Electron transfer in photosystem I

This mini-review focuses on recent experimental results and questions, which came up since the last more comprehensive reviews on the subject. We include a brief discussion of the different techniques used for time-resolved studies of electron transfer in photosystem I (PS I) and relate the kinetic results to new structural data of the PS I reaction centre.

Modulation of primary radical pair kinetics and energetics in photosystem II by the redox state of the quinone electron acceptor Q(A)

Time-resolved photovoltage measurements on destacked photosystem II membranes from spinach with the primary quinone electron acceptor Q(A) either singly or doubly reduced have been performed to monitor the time evolution of the primary radical pair P680(+)Pheo(-). The maximum transient concentration of the primary radical pair is about five times larger and its decay is about seven times slower with doubly reduced compared with singly reduced Q(A). The possible biological significance of these differences is discussed. On the basis of a simple reversible reaction scheme, the measured apparent rate constants and relative amplitudes allow determination of sets of molecular rate constants and energetic parameters for primary reactions in the reaction centers with doubly reduced Q(A) as well as with oxidized or singly reduced Q(A). The standard free energy difference DeltaG degrees between the charge-separated state P680(+)Pheo(-) and the equilibrated excited state (Chl(N)P680)* was found to be similar when Q(A) was oxidized or doubly reduced before the flash (approximately -50 meV). In contrast, single reduction of Q(A) led to a large change in DeltaG degrees (approximately +40 meV), demonstrating the importance of electrostatic interaction between the charge on Q(A) and the primary radical pair, and providing direct evidence that the doubly reduced Q(A) is an electrically neutral species, i.e., is doubly protonated. A comparison of the molecular rate constants shows that the rate of charge recombination is much more sensitive to the change in DeltaG degrees than the rate of primary charge separation.

Fusion of chromatophores from photosynthetic bacteria with a supported lipid layer: characterization of the electric units

Direct electrometric measurements of membrane potential changes are a valuable tool for study of vectorial transfer of electrons, protons, and ions. Commonly model membrane systems are created by fusion of lipid/protein vesicles with lipid-coated thin films. We characterized the electric units resulting from this process using chromatophores from the purple bacterium Rhodobacter sphaeroides and either a Mylar film or a planar modified gold electrode as support. Investigation of the shunting activity of the ionophore gramicidin on the flash-induced potential change demonstrates fusion of individual chromatophores to form independent 'blisters', which preserve an interior aqueous compartment. Under current-clamp conditions the photovoltage follows the change of the membrane potential of the individual blisters.

Escape probability and trapping mechanism in purple bacteria: revisited

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.

Spectroscopic and molecular characterization of a long wavelength absorbing antenna of Ostreobium sp

One of the strains of the marine green alga Ostreobium sp. possesses an exceptionally large number of long wavelength absorbing chlorophylls (P. Haldall, Biol. Bull. 134, 1968, 411-424) as evident from a distinct shoulder in the absorption spectrum at around 710 nm while in the other strain this shoulder is absent. Therefore, Ostreobium offers a unique possibility to explore the origin of these red-shifted chlorophylls, because strains with and without these spectral forms can be compared. Here, we characterize these red forms spectroscopically by absorption, fluorescence and CD spectroscopy. In the CD spectra at least three spectroscopic red forms are identified which lead to an unusual room temperature fluorescence spectrum that peaks at 715 nm. The gel electrophoretic pattern from thylakoids of Ostreobium sp. shows an intense band at 22 kDa which correlates with the presence or absence of long wavelength absorbing pigments. By protein sequencing of the N-terminus of the 22-kDa polypeptide and sequence alignments, this was identified as an Lhca1-type light-harvesting complex. The abundance of this polypeptide - and a possibly co-migrating one - in Ostreobium sp. indicates an antenna size of approximately 340 chlorophyll molecules (Chl a and Chl b) per PS IIalpha reaction center, which is significantly larger than in higher plants ( approximately 240). The red forms are more abundant in the interior of the thalli where a 'shade-light' light field is expected than in the white-light exposed surface. This demonstrates that algae exist which may be able to up-regulate the synthesis of large amounts of LHCI and associated red forms under appropriate illumination conditions.

Electron transfer in the heliobacterial reaction center: evidence against a quinone-type electron acceptor functioning analogous to A1 in photosystem I

Membrane fragments from Heliobacillus mobilis were characterized using time resolved optical spectroscopy and photovoltage measurements in order to detect a possible participation of menaquinone (MQ), functioning analogous to the phylloquinone A1 in photosystem I, as intermediate in electron transfer from the primary acceptor A0 to the iron-sulfur cluster FX in the photosynthetic reaction center. The spectroscopic data obtained exclude that electron transfer from a semiquinone anion MQ- to FX occurred in the time window from 2 ns to 4 micros, where it would be expected in analogy to photosystem I. In the case of a prereduction of FX, only the primary pair P798+A0- was formed. The photovoltage data yielded a single kinetic phase with a time constant of 700 ps for the transmembrane electron transfer beyond A0; the relative amplitude of this phase suggests that it reflects electron transfer from A0- to FX.

Antenna organization in purple bacteria investigated by means of fluorescence induction curves

Fluorescence induction curves of purple bacteria (Rs. rubrum, Rps. viridis and Rb. capsulatus) were measured in the sub-millisecond time range employing a xenon flash technique. The induction curves of all three species displayed a sigmoidal shape. Analysis of the curves showed that none of the species examined had an antenna organization of a lake (i.e. unrestricted energy transfer between photosynthetic units). The apparent time constants of inter-unit exciton transfer were estimated to be approximately 24 ps in the case of LHC 1-containing species (Rs. rubrum and Rps. viridis) and 40 ps in the case of the LHC 2-containing species Rb. capsulatus. This result demonstrates that LHC 2 (B800-850) acts as a sort of insulator between photosynthetic units. Assuming a coordination number of 6 in the LHC 1-containing species the mean single step energy transfer time between adjacent LHC 1 can be estimated to be 4-5 ps. This is not perfectly compatible with the much faster Förster transfer rate of <1ps that follows from the minimal chromophore-chromophore distances estimated from digital image processing of micrographs from stained membranes. It thus may be concluded that the photosynthetic units (reaction center plus LHC 1) are loosely arranged in the photosynthetic membrane, like in the fluid-mosaic-membrane model, rather than in a hexagonally crystalline configuration.