Energy Equilibration Processes Research Articles

Coloring Outside the Lines: Exploiting Pigment-Protein Synergy for Far-Red Absorption in Plant Light-Harvesting Complexes.

Plants are designed to utilize visible light for photosynthesis. Expanding this light absorption toward the far-red could boost growth in low-light conditions and potentially increase crop productivity in dense canopies. A promising strategy is broadening the absorption of antenna complexes to the far-red. In this study, we investigated the capacity of the photosystem I antenna protein Lhca4 to incorporate far-red absorbing chlorophylls d and f and optimize their spectra. We demonstrate that these pigments can successfully bind to Lhca4, with the protein environment further red-shifting the chlorophyll d absorption, markedly extending the absorption range of this complex above 750 nm. Notably, chlorophyll d substitutes the canonical chlorophyll a red-forms, resulting in the most red-shifted emission observed in a plant light-harvesting complex. Using ultrafast spectroscopy, we show that the introduction of these novel chlorophylls does not interfere with the excited state decay or the energy equilibration processes within the complex. The results demonstrate the feasibility of engineering plant antennae to absorb deeper into the far-red region while preserving their functional and structural integrity, paving the way for innovative strategies to enhance photosynthesis.

Energy transfer dynamics in trimers and aggregates of light-harvesting complex II probed by 2D electronic spectroscopy.

The pathways and dynamics of excitation energy transfer between the chlorophyll (Chl) domains in solubilized trimeric and aggregated light-harvesting complex II (LHCII) are examined using two-dimensional electronic spectroscopy (2DES). The LHCII trimers and aggregates exhibit the unquenched and quenched excitonic states of Chl a, respectively. 2DES allows direct correlation of excitation and emission energies of coupled states over population time delays, hence enabling mapping of the energy flow between Chls. By the excitation of the entire Chl b Qy band, energy transfer from Chl b to Chl a states is monitored in the LHCII trimers and aggregates. Global analysis of the two-dimensional (2D) spectra reveals that energy transfer from Chl b to Chl a occurs on fast and slow time scales of 240-270 fs and 2.8 ps for both forms of LHCII. 2D decay-associated spectra resulting from the global analysis identify the correlation between Chl states involved in the energy transfer and decay at a given lifetime. The contribution of singlet-singlet annihilation on the kinetics of Chl energy transfer and decay is also modelled and discussed. The results show a marked change in the energy transfer kinetics in the time range of a few picoseconds. Owing to slow energy equilibration processes, long-lived intermediate Chl a states are present in solubilized trimers, while in aggregates, the population decay of these excited states is significantly accelerated, suggesting that, overall, the energy transfer within the LHCII complexes is faster in the aggregated state.

Energy Equilibration Processes of Electrons, Magnons, and Phonons at the Femtosecond Time Scale

We relate the energy dissipation processes at the femtosecond (electron-spin relaxation time tau el-sp) and nanosecond time scale (Gilbert relaxation taualpha) to the microscopic model proposed by Koopmans [Phys. Rev. Lett. 95, 267207 (2005)]. At both time scales, Elliot-Yafet scattering is proposed as the dominant contribution. We controllably manipulate the energy dissipation by transition metal doping (Pd) and rare earth doping (Dy) of a Permalloy film. While a change in taualpha of more than a factor of 2 is observed, tau el-sp remains constant. We explain the discrepancies as due to relaxation channels not considered in the model.

Spectral Inhomogeneity of Photosystem I and Its Influence on Excitation Equilibration and Trapping in the Cyanobacterium Synechocystis sp. PCC6803 at 77 K

Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685–687 nm within few transfer steps (300–400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4–6 ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30–50-ps process of photochemical trapping of the excitation by P 700. These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) ∼ 200–300 cm −1). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683 nm and 678 nm under excitation into the red side of the absorption spectrum (>690 nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll a monomers from the excited-state absorption of the dimeric red pigments.

Excitation energy transfer in Photosystem I from oxygenic organisms.

This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms. In the trimeric PS I core from cyanobacteria, the efficiency of solar energy conversion is largely determined by ultrafast excitation transfer processes in the core chlorophyll a (Chl a) antenna network and efficient photochemical trapping in the reaction center (RC). The role of clusters of Chl a in energy equilibration and photochemical trapping in the PS I core is discussed. Dimers of the longest-wavelength absorbing (red) pigments with strongest excitonic interactions localize the excitation in the PS I core antenna. Those dimers that are located closer to the RC participate in a fast energy equilibration with coupled pigments of the RC. This suggests that the function of the red pigments is to concentrate the excitation near the RC. In the PS I holocomplex from algae and higher plants, in addition to the red pigments of the core antenna, spectrally distinct red pigments are bound to the peripheral Chl a/b-binding light-harvesting antenna (LHC I), specifically to the Lhca4 subunit of the LHC I-730 complex. Intramonomeric energy equilibration between pools of Chl b and Chl a in Lhca1 and Lhca4 monomers of the LHC I-730 heterodimer are as fast as the energy equilibration processes within the PS I core. In contrast to the structural stability of the PS I core, the flexible subunit structure of the LHC I would probably determine the observed slow excitation energy equilibration processes in the range of tens of picoseconds. The red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I.

Ultrafast excitation dynamics of low energy pigments in reconstituted peripheral light-harvesting complexes of photosystem I

Ultrafast dynamics of a reconstituted Lhca4 subunit from the peripheral LHCI-730 antenna of photosystem I of higher plants were probed by femtosecond absorption spectroscopy at 77 K. Intramonomeric energy transfer from chlorophyll (Chl) b to Chl a and energy equilibration between Chl a molecules observed on the subpicosecond time scale are largely similar to subpicosecond energy equilibration processes within LHCII monomers. However, a 5 ps equilibration process in Lhca4 involves unique low energy Chls in LHCI absorbing at 705 nm. These pigments localize the excitation both in the Lhca4 subunit and in LHCI-730 heterodimers. An additional 30–50 ps equilibration process involving red pigments of Lhca4 in the heterodimer, observed by transient absorption and picosecond fluorescence spectroscopy, was ascribed to intersubunit energy transfer.

Excitation Dynamics and Heterogeneity of Energy Equilibration in the Core Antenna of Photosystem I from the Cyanobacterium Synechocystis sp. PCC 6803

Energy equilibration in the photosystem I core antenna from the cyanobacterium Synechocystis sp. PCC 6803 was studied using femtosecond transient absorption spectroscopy at 298 K. The photosystem I core particles were excited at 660, 693, and 710 nm with 150 fs spectrally narrow laser pulses (fwhm = 5 nm). Global analysis revealed three kinetic processes in the core antenna with lifetimes of 250-500 fs, 1.5-2.5 ps, and 20-30 ps. The first two components represent strongly excitation wavelength-dependent energy equilibration processes while the 20-30 ps phase reflects the trapping of energy by the reaction center. Excitation into the blue and red edge of the absorption band induces downhill and uphill energy flows, respectively, between different chlorophyll a spectral forms of the core. Excitation at 660 nm induces a 500 fs downhill equilibration process within the bulk of antenna while the selective excitation of long-wavelength-absorbing chlorophylls at 710 nm results in a 380 fs uphill energy transfer to the chlorophylls absorbing around 695-700 nm, presumably reaction center pigments. The 1.5-2.5 ps phases of downhill and uphill energy transfer are largely equivalent but opposite in direction, indicating energy equilibration between bulk antenna chlorophylls at 685 nm and spectral forms absorbing below 700 nm. Transient absorption spectra with excitation at 693 nm exhibit spectral evolution within approximately 2 ps of uphill energy transfer to major spectral forms at 680 nm and downhill energy transfer to red pigments at 705 nm. The 20-30 ps trapping component and P(700) photooxidation spectra derived from data on the 100 ps scale are largely excitation wavelength independent. An additional decay component of red pigments at 710 nm can be induced either by selective excitation of red pigments or by decreasing the temperature to 264 K. This component may represent one of the phases of energy transfer from inhomogeneously broadened red pigments to P(700). The data are discussed based on the available structural model of the photosystem I reaction center and its core antenna.

The mechanism and kinetics of evaporation from laser irradiated UO2 surfaces

A theoretical analysis of laser evaporation experiments for the determination of the high temperture saturated vapor pressure of uranium dioxide is presented. The interaction of laser radation with the condensed phase is discussed and the timescales involved in the energy equilibration processes are given. The mechanism of evaporation is described within the framework of the Terrace–Ledge–Kink model, where it is shown that by using symmetry properties of ionic crystal lattices, the surface Madelung potentials and binding energies of ions and neutral ionic units may be determined. Results are presented for NaCl, CsCl, and UO2 surfaces. Based on these calculations, general conclusions are drawn on the nature of the evaporating species. In addition, it is shown that the multispecies evaporation from uranium dioxide is fundamentally related to the amount of atomic and electronic disorder in the surface layer. The effects of the kinetics of surface reactions on the rate of evaporation are investigated. Parallel reaction paths such as direct evaporation from kink sites are treated in addition to surface diffusion followed by desorption series reactions. Finally, the possibility of producing superheated metastable surface layers under pulsed laser heating conditions is investigated using models for the kinetics of atomic and electronic disorder in the condensed phase.