• Energy Research

Photosystem I (PSI) plays a major role in the light reactions of photosynthesis. In higher plants, PSI is composed of a core complex and four outer antennas that are assembled as two dimers, Lhca1/4 and Lhca2/3. Time-resolved fluorescence measurements on the isolated dimers show very similar kinetics. The intermonomer transfer processes are resolved using target analysis. They occur at rates similar to those observed in transfer to the PSI core, suggesting competition between the two transfer pathways. It appears that each dimer is adopting various conformations that correspond to different lifetimes and emission spectra. A special feature of the Lhca complexes is the presence of an absorption band at low energy, originating from an excitonic state of a chlorophyll dimer, mixed with a charge-transfer state. These low-energy bands have high oscillator strengths and they are superradiant in both Lhca1/4 and Lhca2/3. This challenges the view that the low-energy charge-transfer state always functions as a quencher in plant Lhc's and it also challenges previous interpretations of PSI kinetics. The very similar properties of the low-energy states of both dimers indicate that the organization of the involved chlorophylls should also be similar, in disagreement with the available structural data.

Excitation energy transfer (EET) and trapping in Synechococcus WH 7803 whole cells and isolated photosystem I (PSI) complexes have been studied by time-resolved emission spectroscopy at room temperature (RT) and at 77 K. With the help of global and target analysis, the pathways of EET and the charge separation dynamics have been identified. Energy absorbed in the phycobilisome (PB) rods by the abundant phycoerythrin (PE) is funneled to phycocyanin (PC645) and from there to the core that contains allophycocyanin (APC660 and APC680). Intra-PB EET rates have been estimated to range from 11 to 68/ns. It was estimated that at RT, the terminal emitter of the phycobilisome, APC680, transfers its energy at a rate of 90/ns to PSI and at a rate of 50/ns to PSII. At 77 K, the redshifted Chl a states in the PSI core were heterogeneous, with maximum emission at 697 and 707 nm. In 72% of the PSI complexes, the bulk Chl a in equilibrium with F697 decayed with a main trapping lifetime of 39 ps.

The light-harvesting complexes (LHCs) of plants can regulate the energy flux to the reaction centers in response to fluctuating light by virtue of their vast conformational landscape. They do so by switching from a light-harvesting state to a quenched state, dissipating the excess absorbed energy as heat. However, isolated LHCs are prevalently in their light-harvesting state, which makes the identification of their photoprotective mechanism extremely challenging. Here, ensemble time-resolved fluorescence experiments show that monomeric CP29, a minor LHC of plants, exists in various emissive states with identical spectra but different lifetimes. The photoprotective mechanism active in a subpopulation of strongly quenched complexes is further investigated via ultrafast transient absorption spectroscopy, kinetic modeling, and mutational analysis. We demonstrate that the observed quenching is due to excitation energy transfer from chlorophylls to a dark state of the centrally bound lutein.

Blue and green sunlight become available for photosynthetic energy conversion through the light-harvesting (LH) function of carotenoids, which involves transfer of carotenoid singlet excited states to nearby (bacterio)chlorophylls (BChls). The excited-state manifold of carotenoids usually is described in terms of two singlet states, S 1 and S 2 , of which only the latter can be populated from the ground state by the absorption of one photon. Both states are capable of energy transfer to (B)Chl. We recently showed that in the LH1 complex of the purple bacterium Rhodospirillum rubrum , which is rather inefficient in carotenoid-to-BChl energy transfer, a third additional carotenoid excited singlet state is formed. This state, which we termed S*, was found to be a precursor on an ultrafast fission reaction pathway to carotenoid triplet state formation. Here we present evidence that S* is formed with significant yield in the LH2 complex of Rhodobacter sphaeroides , which has a highly efficient carotenoid LH function. We demonstrate that S* is actively involved in the energy transfer process to BChl and thus have uncovered an alternative pathway of carotenoid-to-BChl energy transfer. In competition with energy transfer to BChl, fission occurs from S*, leading to ultrafast formation of carotenoid triplets. Analysis in terms of a kinetic model indicates that energy transfer through S* accounts for 10–15% of the total energy transfer to BChl, and that inclusion of this pathway is necessary to obtain a highly efficient LH function of carotenoids.

Neorhodopsin (NeoR) is a newly discovered fungal bistable rhodopsin that reversibly photoswitches between UV- and near-IR absorbing states denoted NeoR367 and NeoR690, respectively. NeoR367 represents a deprotonated retinal Schiff base (RSB), while NeoR690 represents a protonated RSB. Cryo-EM studies indicate that NeoR forms homodimers with 29 Å center-to-center distance between the retinal chromophores. UV excitation of NeoR367 takes place to an optically allowed S3 state of 1Bu+ symmetry, which rapidly converts to a low-lying optically forbidden S1 state of 2Ag- symmetry in 39 fs, followed by a multiexponential decay to the ground state on the 1-100 ps time scale. A theoretically predicted nπ* (S2) state does not get populated in any appreciable transient concentration during the excited-state relaxation cascade. We observe an intradimer retinal to retinal excitation energy transfer (EET) process from the NeoR367 S1 state to NeoR690, in competition with photoproduct formation. To quantitatively assess the EET mechanism and rate, we experimentally addressed and modeled the EET process under varying NeoR367-NeoR690 photoequilibrium conditions and determined the EET rate at (200 ps)-1. The NeoR367 S1 state shows a weak stimulated emission band in the near-IR around 700 nm, which may result from mixing with an intramolecular charge-transfer (ICT) state, enhancing the transition dipole moment of the S1-S0 transition and possibly facilitating the EET process. We suggest that EET may bear general relevance to the function of bistable multiwavelength rhodopsin oligomers.

Photosynthetic activity and respiration share the thylakoid membrane in cyanobacteria. We present a series of spectrally resolved fluorescence experiments where whole cells of the cyanobacterium Synechocystis sp. PCC 6803 and mutants thereof underwent a dark-to-light transition after different dark-adaptation (DA) periods. Two mutants were used: (i) a PSI-lacking mutant (ΔPSI) and (ii) M55, a mutant without NAD(P)H dehydrogenase type-1 (NDH-1). For comparison, measurements of the wild-type were also carried out. We recorded spectrally resolved fluorescence traces over several minutes with 100 ms time resolution. The excitation light was at 590 nm so as to specifically excite the phycobilisomes. In ΔPSI, DA time has no influence, and in dichlorophenyl-dimethylurea (DCMU)-treated samples we identify three main fluorescent components: PB-PSII complexes with closed (saturated) RCs, a quenched or open PB-PSII complex, and a PB-PSII 'not fully closed.' For the PSI-containing organisms without DCMU, we conclude that mainly three species contribute to the signal: a PB-PSII-PSI megacomplex with closed PSII RCs and (i) slow PB → PSI energy transfer, or (ii) fast PB → PSI energy transfer and (iii) complexes with open (photochemically quenched) PSII RCs. Furthermore, their time profiles reveal an adaptive response that we identify as a state transition. Our results suggest that deceleration of the PB → PSI energy transfer rate is the molecular mechanism underlying a state 2 to state 1 transition.