A. Yu. Borisov

Minor component B-905 of light-harvesting antenna in Rhodospirillum rubrum chromatophores and the mechanism of singlet—singlet annihilation as studied by difference selective picosecond spectroscopy

1. Introduction of non-linear processes (single-singlet annihilation) in the light-harvesting antenna was also advanced. Primary photophysical events in the photosynthetic apparatus include: (i) transfer of photo-induced exci- tation along the light-harvesting chlorophyll-like antenna towards the reaction centers (RCs); (ii) con- version of this excitation energy into the energy of separated charges in the RCs [ 1,2]. 2. Materials and methods Laser studies on excitation transfer along antenna pigments have been performed only with the fluores- cence picosecond spectrometers because chromato- phores, thylakoids, chloroplasts, and intact cells are characterized by a relatively high fluorescence yield [3-51. Attempts at employment of absorption-pico- second spectroscopy for this purpose have not yielded any significant results due to difficulties in reliable registration of very small amplitudes of absorbancy changes (-1O-2-1O-3) in those intact preparations. Chromatophores were isolated from purple non- sulphur photosynthetic bacterium

The study of excitation transfer between light‐harvesting antenna and reaction center in chromatophores from purple bacterium Rhodospirillum rubrum by selective picosecond spectroscopy

The primary photophysical processes of photosynthesis are usually subdivided into: (i) Delivery of excitation from light-harvesting antenna pigments to reaction centers (RCs); (ii) Conversion of excitation in RC into separated charges. A portion of excitations (l10%) is usually lost en route to RCs giving rise to prompt fluorescence of antenna pigments. According to currently accepted ideas the time course of this fluorescence must reflect the kinetics of excitation delivery to RCs and the risetime of RC photooxidation. On this basis, the first picosecond studies of photosynthetic objects were carried out 11-41. Later it was demonstrated that efficient biexcitonic quenching may be involved in picosecond experiments with powerful laser pulses (see reviews [5,6]), which may obscure considerably the real kinetics of fluorescence decay. Besides, a portion of what was usually considered as fluorescence may just be short-lived delayed emissions [7-91. The third difficulty is due to the fact that some spectra1 forms of antenna chlorophylls exhibit fluorescence with a negligible yield and lifetime [ 10,l 11. All these difficulties called for using the absorbance laser methods for monitoring the excitation transfer in antenna. The difference picosecond laser spectrometer of Vilnius State University [ 12,131 is adequate now for these studies. This work reports the first picosecond absorption data on: (i) Transfer and disappearance of excitations from light-harvesting antenna;

Excitation trapping by different states of photosynthetic reaction centres

Fluorescence of light-harvesting bacteriochlorophyll (BChl) of photosynthetic bacteria displays a variable yield, the reciprocal being dependent linearly on the portion of active reaction centres (RC) [ 1,2] . If the RC is in a photochemically active state, it traps singlet (S) excitation energy from antenna BChl in some tenth of a picosecond [3]. S-S type of energy migration to the RCs predicts the following relation between the quantum yield of primary energy trapping by active RC (+,h) and maximal increase in fluorescence yield (qfl) under transition from active (all RCs are open) to saturated (all RCs are closed) photosynthesis (see, e.g., [3] ):

The fluorescence lifetime and energy migration mechanism in Photosystem I of plants

Abstract 1. 1. The active chloroplasts and subchloroplast particles sedimenting at 10000 and 165000 × g were isolated from pea leaves. The ratios of chlorophyll a belonging to Photosystem I and II were shown to be equal to 2.7, 0.4 and 5.9, respectively. 2. 2. Separate values of fluorescence lifetimes were obtained for chlorophyll a of Photosystem I ( τ ⩽ 0.03ns) and Photosystem II (1.6 ⩽ τ 2 ⩽ 1.8 ns in saturating light conditions). 3. 3. On the basis of the above estimation of the maximal τ 1 value, the minimal chlorophyll a molecular interaction energy is 0.02 eV. These moderate interactions correspond to the excition type of energy migration in Photosystem I.

Energy transfer in bacterial photosynthesis

Four possible explanations are offered to account for low fluorescence increase observed for purple bacteria under transition from active to inhibited photosynthesis. The increase observed is inconsistent with high (≅1.0) yield of primary photosynthetic process of P890 photooxidation. The dependences of fluorescence yield and lifetime on the portion of active reaction centres have been analysed for each case. Experimental investigation carried out favours the existence of background fluorescence together with fluorescence, whose quantum yield correlates with the reaction centre functional state. The important conclusion is made that lifetime of photosynthetic fluorescence is much lower than 1 nsec and energy is transferred to the reaction centres by a mechanism other than inductive-resonance.

One- and two-photon picosecond processes of electron transfer among the porphyrin molecules in bacterial reaction centers

Picosecond (ps) spectroscopy of bacterial reaction centers (RC) has shown that charge separation between bacteriochlorophyll (BChl) and bacteriopheophytin (Bph) molecules is the primary photoprocess in RC [ 1,2]. But ps laser techniques, although providing high time resolution, can cause serious artifacts if applied to photosynthetic objects [3]. Photosynthetic systems are characterized by an efficient migration of excitations in antenna chlorophyll and BChl, which focuses the excitations in the RC and their vicinity. This favours involvement of nonlinear excitation processes that may greatly obscure the kinetics of the primary photosynthetic processes. To minimize such artifacts, we have,used tunable parametric oscillators in order to obtain selective excitation of the RC pigments studied [4] (near the As70 peak, instead of at 530 nm as had been done). The sensitivity of the instrument has been increased greatly, so that the quantity of photons in each pulse could be reduced to the level q 1 photon/RC [5]. In this work oneand two-photon Ass,, has been shown to occur in prereduced Rhodopseudomonas sphaeroides R-26 RC, in agreement with [6,7]. One-photon absorption results in charge separation with formation of P-870” and P-800’in <lo ps. Then fast electron exchange (<lo ps) between P-800 and Bph is observed. This state has some absorption near 880 nm and thus can absorb a second photon. This causes the localization of an electron on the Bph molecule first and then within several ns on the B&l-800 molecule during recombination with P-870”.

Short-lived delayed luminescence of photosynthetic organisms. I. Nanosecond afterglows in purple bacteria at low redox potentials.

A combined study of emissions of purple bacteria Rhodospirillum rubrum, Ectothiorhodospira shaposhnikovii and Thiocapsa roseopersicina was performed under conditions of low potential. It has been shown that a considerable part of the emission represents a delayed luminescence with a lifetime of about 5 ns and an activation energy delta E = 0.05 +/- 0.03 eV. Intensity of this delayed luminescence is approximately equal to that of prompt fluorescence. It diminishes as temperature decreases and also as the intermediate acceptor I becomes reduced after prolonged illumination under low potential conditions. This luminescence represents a radiative decay of the intermediate state, PF, and the luminescence activation energy, delta E, reflects the energy barrier between P*-890 and PF. The value of this barrier determined in the present work is much lower than those obtained previously [3,4,26] for the free-energy release during the primary act of charge separation, basing on redox potential techniques. The reason for this discrepancy is discussed. Delayed luminescence in the picosecond time range is predicted to exist under conditions of active photosynthesis as a result of a small (approx. 0.05 eV) energy barrier between PF and the excited singlet state of reaction center bacteriochlorophyll.

Energy transfer in photoactive complexes obtained from green bacterium Chlorobium limicola

Abstract The pigment · protein complexes enriched with bacteriochlorophyll a were derived from green bacterium Chlorobium limicola . The light dependences of bacteriochlorophyll fluorescence yield and lifetime as well as the portion of photooxidized reaction centers P -840 were recorded. These combined data revealed that in addition to the known fluorescence with a lifetime exceeding 2 ns, the short-living emission exists in a picosecond time scale. The latter belongs to the main portion of bacteriochlorophyll a and its lifetime and yield are governed by the P -840 state. According to the time-lever method developed for phase fluorimetry, its physiological lifetime amounts to no more than 40–100 ps and the limiting value to 10–25 ps. The fluorescence yield and the portion of molecules belonging to different funds are also determined. The above data prove the energy migration in bacteriochlorophyll a to be of the excitonic type at a moderate rate of molecular interaction. The quantum yield of triplet formation in antenna bacteriochlorophyll is negligible in a state of active photosynthesis, as in purple bacteria and Photosystem I of plants.

Determination of the quantum yields of the primary photosynthesis events and the photosynthetic unit types in purple bacteria

Summary1.Photoinduced changes in absorbancy and fluorescence yields were studied inChromatium minutissium andEctothiorhodospira Shaposhnikovii suspensions. These parameters were examined as functions of light intensity in the region of 750–950 nm under aerobic and anaerobic conditions.2.The fluorescence was shown to consist of two spectrally indistinguishable emissions: “photosynthetic” emission with a yield correlating with the state of reaction centres (P890) and “background” emission with constant yield.3.The “photosynthetic” fluorescence dependence on the portion of nonactive P890 was consistent with the multicentral (statistic) model of photosynthetic unit organization.4.In anaerobic conditions the amplitude of the photoinduced absorbancy increase around 910 nm was proportional to the portion of nonactive P890.5.A precise method for determinating the quantum yield of the primary electron donation was proposed. ForChr. minutissimum andE. shaposhnikovii under aerobic conditions these yields were found to be equal to 0·91±0·02 and 0·93±0·02 respectively.

The quantum yield of reaction center photooxidation in subchloroplast fragments enriched with photosystem. I.

Abstract Suspensions of light particles sedimented at 16500×g from pea subchloroplast preparations were investigated. The exciting light readily caused the photooxidation of P700 in a fraction of active particles. The effective energy transfer has been shown to occur between individual P700. The quantum yield of P700 photooxidation was estimated by means of the relative method to be ⩾0.75.