Exciton Description of Chlorosome to Baseplate Excitation Energy Transfer in Filamentous Anoxygenic Phototrophs and Green Sulfur Bacteria

A description of intra-chlorosome and from chlorosome to baseplate excitation energy transfer in green sulfur bacteria and in filamentous anoxygenic phototrophs is presented. Various shapes and sizes, single and multiwalled tubes, cylindrical spirals and lamellae of the antenna elements mimicking pigment organization in chlorosomes were generated by using molecular mechanics calculations, and the absorption, LD, and CD spectra of these were predicted by using exciton theory. Calculated absorption and LD spectra were similar for all modeled antenna structures; on the contrary, CD spectra turned out to be sensitive to the size and pigment orientations in the antenna. It was observed that, bringing two tubular antennae at close enough interaction distance, the exciton density of the lowest energy state became localized on pigments facing each other in the antenna dimer. Calculations predicted for stacked tubular antenna elements extremely fast, faster than 500 fs, intra-chlorosome energy transfer toward the baseplates in the direction perpendicular to the chlorosome long axis. Downhill excitation energy transfer according to our model is driven by interactions of the antennae with their immediate surroundings. Energy transfer from the chlorosome to the baseplate, consisting of 2D lattices of monomeric and dimeric bacteriochlorophyll <i>a</i> molecules, was predicted to occur in 5–15 ps, in agreement with experimental findings. Advancement of excitation through a double tube antenna stack, a model for antenna element organization in chlorosomes of green sulfur bacteria, to a monomeric baseplate was visualized in space and in time

Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and Thermalized Donor States.

Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) into a titanium dioxide nanocrystalline film occurs on the femto- and picosecond time scales. Here we show that the dominating part of the electron transfer proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state, prior to electronic and nuclear relaxation of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies

Electron transfer from the singlet and triplet excited states of Ru(dcbpy)(2)(NCS)(2) into nanocrystalline TiO2 thin films

Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpY)(2)(NCS)(2) (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of similar to30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs(-1) for ET and 1/75 fs(-1) for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from similar to1 to similar to60 ps

Excitation energy transfer in the LHC-II trimer:From carotenoids to chlorophylls in space and time

Exciton model for description of experimentally determined excitation energy transfer from carotenoids to chlorophylls in the LHC-II trimer of spinach is presented. Such an approach allows connecting the excitonic states to the spatial structure of the complex and hence descriptions of advancements of the initially created excitations in space and time. Carotenoids were excited at 490 nm and at 500 nm and induced absorbance changes probed in the Chl Qy region to provide kinetic data that were interpreted by using the results from exciton calculations. Calculations included the 42 chlorophylls and the 12 carotenoids of the complex, Soret, Qx and Qy states of the chlorophylls, and the main absorbing S2 state of the carotenoids. According to the calculations excitation at 500 nm populates mostly a mixed Lut S2 Chl a Soret state, from where excitation is transferred to the Qx and Qy states of the Chl a's on the stromal side. Internal conversion of the mixed state to a mixed Lut S1 and Chl a Qy state provides a channel for Lut S1 to Chl a Q y energy transfer. The results from the calculations support a picture where excitation at 490 nm populates primarily a mixed neoxanthin S 2 Chl b Soret state. From this state excitation from neoxanthin is transferred to iso-energetic Chl b Soret states or via internal conversion to S1 Chl b Qy states. From the Soret states excitation proceeds via internal conversion to Qy states of Chl b's mostly on the lumenal side. A rapid Chl b to Chl a transfer and subsequent transfer to the stromal side Chl a's and to the final state completes the process after 490 nm excitation. The interpretation is further supported by the fact that excitation energy transfer kinetics after excitation of neoxanthin at 490 nm and the Chl b Qy band at 647 nm (Linnanto et al., Photosynth Res 87:267-279, 2006) are very similar

Red spectral forms of chlorophylls in green plant PSI- A site-selective and high-pressure spectroscopy study

One of the special spectroscopic characteristics of photosystem I (PSI) complexes is that they possess absorption and emission bands at lower energy than those of the reaction center. In this paper, the red pigment pools of PSI-200, PSI-core, and LHCI complex from Arabidopsis thaliana have been characterized at low temperatures by means of spectrally selective (hole-burning and fluorescence line-narrowing) and high-pressure spectroscopic techniques. It was shown that the green plant PSI-200 complex has at least three red pigment pools, from which two are located in the PSI-core and one, in the peripheral light-harvesting complex I (LHCI). All of the red pigment pools are characterized by strong electron-phonon coupling. A Huang-Rhys factor of 2.9 found for the red pigments of LHCI is the largest found for any photosynthetic antenna system. This contrasts with the bulk pigments in the main Qy absorption band of chlorophyll a pigments for which the Huang-Rhys factors of less than unity are observed. This electron-phonon coupling difference of the red and bulk pigments is well reflected by the spectral dependence of the hole-burning efficiency, which is significantly reduced in the red absorption region. As a result of extremely low hole-burning efficiency in the red absorption band of LHCI, the hole-burning spectra of the PSI-200 complex mainly originate from the red pigments of the PSI-core complex. At the same time, the source of the red emission in PSI-200 is the red pigments of LHCI, in agreement with previous studies. The hole-burning spectra of PSI-core complexes from green plant and cyanobacteria are similar, both in red and bulk absorption regions. High-pressure spectroscopy data reveal dramatically larger pressure-induced linear shift rates for the redmost absorption and emission bands relative to those of bulk absorption bands. This is interpreted as due mostly to increased conformational mixing between the locally excited and charge transfer configurations of the red pigment aggregates. On the basis of analysis of available experimental data, we suggest that pigment dimers are probably responsible for the redmost states. Consequently, the excited red states can be interpreted as excimer states