On the relation between absorption and fluorescence emission spectra of photosystems: Derivation of a Stepanov relation for pigment culsters

Antenna entropy in plant photosystems does not reduce the free energy for primary charge separation

We have investigated the concept of the so-called "antenna entropy" of higher plant photosystems. Several interesting points emerge: 1. In the case of a photosystemwhich harbours an excited state, the “antenna entropy” is equivalent to the configurational (mixing) entropy of a thermodynamic canonical ensemble. The energy associated with this parameter has been calculated for a hypothetical isoenergetic photosystem, photosystem I and photosystem II, and comes out in the range of 3.5 - 8% of the photon energy considering 680 nm. 2. The “antenna entropy” seems to be a rather unique thermodynamic phenomenon, in as much as it does not modify the free energy available for primary photochemistry, as has been previously suggested. 3. It is underlined that this configurational (mixing) entropy, unlike heat dispersal in a thermal system, does not involve energy dilution. This points out an important difference between thermal and electronic energy dispersal.

Reconstituted CP29: multicomponent fluorescence decay from an optically homogeneous sample

The multiexponential fluorescence decay of the CP29 complex in which the apoprotein and pigments were reconstituted in vitro was examined. Of the three decay components observed only the two dominant ones, with about 3 and 5 ns lifetimes, were studied. The main question addressed was whether the multicomponent decay was associated with sample optical heterogeneity. To this end, we examined the optical absorption and fluorescence of the CP29 sample by means of two different and independent experimental strategies. This approach was used as the wavelength positions of the absorption/fluorescence spectral forms has recently been shown to be a sensitive indicator of the binding site-induced porphyrin ring deformation (Zucchelli et al. Biophys J 93:2240-2254, 2007) and hence of apoprotein conformational changes. The data indicate that this CP29 sample is optically homogeneous. It is hypothesised that the different lifetimes are explained in terms of multiple detergent/CP29 interactions leading to different quenching states, a suggestion that allows for optical homogeneity.

Energetics of primary and secondary electron transfer in Photosystem II membrane particles of spinach revisited on basis of recombination-fluorescence measurements

Photon absorption by one of the roughly 200 chlorophylls of the plant Photosystem II (PSII) results in formation of an equilibrated excited state (Chl200*) and is followed by chlorophyll oxidation (formation of P680+) coupled to reduction of a specific pheophytin (Phe), then electron transfer from Phe- to a firmly bound quinone (QA), and subsequently reduction of P680+ by a redox-active tyrosine residue denoted as Z. The involved free-energy differences (DeltaG) and redox potentials are of prime interest. Oxygen-evolving PSII membrane particles of spinach were studied at 5 degrees C. By analyzing the delayed and prompt Chl fluorescence, we determined the equilibrium constant and thus free-energy difference between Chl200* and the [Z+,QA-] radical pair to be -0.43+/-0.025 eV, at 10 mus after the photon absorption event for PSII in its S(3)-state. On basis of this value and previously published results, the free-energy difference between P680* and [P680+,QA-] is calculated to be -0.50+/-0.04 eV; the free-energy loss associated with electron transfer from Phe to QA is found to be 0.34+/-0.04 eV. The given uncertainty ranges do not represent a standard deviation or likely error, but an estimate of the maximal error. Assuming a QA-/QA redox potential of -0.08 V, the following redox-potential estimates are obtained: +1.25 V for P680/P680+; +1.21 V for Z/Z+ (at 10 mus); -0.42 V for Phe-/Phe; -0.58 V for P680*/P680+.

A temporal model of level-invariant, tone-in-noise detection

Level-invariant detection refers to findings that thresholds in tone-in-noise detection are unaffected by roving-level procedures that degrade energy cues. Such data are inconsistent with ideas that detection is based on the energy passed by an auditory filter. A hypothesis that detection is based on a level-invariant temporal cue is advanced. Simulations of a leaky-integrator model, consisting of a bandpass filter, half-wave rectification, and a lowpass filter, account for thresholds in band-widening experiments. The decision variable is calculated from the discrete Fourier transform of the leaky-integrator output. A counterintuitive finding is the apparent disassociation of the phenomenon of critical bands estimated from band-widening experiments and the theory of auditory filters. Physiological plausibility is demonstrated by showing that a leaky integrator describes the discharge cadence of primary afferents for tone-in-noise stimuli as well as for complex periodic sounds.

Excited state trapping and the Stepanov relation with reference to Photosystem I

It has been previously demonstrated that the Stepanov equation provides a rather good description of the absorption/fluorescence spectra in Photosystem I, even though excited state equilibration is not rapid with respect to the excited state decay. In the present article this apparent contradiction is examined analytically for two-state systems and numerically for many-state systems. It is demonstrated that, in the special case of the trapping process being associated with the initially populated state, neither very rapid excited state equilibration nor a transfer equilibrium, which approximates a true Boltzmann distribution, are prerequisites to obtaining a very close approximation to a correct Stepanov result. This interesting conclusion is discussed in terms of plant Photosystem I (PSI-200). It is concluded that whereas, in compartmental modeling, photochemical trapping may be formally associated with the bulk antenna pigments due to the strong energy coupling between them and the trap pigments, this is not the case for the red spectral forms.

Decay kinetics and quantum yields of fluorescence in photosystem I from Synechococcus elongatus with P700 in the reduced and oxidized state: are the kinetics of excited state decay trap-limited or transfer-limited?

Transfer and trapping of excitation energy in photosystem I (PS I) trimers isolated from Synechococcus elongatus have been studied by an approach combining fluorescence induction experiments with picosecond time-resolved fluorescence measurements, both at room temperature (RT) and at low temperature (5 K). Special attention was paid to the influence of the oxidation state of the primary electron donor P700. A fluorescence induction effect has been observed, showing a approximately 12% increase in fluorescence quantum yield upon P700 oxidation at RT, whereas at temperatures below 160 K oxidation of P700 leads to a decrease in fluorescence quantum yield ( approximately 50% at 5 K). The fluorescence quantum yield for open PS I (with P700 reduced) at 5 K is increased by approximately 20-fold and that for closed PS I (with P700 oxidized) is increased by approximately 10-fold, as compared to RT. Picosecond fluorescence decay kinetics at RT reveal a difference in lifetime of the main decay component: 34 +/- 1 ps for open PS I and 37 +/- 1 ps for closed PS I. At 5 K the fluorescence yield is mainly associated with long-lived components (lifetimes of 401 ps and 1.5 ns in closed PS I and of 377 ps, 1.3 ns, and 4.1 ns in samples containing approximately 50% open and 50% closed PS I). The spectra associated with energy transfer and the steady-state emission spectra suggest that the excitation energy is not completely thermally equilibrated over the core-antenna-RC complex before being trapped. Structure-based modeling indicates that the so-called red antenna pigments (A708 and A720, i.e., those with absorption maxima at 708 nm and 720 nm, respectively) play a decisive role in the observed fluorescence kinetics. The A720 are preferentially located at the periphery of the PS I core-antenna-RC complex; the A708 must essentially connect the A720 to the reaction center. The excited-state decay kinetics turn out to be neither purely trap limited nor purely transfer (to the trap) limited, but seem to be rather balanced.

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.

Uphill energy transfer in LH2-containing purple bacteria at room temperature

Uphill energy transfer in the LH2-containing purple bacteria Rhodopseudomonas acidophila, Rhodopseudomonas palustris, Rhodobacter sphaeroides, Chromatium vinosum and Chromatium purpuratum was studied by stationary fluorescence spectroscopy at room temperature upon selective excitation of the B800 pigments of LH2 and the B880 pigments of LH1 at 803 nm and 900 nm, respectively. The resulting fluorescence spectra differed significantly at wavelengths shorter than the fluorescence maximum but agreed at longer wavelengths. The absorption spectra of the species studied were decomposed into five bands at approx. 800, 820, 830, 850 and 880 nm using the shapes of the absorption spectra of the LH1-RC only species Rhodospirillum rubrum and the isolated B800-850 complex from Rps. acidophila strain 10050 as guide spectra. This allowed a quantification of the number of pigments in each pigment group and, consequently, the antenna size of the photosynthetic unit assuming 36 bacteriochlorophyll a molecules in an LH1-RC complex. In most of the LH2-containing purple bacterial strains the number of LH2 rings per LH1-RC was less than the idealized number of eight (Papiz et al., Trends Plant Sci. 1 (1996) 198-206), which was achieved only by C. purpuratum. Uphill energy transfer was assayed by comparing the theoretical fluorescence spectrum obtained from a Boltzmann equilibrium with the measured fluorescence spectrum obtained by 900 nm excitation. The good match of both spectra in all the purple bacteria studied indicates that uphill energy transfer occurs practically up to its thermodynamically maximal possible extent. All strains studied contained a small fraction of either poorly connected or unconnected LH2 complexes as indicated by higher fluorescence yields from the peripheral complexes than predicted by thermal equilibration or kinetic modeling. This impedes generally the quantitative analysis of blue-excited fluorescence spectra.

The effect of excited state population in photosystem II on the photoinhibition-induced changes in chlorophyll fluorescence parameters

The photoinhibition-induced changes in Photosystem II fluorescence parameters of spinach thylakoids were only slightly sensitive to the excited state population in Photosystem II antenna, as modulated by either quinone quenching or energy spillover. The possibility that this may be due to a small fraction of chlorophyll molecules which are poorly coupled to the antenna is discussed.

Light-harvesting in Acaryochloris marina--spectroscopic characterization of a chlorophyll d-dominated photosynthetic antenna system

Oxygenic photosynthesis of the prokaryote Acaryochloris marina involves chlorophyll d (Chl d) as the major pigment [Miyashita et al. (1996) Nature 383, 402]. Four spectral forms of Chl d (peak wavelengths: 694, 714, 726 and 740 nm) are resolvable by low-temperature absorption spectroscopy on intact cells. Based on fluorescence spectra (at 290 K and 77 K) and on analysis of fluorescence induction curves we conclude: (1) excitation energy is efficiently transferred between the various spectral forms of Chl d and the PS II reaction center; (2) Chl d serves as a light-harvesting pigment for both, Photosystem II (PS II) and PS I; (3) excitation energy transfer between PS II units occurs.