the relevance of photoheterotrophic metabolism, which since the discovery of proteorhodopsins is considered the main contributor to solar energy capture on the surface of the oceans. Solar salterns constitute an exceptional system for the simultaneous study of several salt concentrations, ranging from seawater, the most abundant environment on Earth, to saturated brine, one of the most extreme, which has been scarcely studied. In this study, pigment composition across the salinity gradient has been analyzed by spectrophotometry and RP-HPLC, and the influence of salinity on microbial diversity of the three domains of life has been evaluated by a metataxonomic study targeting hypervariable regions of 16S and 18S rRNA genes. Furthermore, based on the chlorophyll and retinal content, we have estimated the relative abundance of rhodopsins and photosynthetic reaction centers, concluding that there is a strong correlation between the retinal/chlorophyll a ratio and salinity. Retinal-based photoheterotrophy is particularly important for prokaryotic survival in hypersaline environments, surpassing the sunlight energy captured by photosynthesis, and being more relevant as salinity increases. This has implications for understanding the survival of microorganisms in extreme conditions and the energy dynamics in solar salter ponds.

Protein Medium Facilitates Electron Transfer in Photosynthetic Heliobacterial Reaction Center.

This computational study addresses the question of how large membrane-bound proteins of electron transport chains facilitate fast vector-based charge transport. We study electron transfer reactions following ultrafast initial charge separation induced by absorption of light by P800 primary pair and leading to the electron localization at the A0 cofactor. Two subsequent, much slower reactions, electron transfer to the iron-sulfur cluster Fx and reduction of the menaquinone (MQ) cofactor, are studied by combining molecular dynamics simulations, electronic structure calculations, and theoretical modeling. The low value of the electronic coupling between A0 and Fx brings this reaction to the microsecond time scale even at the zero activation barrier. In contrast, A0-MQ electron transfer occurs on a subnanosecond time scale and might become the preferred route for charge transport. We elucidate mechanistic properties of the protein medium allowing fast, vectorial charge transfer. The electric field is high and inhomogeneous inside the protein and is coupled to high polarizabilities of cofactors to significantly lower the reaction barrier. The A0-MQ separation puts this reaction at the edge between the plateau characterizing the reaction dynamical control and exponential falloff due to electronic tunneling. A strong separation in relaxation times of the medium dynamics for the forward and backward reactions promotes vectorial charge transfer.

Acidisoma cladoniae sp. nov., an acidotolerant bacterium isolated from an Antarctic lichen.

Gram-staining-negative, aerobic, white-cream-pearly colony, coccobacilli, and non-motile bacterial strain, PAMC 29798T was isolated from an Antarctic lichen. The strain was acidotolerant and psychrotolerant growing at pH 4.0-7.5 (optimally at pH 4.0-6.5) and 0-25°C (optimally at 10-20°C). The major fatty acids are Summed Feature 8, C18:1 2OH, and C19:0 cyclo ω8c. The major respiratory quinone was Q-10. Phylogenetic and phylogenomic analyses indicated that strain PAMC 29798T belonged to the genus Acidisoma and 16S rRNA gene sequences of PAMC 29798T were closely related to Acidisoma silvae (97.7% sequence similarity), Acidisoma cellulosilyticum (96.5%), Acidisoma tundrae (96.5%), and Acidisoma sibiricum (96.3%). Genomic relatedness analyses showed that strain PAMC 29798T was clearly distinguished from type strains of the genus Acidisoma based on values of average nucleotide identity (< 75%) and the digital DNA-DNA hybridization (< 19.6%). Genome analysis revealed that the genome size of PAMC 29798T is approximately 5.0 Mb with a G+C content of 63.4%. The complete genome comprises 5 contigs containing 4636 protein-coding genes, 46 tRNA genes, and 2 rRNA operons. The genome possesses genes for light-harvesting complexes, type-II photosynthetic reaction center, and C-P lyase to solubilize organic phosphates, while genes encoding nitrogenase iron protein involved in the nitrogen fixation were not present. Based on the results of phylogenetic, genome-based relatedness, and physiological and genomic analyses, strain PAMC 29798T is proposed to represent a novel species of the genus Acidisoma, with the name Acidisoma cladoniae. The type strain is PAMC 29798T (= KCTC 82159T = JCM 35634T).

Photosynthetic biohybrid systems for solar fuels catalysis.

Photosynthetic reaction center (RC) proteins are finely tuned molecular systems optimized for solar energy conversion. RCs effectively capture and convert sunlight with near unity quantum efficiency utilizing light-induced directional electron transfer through a series of molecular cofactors embedded within the protein core to generate a long-lived charge separated state with a useable electrochemical potential. Of current interest are new strategies that couple RC chemistry to the direct synthesis of energy-rich compounds. This Feature Article highlights recent work from our lab on RC and RC-inspired hybrid systems that capture the Sun's energy and convert it to chemical energy in the form of H2, a carbon-neutral energy source derived from water. Biohybrids made from the Photosystem I (PSI) RC are among the best photocatalytic H2-producing protein hybrids to date. Targeted self-assembly strategies that couple abiotic catalysts to PSI translate to catalyst incorporation at intrinsic PSI sites within thylakoid membranes to achieve complete solar water-splitting systems. RC-inspired biohybrids interface synthetic photosensitizers and molecular catalysts with small proteins to create photocatalytic systems and enable the spectroscopic discernment of the structural features and electron transfer processes that underpin solar-driven proton reduction. In total, these studies showcase the incredible scientific opportunities photosynthetic biohybrid research provides for harnessing the optimal qualities of both artificial and natural photosynthetic systems and developing materials that capture, convert, and store solar energy as a fuel.

Extended perturbative approach including Redfield and Förster limits for qualitative analysis of exciton dynamics in any photosynthetic light harvesting and reaction center

According to many reports, the various structures of photosynthetic light-harvesting/reaction-center complexes and their molecular-dynamics simulations necessitate a numerically efficient and quality-conserved theory of excitation energy transfer and exciton relaxation in large pigment systems. Although exciton dynamics depend on various parameters, such as exciton coupling strength, exciton–phonon coupling, site energy values for each pigment, and temperature, classifying the transition mechanism for any Hamiltonian into perturbatively delocalized or localized theories is challenging. In this study, perturbative quantum master equations of a reduced density matrix for any orthogonal transformation similar to the coherent modified Redfield theory are derived. Our approach qualitatively conserves the dynamics of relevant perturbative approximations in each limiting case. As an application, any orthogonal transformation of a relevant system is optimized using the average of the square of interactions between orthogonal state transitions. The numerical results for two pigment systems are compared with the limiting formalisms of the modified Redfield and Förster theory.

Tunneling Mechanisms of Quinones in Photosynthetic Reaction Center–Light Harvesting 1 Supercomplexes

In photosynthesis, light energy is absorbed and transferred to the reaction center, ultimately leading to the reduction of quinone molecules through the electron transfer chain. The oxidation and reduction of quinones generate an electrochemical potential difference used for adenosine triphosphate synthesis. The trafficking of quinone/quinol molecules between electron transport components has been a long‐standing question. Here, an atomic‐level investigation into the molecular mechanism of quinol dissociation in the photosynthetic reaction center–light‐harvesting complex 1 (RC–LH1) supercomplexes from Rhodopseudomonas palustris, using classical molecular dynamics (MD) simulations combined with self‐random acceleration MD‐MD simulations and umbrella sampling methods, is conducted. Results reveal a significant increase in the mobility of quinone molecules upon reduction within RC–LH1, which is accompanied by conformational modifications in the local protein environment. Quinol molecules have a tendency to escape from RC–LH1 in a tail‐first mode, exhibiting channel selectivity, with distinct preferred dissociation pathways in the closed and open LH1 rings. Furthermore, comparative analysis of free energy profiles indicates that alternations in the protein environment accelerate the dissociation of quinol molecules through the open LH1 ring. In particular, aromatic amino acids form π‐stacking interactions with the quinol headgroup, resembling the key components in a conveyor belt system. This study provides insights into the molecular mechanisms that govern quinone/quinol exchange in bacterial photosynthesis and lays the framework for tuning electron flow and energy conversion to improve metabolic performance.

Nitroxide Spin Labels for Exploring Relationships Between Molecular Structure, Microenvironment and EPR Parameters: A Mini-review Dedicated to Carlo Corvaja

AbstractThis mini-review is dedicated to Carlo Corvaja (University of Padova) in recognition of his important contributions to the study of biomimetic donor–acceptor model dyads and triads and to the understanding of spin exchange in excited fullerene–nitroxide derivatives. We report on attractive examples of multi-frequency and multi-resonance EPR spectroscopy, highlighting recent work in Padova and Berlin/Mülheim. The examples selected include TR-EPR, ENDOR, and EDNMR experiments on photoexcited spin-labeled macromolecules, such as fullerene–nitroxide complexes or photosynthetic bacterial reaction centers, which were optionally NO spin-labeled. From the spin interaction parameters measured, detailed information about structure and dynamics of macromolecules embedded in liquid-solution or solid-state microenvironments could be extracted.

Anomalous Temperature Dependence of the Triplet-Triplet Energy Transfer in Cereibacter sphaeroides I(L177)H Mutant Reaction Centers.

In photosynthetic reaction centers, quenching of the primary donor triplet state by energy transfer to the carotenoid molecule provides efficient suppression of generation of singlet-excited oxygen, potent chemical oxidant. This process in the Cereibacter sphaeroides reaction centers is thermoactivated, and discontinues at temperatures below 40K. In these reaction centers, substitution of amino acid residue isoleucine at the 177position of the L-subunit with histidine results in the sharp decrease of activation energy, so that the carotenoid triplets are populated even at 10K. Activation energy of the T-T energy transfer was estimated as 7.5cm-1, which is more than 10-fold lower than activation energy in the wild type reaction centers. At certain temperatures, the energy transfer in the mutant is decelerated, which is related to the increase of effective distance of the triplet-triplet transfer. To the best of our knowledge, the described mutation presents the first reaction center modification leading to the significant decrease in activation energy of the T-T energy transfer to carotenoid molecule. TheI(L177)H mutant reaction centers present a considerable interest for further studies of the triplet state quenching mechanisms, and of other photophysical and photochemical processes in the reaction centers of bacterial photosynthesis.

Absence of a link between stabilized charge-separated state and structural changes proposed from crystal structures of a photosynthetic reaction center

Structural differences between illuminated and unilluminated crystal structures led to the proposal that the charge-separated state was stabilized by structural changes in its membrane extrinsic protein subunit H in a bacterial photosynthetic reaction center [Katona, G. et al. Nat. Struct. Mol. Biol. 2005, 12, 630–631]. Here, we explored the proposal by titrating all titratable sites and calculating the redox potential (Em) values in these crystal structures. Contrary to the expected charge-separated states, Em for quinone, Em(QA/QA•–), is even lower in the proposed charge-separated structure than in the ground-state structure. The subunit-H residues, which were proposed to exhibit electron-density changes in the two crystal structures, contribute to an Em(QA/QA•–) difference of only <0.5 mV. Furthermore, the protonation states of the titratable residues in the entire reaction center are practically identical in the two structures. These findings indicate that the proposed structural differences are irrelevant to explaining the significant prolongation of the charge-separated-state lifetime.

Superexchange Electron Transfer and Protein Matrix in the Charge-Separation Process of Photosynthetic Reaction Centers.

In type-II reaction centers, such as photosystem II (PSII) and reaction centers from purple bacteria (PbRC), light-induced charge separation involves electron transfer from pheophytin (PheoD1) to quinone (QA), occurring near a conserved tryptophan residue (D2-Trp253 in PSII and Trp-M252 in PbRC). This study investigates the route of the PheoD1-to-QA electron transfer, focusing on the superexchange coupling (|HPheoD1···QA|) in the PSII protein environment. |HPheoD1···QA| is significantly larger for the PheoD1-to-QA electron transfer via the unoccupied molecular orbitals of D2-Trp253 ([Trp]•--like intermediate state, 0.73 meV) compared to direct electron transfer (0.13 meV), suggesting that superexchange is the dominant mechanism in the PSII protein environment. While the overall impact of the protein environment is limited, local interactions, particularly H-bonds, enhance superexchange electron transfer by directly affecting the delocalization of molecular orbitals. The D2-W253F mutation significantly decreases the electron transfer rate. The conservation of D2-Trp253/D1-Phe255 (Trp-M252/Phe-L216 in PbRC) in the two branches appears to differentiate superexchange coupling, contributing to the branches being either active or inactive in electron transfer.

In situ structural determination of cyanobacterial phycobilisome–PSII supercomplex by STAgSPA strategy

Photosynthesis converting solar energy to chemical energy is one of the most important chemical reactions on earth. In cyanobacteria, light energy is captured by antenna system phycobilisomes (PBSs) and transferred to photosynthetic reaction centers of photosystem II (PSII) and photosystem I (PSI). While most of the protein complexes involved in photosynthesis have been characterized by in vitro structural analyses, how these protein complexes function together in vivo is not well understood. Here we implemented STAgSPA, an in situ structural analysis strategy, to solve the native structure of PBS–PSII supercomplex from the cyanobacteria Arthrospira sp. FACHB439 at resolution of ~3.5 Å. The structure reveals coupling details among adjacent PBSs and PSII dimers, and the collaborative energy transfer mechanism mediated by multiple super-PBS in cyanobacteria. Our results provide insights into the diversity of photosynthesis-related systems between prokaryotic cyanobacteria and eukaryotic red algae but are also a methodological demonstration for high-resolution structural analysis in cellular or tissue samples.

Light-induced H2 generation in a photosystem I-O2-tolerant [FeFe] hydrogenase nanoconstruct

The fusion of hydrogenases and photosynthetic reaction centers (RCs) has proven to be a promising strategy for the production of sustainable biofuels. Type I (iron-sulfur-containing) RCs, acting as photosensitizers, are capable of promoting electrons to a redox state that can be exploited by hydrogenases for the reduction of protons to dihydrogen (H2). While both [FeFe] and [NiFe] hydrogenases have been used successfully, they tend to be limited due to either O2 sensitivity, binding specificity, or H2 production rates. In this study, we fuse a peripheral (stromal) subunit of Photosystem I (PS I), PsaE, to an O2-tolerant [FeFe] hydrogenase from Clostridium beijerinckii using a flexible [GGS]4 linker group (CbHydA1-PsaE). We demonstrate that the CbHydA1 chimera can be synthetically activated in vitro to show bidirectional activity and that it can be quantitatively bound to a PS I variant lacking the PsaE subunit. When illuminated in an anaerobic environment, the nanoconstruct generates H2 at a rate of 84.9 ± 3.1 µmol H2 mgchl-1 h-1. Further, when prepared and illuminated in the presence of O2, the nanoconstruct retains the ability to generate H2, though at a diminished rate of 2.2 ± 0.5 µmol H2 mgchl-1 h-1. This demonstrates not only that PsaE is a promising scaffold for PS I-based nanoconstructs, but the use of an O2-tolerant [FeFe] hydrogenase opens the possibility for an in vivo H2 generating system that can function in the presence of O2.

Symmetry Breaking Charge Separation in Far-Red Absorbing Bisstyryl-Bodipy Dimers

Symmetry breaking charge transfer is one of the important photo-events occurring in photosynthetic reaction centers that is responsible for initiating electron transfer leading to a long-lived charge-separated state and has been successfully employed in light-to-electricity converting optoelectronic devices. Here, we report the synthesis of far-red absorbing and emitting BODIPY-dimer to undergo symmetry-breaking charge transfer leading to charge-separated states of appreciable lifetimes in polar solvents. Compared to its monomer analog, both steady-state and time-resolved fluorescence originating from the S1 state of the dimer revealed quenching which increased with an increase in solvent polarity. The electrostatic potential map from DFT and the time-dependent DFT calculations suggested the existence of a quadrupolar type charge transfer state in polar solvents, and the singlet excited state to be involved in the charge separation process. The electrochemically determined redox gap to be smaller than the energy of the S1 state supporting thermodynamic feasibility of the envisioned symmetry-breaking charge transfer and separation. The spectrum of the charge-separated state arrived from spectroelectrochemical studies, revealing diagnostic peaks helpful for transient spectral interpretation. Finally, ultrafast transient pump-probe spectroscopy provided conclusive evidence of diabatic charge separation in polar solvents by far-red pulsed laser light irradiation. The measured lifetime of the final charge-separated states was found to be 165 ps in dichlorobenzene, 140 ps in benzonitrile, and 43 ps in dimethyl sulfoxide, revealing their significance in light energy harvesting, especially from the less-explored far-red region.

Single Cells Polydopamine Coating of Rhodobacter Sphaeroides for Enhanced Electron Transfer

Photosynthetic bacteria are anoxygenic microorganisms with highly versatile metabolism, as they use sunlight to oxidize a broad variety of organic compounds in addition to heterotrophic and photoautotropic alternative metabolisms. Recent advances in coupling light-harvesting microorganisms with electronic components has led to a new generation of biohybrid devices based on microbial photocatalysts. These devices are limited by the poorly conductive interface between phototrophs and synthetic materials that inhibit charge transfer.Polydopamine (PDA), produced by self-assembly of dopamine, is a very versatile and bioinspired polymer with widespread applications1 mostly due its ability to adhere and cover surfaces of different chemical composition. The oxidative conditions employed for the formation of this dark insoluble polymer are mild and biocompatible and have inspired scientists to develop novel nanomaterials. Post-functionalization of PDA2 also enables fine tuning of properties. Furthermore, the ability of this monomer to self-assemble and polymerize in the bacterial growth medium was considered one of the requirements of the polymer to be used as coating material beside the tunable conductive properties and the flexible structure.Biocompatibility of dopamine was tested by in vivo addition in the growth media of the photosynthetic purple non sulphur Rhodobacter (R.) sphaeroides 3 in anoxygenic conditions. This study focuses on overcoming the bottleneck of biohybrid devices through the metabolically-driven encapsulation of photosynthetic cells with a bio-inspired conductive polymer. The treated cells show preserved light absorption of the photosynthetic pigments in the presence of dopamine concentrations ranging between 0.05 – 3.5 mM. The thickness and nanoparticle formation of the membrane-associated PDA matrix was further shown to vary with the dopamine concentrations in this range. Compared to uncoated cells, the encapsulated cells show up to a 20-fold enhancement in transient photocurrent measurements under mediatorless conditions. The biologically synthesized PDA can thus act as a matrix for electronically coupling the light-harvesting metabolisms of cells with conductive surfaces. 1Liu, Y.; Ai, K.; Lu, L. Polydopamine and its derivative materials: Synthesis and promising applications in energy, environmental, and biomedical fields. Chem. Rev. 2014, 114, 5057–5115. 2Buscemi, G., Vona, D., Ragni, R., Comparelli, R., Trotta, M., Milano, F., Farinola, G.M., Polydopamine/Ethylenediamine Nanoparticles Embedding a Photosynthetic Bacterial Reaction Center for Efficient Photocurrent Generation. Adv. Sustainable Syst. 2021, 2000303. 3 Labarile, R.; Varsalona, M.; Vona, D.; Stufano, P.; Grattieri, M.; Farinola, G. M.; Trotta, M., A novel route for anoxygenic polymerization of dopamine via purple photosynthetic bacteria metabolism. MRS Advances 2023