A. P. Razjivin

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

Excitonic interactions in the light-harvesting antenna of photosynthetic purple bacteria and their influence on picosecond absorbance difference spectra

A new model of the light‐harvesting antenna (core complex) of purple photosynthetic bacteria is proposed based on excitonic interactions in circular aggregates of bacteriochlorophyll molecules. The calculated absorbance difference spectra of circular aggregates demonstrate all special features observed in the experimental spectra of purple bacteria. In particular, the absorption changes with high amplitude of bleaching at the longwavelength side of the absorption band at different excitation energy are predicted.

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;

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”.

The theory of Forster-type migration between clusters of strongly interacting molecules: application to light-harvesting complexes of purple bacteria

Abstract Excitation dynamics in the light-harvesting core antenna of purple bacteria was studied theoretically using a model of excitation delocalization over the circular aggregate of strongly interacting bacteriochlorophyll molecules (‘DCA model’) (V.I. Novoderzhkin and A.P. Razjivin, FEBS Lett. 330 (1993) 5). Energy transfer from the core antenna complex to the reaction center (RC) and between core antenna complexes was treated as a Forster-type process. The assumption of the DCA model for the core antenna offers an explanation of the experimental data on the excitation trapping by RCs (60 ps antenna excitation decay; quasi-irreversible trapping). The rate of excitation migration between neighboring antenna core complexes is well suited to the realization of the ‘lake model’. The advantages of the proposed model as compared with other models are discussed.

Excitation delocalization over the whole core antenna of photosynthetic purple bacteria evidenced by non‐linear pump‐probe spectroscopy

Anomalously high values of photoinduced absorption changes were revealed in the antenna of photosynthetic purple bacteria. They were found to be 4–16 times greater at the bleaching peak of the antenna than at the bleaching peak of the BChl dimer of the reaction center. This is direct proof of excitation delocalization over many pigment molecules. Calculations according to the model of exciton delocalization over all core antenna BChls allow one to explain the observed phenomenon.

Fluorescence of native and carotenoid-depleted LH2 from Chromatium minutissimum, originating from simultaneous two-photon absorption in the spectral range of the presumed (optically ‘dark’) S1 state of carotenoids

Native and carotenoid‐depleted peripheral purple bacterial light‐harvesting complex (LH2) were investigated by simultaneous two‐photon excited (between 1300–1500 nm) fluorescence (TPF). TPF results from direct bacteriochlorophyll excitation in both samples. The spectral position of the 2Ag − state of rhodopsin is indicated by a diminuition of the bacteriochlorophyll TPF in native LH2. In conclusion, comparison to carotenoid‐depleted samples is a conditio sine qua non for unambiguous interpretation of similar experiments.

Exciton delocalization in the antenna of purple bacteria: Exciton spectrum calculations using X‐ray data and experimental site inhomogeneity

Electron absorption and circular dichroism spectra of the peripheral light‐harvesting complex (LH2) of photosynthetic purple bacteria were calculated taking into account the real‐life spatial arrangement and experimental inhomogeneous broadening of bacteriochlorophyll molecules. It was shown that strong excitonic interactions between 18 bacteriochlorophyll molecules (BChl850) within the circular aggregate of the LH2 complex result in an exciton delocalization over all these pigment molecules. The site inhomogeneity (spectral disorder) practically has no influence on exciton delocalization. The splitting between two lowest exciton levels corresponds to experimentally revealed splitting by hole‐burning studies of the LH2 complex.

Excition theory of spectra and energy transfer in photosynthesis: spectral hole burning in the antenna of purple bacteria

Abstract A theory of hole burning in the antenna of photosynthetic purple bacteria has been developed. The theory is based on the model of excitation delocalization over the circular aggregate of pigment molecules (DCA model). A good agreement between the theoretical spectra and experimental hole profiles for purple bacteria was obtained. The advantages of the DCA model for interpretation of spectral and kinetic data are discussed.

Two-Photon Excitation Spectroscopy of Carotenoid-Containing and Carotenoid-Depleted LH2 Complexes from Purple Bacteria

We applied two-photon fluorescence excitation spectroscopy to LH2 complex from purple bacteria Allochromatium minutissimum and Rhodobacter sphaeroides . Bacteriochlorophyll fluorescence was measured under two-photon excitation of the samples within the 1200-1500 nm region. Spectra were obtained for both carotenoid-containing and -depleted complexes of each bacterium to allow their direct comparison. The depletion of carotenoids did not alter the two-photon excitation spectra of either bacteria. The spectra featured a wide excitation band around 1350 nm (2x675 nm, 14,800 cm(-1)) which strongly resembled two-photon fluorescence excitation spectra of similar complexes published by other authors. We consider obtained experimental data to be evidence of direct two-photon excitation of bacteriochlorophyll excitonic states in this spectral region.