Downhill Energy Transfer Research Articles

Excitation Dynamics in the Core Antenna of PS I from Chlamydomonas reinhardtii CC 2696 at Room Temperature

Photosystem I particles from a eukaryotic organism, the green alga Chlamydomonas reinhardtii CC 2696, were studied by transient hole-burning spectroscopy at room temperature. Global analysis of the spectra recorded after excitation of chlorophyll a molecules in Photosystem I at selected wavelengths between 670 and 710 nm reveals excitation dynamics with subpicosecond, 2−3 ps, and 20−23 ps components. The subpicosecond and 2−3 ps components are ascribed to energy equilibration within the core antenna, whereas the 20−23 ps component is ascribed to energy trapping by the reaction center. Energy equilibration components describe both uphill and downhill energy transfer depending of the excitation wavelength. The initial transient absorbance bands after direct excitation of the red tail of the Qy transition band of chlorophyll a (at 700, 705, and 710 nm) are 25 nm wide and structured, revealing strongly coupled excited states among a group of molecules, most likely reaction center chlorophyll molecules. Excita...

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.

Kinetics of absorbance and anisotropy upon excited state relaxation in the reaction center core complex of a green sulfur bacterium.

Properties of the excited states in reaction center core (RCC) complexes of the green sulfur bacterium Prosthecochloris aestuarii were studied by means of femtosecond time-resolved isotropic and anisotropic absorption difference spectroscopy at 275 K. Selective excitation of the different transitions of the complex resulted in the rapid establishment of a thermal equilibrium. At about 1 ps after excitation, the energy was located at the lowest energy transition, BChl a 835. Time constants varying between 0.26 and 0.46 ps were observed for the energy transfer steps leading to this equilibrium. These transfer steps were also reflected in changes in polarization. Our measurements indicate that downhill energy transfer towards excited BChl a 835 occurs via the energetically higher spectral forms BChl a 809 and BChl a 820. Low values of the anisotropy of about 0.07 were found in the 'two-color' measurements at 820 and 835 nm upon excitation at 800 nm, whereas the 'one-color' kinetics showed much higher anisotropies. Charge separation occurred with a time constant varying between 20 and 30 ps.

Photobleaching and spectral diffusion in disordered media

The photochemical bleaching of an energetically disordered molecular system is studied theoretically. Spectral diffusion is taken into account, which is marked by downhill energy transfer of excitons between the molecular sites. The resulting redshift in the excited-state occupation gives rise to a relatively stronger bleaching of the lower-lying states, which is observed in the blueshift of both the absorption spectrum and the photoluminescence spectrum of the bleached system. During the photochemical bleaching reaction, quenching sites are formed, at which the excitons decay nonradiatively. These quenching sites modify the excited-state dynamics and thus the bleaching process. The relevance of our model to the photobleaching of poly$(p$-phenylene-vinylene) is pointed out.

Spectral equilibration and primary photochemistry in Heliobacillus mobilis at cryogenic temperature.

We performed multicolor femtosecond transient absorption measurements on membranes of the photosynthetic bacterium Heliobacillus mobilis at 20 K, by selective excitation at either the red or the blue extreme of the bacteriochlorophyll g Q(Y) band, which is split in three spectral forms (Bchl g 778, 793, and 808) at low temperature. In contrast to room temperature, there is no observable uphill energy transfer upon excitation at the red extreme. This provides a direct experimental confirmation of the expected strong temperature dependence of uphill energy transfer in multichromophore systems. Upon excitation at the blue edge, downhill energy transfer is observed on time ranges varying over 2 orders of magnitude and is discussed in terms of four distinct energy transfer processes: Bchl g 778* --> Bchl g 793* (approximately 50 fs); Bchl g 778* --> Bchl g 808* (approximately 400 fs); Bchl g 793* --> Bchl g 808* (approximately 1.4 ps); and within Bchl g 808* (approximately 7 ps). Surprisingly, the amount of oxidized primary donor P798+ formed on the time scale of picoseconds and tens of picoseconds was found to depend on the excitation conditions: trapping occurs mainly in approximately 80 ps and slower from directly excited Bchl g 808* and can additionally occur in a few picoseconds from Bchl g 778* and Bchl g 793* upon blue excitation. This finding implies that spectral equilibration is not complete prior to charge separation and furthermore is inconsistent with a funnel model, in which P798 is surrounded by long-wavelength pigments. More generally, we discuss to what extent our data bring constraints on the spatial distribution of the different spectral forms of the pigments.

Energy and electron transfer upon selective femtosecond excitation of pigments in membranes of Heliobacillus mobilis.

Excitation energy transfer steps in membranes of Heliobacillus mobilis were directly monitored by transient absorption spectroscopy with a time resolution of 30 fs under selective excitation within the inhomogeneously broadened bacteriochlorophyll g QY band. The initial anisotropy was found to be > 0.4, indicating that the pigments are excitonically coupled. After initial decay of this anisotropy in < 50 fs, major sub-picosecond components associated with spectral equilibration were identified, corresponding to uphill energy transfer with a 300 fs time constant (812 nm excitation) and downhill energy transfer with 100 and 500 fs components (770 nm excitation). These equilibrations are ascribed predominantly to single excitation transfer steps, as anisotropy measurements showed that equilibration within spectrally similar pigments occurs on the same time scale as spectral equilibration, a situation which contrasts with that in photosystem I. Downhill energy transfer occurs to a significant extent directly to an energetically heterogeneous population of excited states as well as in a sequential way via gradually lower-lying pools of bacteriochlorophyll g. This finding supports a description in which all pigments, including the bluemost absorbing, are spatially organized in a random way rather than in clusters of spectrally similar species. Spectral equilibration is not entirely completed prior to formation of the primary radical pair P798 + A0-, which was found to proceed in a multiexponential way (time constants of 5 and 30 ps). No indication for the formation of radical species other than P798 + A0- on the time scale up to 100 ps was found.

Subpicosecond Transient Absorption Difference Spectroscopy on the Reaction Center of Photosystem II: Radical Pair Formation at 77 K

To further characterize the processes of charge separation and excitation energy transfer in the isolated photosystem II reaction center complex (the D1-D2-cytochrome b-559 complex), subpicosecond absorption difference spectra were measured at 77 K. The preparations were excited with a repetition rate of 30 Hz by laser pulses with a duration of 300-400 fs at 688, 681, 672, and 670.5 nm. Upon excitation at 688 and 681 nm, a biphasic decay of the induced signals with time constants of 1-2 ps and 80-100 ps was observed. The 1-2 ps component is interpreted as the intrinsic charge separation time from the singlet-excited state of P680, the primary electron donor of photosystem II. The 80-100 ps time is interpreted as charge separation limited by slow energy transfer from a trap state in the photosystem II reaction center degenerate with P680, earlier identified by low-temperature fluorescence experiments. In addition, upon excitation at 672 and 670.5 nm energetically downhill energy transfer processes of 400-500 fs and nearly 14 ps were observed, which according to the associated spectral changes, originate from states in the reaction center absorbing at different energies. 40 refs., 6 figs.

Ultrafast energy transfer in LHC-II trimers from the Chl a/b light-harvesting antenna of Photosystem II

Time-resolved absorption difference profiles were obtained for LHC-II trimers, isolated from Photosystem II in spinach with n-dodecyl β- d-maltoside, using one-color and two-color pump-probe techniques. The one-color isotropic signals are predominantly excited state absorption at 640 nm, and a combination of photobleaching and stimulated emission at wavelengths ≥ 665nm. At intermediate wavelengths, dynamic red-shifting due to downhill energy transfer among the chlorophyll (Chl) spectral forms produces a bipolar signal, in which prompt photo-bleaching/stimulated emission is superseded at later times by excited state absorption. Triexponential analyses of these profiles yield the lifetime components 2–6 ps (associated with the spectral shifting), 14–36 ps (possibly due to energy transfer between LHC-II monomers), and several hundred picoseconds. The one-color anisotropy decays are resolvable at 665–675 nm, with lifetimes of 4.3 to 6.3 ps. They are unresolvably fast (i.e., exhibit subpicosecond lifetimes) at 640–650 nm. The two-color isotropic absorption difference signals show clear spectral evolution arising from equilibration among the LHC-II spectral components for excitation wavelengths shorter than 670 nm. However, most of this spectral evolution occurs within less than 2.5 ps. No resolvable anisotropy decay was observed in the two-color experiments. Taken together, the one-color and two-color experiments indicate that both picosecond and subpicosecond energy transfer steps occur in this antenna. The faster processes appear to dominate the spectral equilibration; slower processes occur in isoenergetic energy transfers among the longer-wavelength Chl a spectral forms that absorb between 665 and 675 nm. The values of the long-time anisotropic r( x), measured in the one-color and two-color experiments, are qualitatively consistent with static linear dichroism spectra of these preparations.