Energy transfer and charge separation in the purple non-sulfur bacterium Roseospirillum parvum

The antenna reaction centre system of the recently described purple non-sulfur bacterium Roseospirillum parvum strain 930I was studied with various spectroscopic techniques. The bacterium contains bacteriochlorophyll (BChl) a, 20% of which was esterified with tetrahydrogeranylgeraniol. In the near-infrared, the antenna showed absorption bands at 805 and 909 nm (929 nm at 6 K). Fluorescence bands were located at 925 and 954 nm, at 300 and 6 K, respectively. Fluorescence excitation spectra and time resolved picosecond absorbance difference spectroscopy showed a nearly 100% efficient energy transfer from BChl 805 to BChl 909, with a time constant of only 2.6 ps. This and other evidence indicate that both types of BChl belong to a single LH1 complex. Flash induced difference spectra show that the primary electron donor absorbs at 886 nm, i.e. at 285 cm(-1) higher energy than the long wavelength antenna band. Nevertheless, the time constant for trapping in the reaction centre was the same as for almost all other purple bacteria: 55+/-5 ps. The shape as well as the amplitude of the absorbance difference spectrum of the excited antenna indicated exciton interaction and delocalisation of the excited state over the BChl 909 ring, whereas BChl 805 appeared to have a monomeric nature.     

Dynamics of energy conversion in reaction center core complexes of the green sulfur bacterium Prosthecochloris aestuarii at low temperature.

Excited-state and electron-transfer dynamics at cryogenic temperature in reaction center core (RCC) complexes of the photosynthetic green sulfur bacterium Prosthecochloris aestuarii were studied by means of time-resolved absorption spectroscopy, using selective excitaton of bacteriochlorophyll (BChl) a and of chlorophyll (Chl) a 670. The results indicate that the BChls a of the RCC complex form an excitonically coupled system. Relaxation of the excitation energy within the ensemble of BChl a molecules occurred within 2 ps. A time constant of about 25 ps was ascribed to charge separation. Absorption changes in the 670 nm region, where Chl a 670 absorbs, were fairly complicated. They showed various time constants and were dependent on the wavelength of excitation and they did not lead to a simple picture of the electron acceptor reaction. Energy transfer from Chl a 670 to BChl a occurred with a time constant of 1.5 ps. However, upon excitation of Chl a 670 the amount of oxidized primary electron donor, P840(+), formed relative to that of excited BChl a was considerably larger than upon direct excitation of BChl a. This indicates the existence of an alternative pathway for charge separation which does not involve excited BChl a.     

Primary photosynthetic processes in Heliobacterium chlorum at 15K

Absorbance changes induced by 25-ps laser flashes were measured in membranes of Heliobacterium chlorum at 15 K. Absorbance difference spectra, measured at various times after the flash showed negative bands in the Q_y region at 812, 793 and 665 nm. The first of these bands was attributed to the formation of excited singlet states of a long-wavelength form of antenna bacteriochlorophyll g (BChl g 808). Absorbance changes of shorter wavelength absorbing antenna BChls g were at least an order of magnitude smaller, indicating rapid excitation energy transfer (i.e. within the time resolution of the apparatus) from these BChls to BChl g 808. Excited BChl g 808 showed a bi-exponential decay with time constants of 50 and 200 ps. The bands at 793 and 665 nm may be attributed to the primary charge separation and reflect the photooxidation of the primary electron donor P-798 and photoreduction of a primary electron acceptor absorbing near 670 nm, presumably a BChl c or Chl a-like pigment. The bleaching of this pigment reversed with a time constant of 300 ps at 15 K and of 800 ps at 300 K. This indicates that electron transfer from the primary to the secondary electron acceptor is approximately 2.5 times faster at 15 K than at room temperature.Abbreviations BChl bacteriochlorophyll - FWHM full width at half maximum - P-798 primary electron donor - Tris tris(hydroxymethyl)amino methane     

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. This revised version was published online in June 2006 with corrections to the Cover Date.     

Picosecond time-resolved absorption and fluorescence dynamics in the artificial bacteriorhodopsin pigment BR6.11.

The picosecond molecular dynamics in an artificial bacteriorhodopsin (BR) pigment containing a structurally modified all-trans retinal chromphore with a six-membered ring bridging the C11=C12-C13 positions (BR6.11) are measured by picosecond transient absorption and picosecond time-resolved fluorescence spectroscopy. Time-dependent intensity and spectral changes in absorption in the 570-650-nm region are monitored for delays as long as 5 ns after the 7-ps, 573-nm excitation of BR6.11. Two intermediates, J6.11 and K6.11/1, both with enhanced absorption to the red (> 600 nm) of the BR6.11 spectrum are observed within approximately 50 ps. The J6.11 intermediate decays with a time constant of 12 +/- 3 ps to form K6.11/1. The K6.11/1 intermediate decays with an approximately 100-ps time constant to form a third intermediate, K6.11/2, which is observed through diminished 650-nm absorption (relative to that of K6.11/1). No other transient absorption changes are found during the remainder of the initial 5-ns period of the BR6.11 photoreaction. Fluorescence in the 650-900-nm region is observed from BR6.11, K6.11/1, and K6.11/2, but no emission assignable to J6.11 is found. The BR6.11 fluroescence spectrum has a approximately 725-nm maximum which is blue-shifted by approximately 15 nm relative to that of native BR-570 and is 4.2 +/- 1.5 times larger in intensity (same sample optical density). No differences in the profile of the fluorescence spectra of BR6.11 and the intermediates K6.11/1 and K6.11/2 are observed. Following ground-state depletion of the BR6.11 population, the time-resolved fluroescence intensity monitored at 725 nm increases with two time constants, 12 +/- 3 and approximately 100 ps, both of which correlate well with changes in the picosecond transient absorption data.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antenna organization in green photosynthetic bacteria. 1. Oligomeric bacteriochlorophyll c as a model for the 740 nm absorbing bacteriochlorophyll c in Chloroflexus aurantiacus chlorosomes

Bacteriochlorophyll (BChl) c was extracted from Chloroflexus aurantiacus and purified by reverse-phase high-pressure liquid chromatography. This pigment consists of a complex mixture of homologues, the major component of which is 4-ethyl-5-methylbacteriochlorophyll c stearyl ester. Unlike previously characterized BChls c, the pigment from C. aurantiacus is a racemic mixture of diastereoisomers with different configurations at the 2a chiral center. Diluting a concentrated methylene chloride solution of BChl c with hexane produces an oligomer with absorption maxima at 740-742 and at 460-462 nm. Both the absorption spectrum and the fluorescence emission spectrum (maximum at 750 nm) of this oligomer closely match those of BChl c in chlorosomes. Further support for this model comes from the ability of alcohols, which disrupt BChl c oligomers by ligating the central Mg atom, to convert BChl c in chlorosomes to a monomeric form when added in low concentrations. The lifetime of fluorescence from the 740 nm absorbing BChl c oligomer is about 80 ps. Although exciton quenching might be unusually fast in the in vitro BChl c oligomer because of its large size and/or the presence of minor impurities, this result suggests that energy transfer from the BChl c antenna in chlorosomes must be very fast if it is to be efficient.     

Excitation Energy Transfer Dynamics and Excited-State Structure in Chlorosomes of Chlorobium phaeobacteroides

The excited-state relaxation within bacteriochlorophyll (BChl) e and a in chlorosomes of Chlorobium phaeobacteroides has been studied by femtosecond transient absorption spectroscopy at room temperature. Singlet-singlet annihilation was observed to strongly influence both the isotropic and anisotropic decays. Pump intensities in the order of 10¹¹ photons × pulse⁻¹ × cm⁻² were required to obtain annihilation-free conditions. The most important consequence of applied very low excitation doses is an observation of a subpicosecond process within the BChl e manifold (~200–500 fs), manifesting itself as a rise in the red part of the Q_y absorption band of the BChl e aggregates. The subsequent decay of the kinetics measured in the BChl e region and the corresponding rise in the baseplate BChl a is not single-exponential, and at least two components are necessary to fit the data, corresponding to several BChl e→BChl a transfer steps. Under annihilation-free conditions, the anisotropic kinetics show a generally slow decay within the BChl e band (10–20 ps) whereas it decays more rapidly in the BChl a region (~1 ps). Analysis of the experimental data gives a detailed picture of the overall time evolution of the energy relaxation and energy transfer processes within the chlorosome. The results are interpreted within an exciton model based on the proposed structure.     

New photocycle intermediates in the photoactive yellow protein from Ectothiorhodospira halophila: picosecond transient absorption spectroscopy.

Previous studies have shown that the room temperature photocycle of the photoactive yellow protein (PYP) from Ectothiorhodospira halophila involves at least two intermediate species: I1, which forms in <10 ns and decays with a 200-micros lifetime to I2, which itself subsequently returns to the ground state with a 140-ms time constant at pH 7 (Genick et al. 1997. Biochemistry. 36:8-14). Picosecond transient absorption spectroscopy has been used here to reveal a photophysical relaxation process (stimulated emission) and photochemical intermediates in the PYP photocycle that have not been reported previously. The first new intermediate (I0) exhibits maximum absorption at approximately 510 nm and appears in </=3 ps after 452 nm excitation (5 ps pulse width) of PYP. Kinetic analysis shows that I0 decays with a 220 +/- 20 ps lifetime, forming another intermediate (Idouble dagger0) that has a similar difference wavelength maximum, but with lower absorptivity. Idouble dagger0 decays with a 3 +/- 0.15 ns time constant to form I1. Stimulated emission from an excited electronic state of PYP is observed both within the 4-6-ps cross-correlation times used in this work, and with a 16-ps delay for all probe wavelengths throughout the 426-525-nm region studied. These transient absorption and emission data provide a more detailed understanding of the mechanistic dynamics occurring during the PYP photocycle.     

Evidence of excited state absorption in PS II membrane fragments

Utilizing a two-beam technique in the frequency domain, the pumped absorption of PS II membrane fragments from spinach and of acetonic chlorophyll-a solutions was measured at room temperature. In a very narrow wavelength region (0.2 nm around the pump pulse wavelength) the relative test beam transmission exhibited either a decrease or an increase, respectively, dependent on the intensity of a strong pump beam. In contrast, the transmission changes of chl-a solutions were not affected by the wavelength mistuning between pump and test beam. The data obtained for PS II membrane fragments were interpreted in terms of excited state absorption of pigment-protein clusters within the light-harvesting complex of PS II. The interpretation of the small absorption band as a homogeneously broadened line led to a transversal relaxation time for chlorophyll in vivo of about 1 ps.Abbreviations PS I photosystem I of green plants - PS II photosystem II of green plants - P700 primary donor of PS I - P680 primary donor of PS II     

Observation of pheophytin reduction in photosystem two reaction centers using femtosecond transient absorption spectroscopy.

Photosystem two reaction centers have been studied using a sensitive femtosecond transient absorption spectrometer. Measurements were performed at 295 K using different excitation wavelengths and excitation intensities which are shown to avoid multiphoton absorption by the reaction centers. Analyses of results collected over a range of time scales and probe wavelengths allowed the resolution of two exponential components in addition to those previously reported [Durrant, J. R., Hastings, G., Hong, Q., Barber, J., Porter, G., & Klug, D. R. (1992) Chem. Phys. Lett. 188, 54-60], plus the long-lived radical pair itself. A 21-ps component was observed. The process(es) responsible for this component was (were) found to produce bleaching of a pheophytin ground-state absorption band at 545 nm and the simultaneous appearance of a pheophytin anion absorption band at 460 nm resulting in a transient spectrum which was that of the radical pair P680+Ph-. This component is assigned to the production of reduced pheophytin. A lower limit of 60% of the final pheophytin reduction was found to occur at this rate. Despite subtle differences in transient spectra, the lifetime and yield of this pheophytin reduction are essentially independent of excitation wavelength within the signal to noise limitations of these experiments. A long-lived species was also observed. This species is produced by those processes which result in the 21-ps component, and it has a spectrum which is found to be independent of excitation wavelength. This spectrum is characteristic of the primary radical pair state P680+Ph-. In addition, a 200-ps component was found which is tentatively assigned to a slow energy-transfer/trapping process. This component was absent if P680 was excited directly and is therefore not integral to primary radical pair formation. Overall, it is concluded that the rate of pheophytin reduction is limited to (21 ps)-1, even when P680 is directly excited.     

Subpicosecond and picosecond studies of electron transfer intermediates in Rhodopseudomonas sphaeroides reaction centers

The primary electron transfer processes in isolated reaction centers of Rhodopseudomonas sphaeroides have been investigated with subpicosecond and picosecond spectroscopic techniques. Spectra and kinetics of the absorbance changes following excitation with 0.7-ps 610-nm pulses, absorbed predominantly by bacteriochlorophyll (BChl), indicate that the radical pair state P⁺BPh^?, in which an electron has been transferred from the BChl dimer (P) to a bacteriopheophytin (BPh), is formed with a time constant no greater than 4 ps. The initial absorbance changes also reveal an earlier state, which could be an excited singlet state, or a P⁺BChl^? radical pair.The bleaching at 870 nm produced by 7 ps excitation pulses at 530 nm (absorbed by BPh) or at 600 nm (absorbed predominantly by BChl) shows no resolvable delay with respect to standard compounds in solution, suggesting that the time for energy transfer from BPh to P is less than 7 ps. However, the bleaching in the BPh band at 545 nm following 7-ps 600-nm excitation, exhibits an 8- to 10-ps lag with respect to standard compounds. This finding is qualitatively similar to the 35-ps delay previously observed at 760 nm by Shuvalov at al. (Shuvalov, V.A., Klevanik, A.V., Sharkov, A.V., Matveetz, Y.A. and Kryukov, P.G. (1978) FEBS Lett. 91, 135–139) when 25-ps 880-nm excitation flashes were used. A delay in the bleaching approximately equal to the width of the excitation flash can be explained in terms of the opposing effects of bleaching due to the reduction of BPh, and absorbance increases due to short-lived excited states (probably of BChl) that turn over rapidly during the flash.The decay of the initial bleaching at 800 nm produced by 7-ps 530- or 600-nm excitation flashes shows a fast component with a 30-ps time constant, in addition to a slower component having the 200-ps kinetics expected for the decay of P⁺BPh^?. The dependence on excitation intensity of the absorbance changes due to the 30-ps component indicate that the quantum yield of the state responsible for this step is lower than that observed for the primary electron transfer reactions. This suggests that at least part of the transient bleaching at 800 nm is due to a secondary process, possibly caused by excitation with an excessive number of photons. If the 800-nm absorbing BChl (B) acts as an intermediate electron carrier in the primary photochemical reaction, electron transfer between B and the BPh must have a time constant no greater than 4 ps.     

Engineering of B800 bacteriochlorophyll binding site specificity in the Rhodobacter sphaeroides LH2 antenna

The light-harvesting 2 complex (LH2) of the purple phototrophic bacterium Rhodobacter sphaeroides is a highly efficient, light-harvesting antenna that allows growth under a wide-range of light intensities. In order to expand the spectral range of this antenna complex, we first used a series of competition assays to measure the capacity of the non-native pigments 3-acetyl chlorophyll (Chl) a, Chl?d, Chl?f or bacteriochlorophyll (BChl) b to replace native BChl?a in the B800 binding site of LH2. We then adjusted the B800 site and systematically assessed the binding of non-native pigments. We find that Arg_?10 of the LH2 β polypeptide plays a crucial role in binding specificity, by providing a hydrogen-bond to the 3-acetyl group of native and non-native pigments. Reconstituted LH2 complexes harbouring the series of (B)Chls were examined by transient absorption and steady-state fluorescence spectroscopies. Although slowed 10-fold to ~6?ps, energy transfer from Chl?a to B850 BChl?a remained highly efficient. We measured faster energy-transfer time constants for Chl?d (3.5?ps) and Chl?f (2.7?ps), which have red-shifted absorption maxima compared to Chl?a. BChl?b, red-shifted from the native BChl?a, gave extremely rapid (≤0.1?ps) transfer. These results show that modified LH2 complexes, combined with engineered (B)Chl biosynthesis pathways in vivo, have potential for retaining high efficiency whilst acquiring increased spectral range.     

Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I

Picosecond transient absorption difference spectroscopy in the blue wavelength region (380-500 nm) was used to study the early electron acceptors in photosystem I. Samples were photosystem I core particles with about 100 chlorophylls per reaction center isolated from the cyanobacterium Synechocystis sp. PCC 6803. After excitation at 590 nm at room temperature, decay-associated spectra (DAS) were determined from global analysis in the blue region, yielding two transient components and one nondecaying component. A 3 ps decay phase is interpreted as primarily due to antenna excited-state redistribution. A 28 ps decay phase is interpreted as due to overall excited-state decay by electron transfer. The nondecaying component is ascribed to the difference spectrum of P(700) and the quinone or A(1) electron acceptor (P(700)(+)A(1)(-) - P(700)A(1)). Decay curves on the millisecond time scale at different wavelengths were measured with an autoxidizable artificial electron acceptor, benzyl viologen, and the (P(700)(+) - P(700)) difference spectrum was constructed. The (A(1)(-) - A(1)) difference spectrum was obtained by taking the difference between the above two difference spectra. A parallel picosecond experiment under strongly reducing conditions was also done as a control experiment. These conditions stabilize the electron on an earlier acceptor, A(0). The nondecaying component of the DAS at low potential was assigned to (P(700)(+)A(0)(-) - P(700)A(0)) since the electron-transfer pathway from A(0) to A(1) was blocked. The [(P(700)(+)A(0)(-) - P(700)A(0)) - (P(700)(+) - P(700))] subtraction gives a spectrum, interpreted as the (A(0)(-) - A(0)) difference spectrum of a chlorophyll a molecule, consistent with previous studies. The (A(1)(-) - A(1)) spectrum resolved on the picosecond time scale shows significant differences with similar spectra measured on longer time scales. These differences may be due to electrochromic effects and spectral evolution.     

Redox effects on the excited-state lifetime in chlorosomes and bacteriochlorophyll c oligomers.

Oligomers of [E,E] BChl CF (8, 12-diethyl bacteriochlorophyll c esterified with farnesol (F)) and [Pr,E] BChl CF (analogously, M methyl, Pr propyl) in hexane and aqueous detergent or lipid micelles were studied by means of steady-state absorption, time-resolved fluorescence, and electron spin resonance spectroscopy. The maximum absorption wavelength, excited-state dynamics, and electron spin resonance (EPR) linewidths are similar to those of native and reconstituted chlorosomes of Chlorobium tepidum. The maximum absorption wavelength of oligomers of [E,E] BChl CF was consistently blue-shifted as compared to that of [Pr,E] BChl CF oligomers, which is ascribed to the formation of smaller oligomers with [E,E] BChl CF than [Pr,E] BChl CF. Time-resolved fluorescence measurements show an excited-state lifetime of 10 ps or less in nonreduced samples of native and reconstituted chlorosomes of Chlorobium tepidum. Under reduced conditions the excited-state lifetime increased to tens of picoseconds, and energy transfer to BChl a or long-wavelength absorbing BChl c was observed. Oligomers of [E,E] BChl CF and [Pr,E] BChl CF in aqueous detergent or lipid micelles show a similar short excited-state lifetime under nonreduced conditions and an increase up to several tens of picoseconds upon reduction. These results indicate rapid quenching of excitation energy in nonreduced samples of chlorosomes and aqueous BChl c oligomers. EPR spectroscopy shows that traces of oxidized BChl c radicals are present in nonreduced and absent in reduced samples of chlorosomes and BChl c oligomers. This suggests that the observed short excited-state lifetimes in nonreduced samples of chlorosomes and BChl c oligomers may be ascribed to excited-state quenching by BChl c radicals. The narrow EPR linewidth suggests that the BChl c are arranged in clusters of 16 and 6 molecules in chlorosomes of Chlorobium tepidum and Chloroflexus aurantiacus, respectively.     

Excitation energy transfer and trapping dynamics in the core complex of the filamentous photosynthetic bacterium Roseiflexus castenholzii

The light-harvesting core complex of the thermophilic filamentous anoxygenic phototrophic bacterium Roseiflexus castenholzii is intrinsic to the cytoplasmic membrane and intimately bound to the reaction center (RC). Using ultrafast transient absorption and time-resolved fluorescence spectroscopy with selective excitation, energy transfer, and trapping dynamics in the core complex have been investigated at room temperature in both open and closed RCs. Results presented in this report revealed that the excited energy transfer from the BChl 800 to the BChl 880 band of the antenna takes about 2?ps independent of the trapping by the RC. The time constants for excitation quenching in the core antenna BChl 880 by open and closed RCs were found to be 60 and 210?ps, respectively. Assuming that the light harvesting complex is generally similar to LH1 of purple bacteria, the possible structural and functional aspects of this unique antenna complex are discussed. The results show that the core complex of Roseiflexus castenholzii contains characteristics of both purple bacteria and Chloroflexus aurantiacus.     

Sequential photoisomerisation dynamics of the push-pull azobenzene Disperse Red 1

The ultrafast dynamics of the push-pull azobenzene Disperse Red 1 following photoexcitation at λ(pump) = 475 nm in solution in 2-fluorotoluene have been probed by broadband transient absorption spectroscopy and fluorescence up-conversion spectroscopy. The measured two-dimensional spectro-temporal absorption map features a remarkable "fast" excited-state absorption (ESA) band at λ ≈ 570 nm appearing directly with the excitation laser pulse and showing a sub-100 fs lifetime with a rapid spectral blue-shift. Moreover, its ultrafast decay is paralleled by rising distinctive ESA at other wavelengths. Global fits to the absorption-time profiles using a consecutive kinetic model yielded three time constants, τ(1) = 0.08 ± 0.03 ps, τ(2) = 0.99 ± 0.02 ps, and τ(3) = 6.0 ± 0.1 ps. Fluorescence-time profiles were biexponential with time constants τ(1)' = 0.12 ± 0.06 ps and τ(2)' = 0.70 ± 0.10 ps, close to the absorption results. Based on the temporal evolution of the transient spectra, especially the "fast" excited-state absorption band at λ ≈ 570 nm, and on the global kinetic analysis of the time profiles, τ(1) is assigned to an ultrafast transformation of the optically excited ππ* state to an intermediate state, which may be the nπ* state, τ(2) to the subsequent isomerisation and radiationless deactivation time to the S(0) electronic ground state, and τ(3) to the eventual vibrational cooling of the internally "hot" S(0) molecules.     

Torsional motion of eosin-labeled F-actin as detected in the time-resolved anisotropy decay of the probe in the sub-millisecond time range

The internal motion of F-actin in the time range from 10(-6) to 10(-3) second has been explored by measuring the transient absorption anisotropy of eosin-labeled F-actin using laser flash photolysis. The transient absorption anisotropy of eosin-F-actin at 20 degrees C has a component that decays in the submicrosecond time scale to an anisotropy of about 0.3. This anisotropy then decays with a relaxation time of about 450 microseconds to a residual anisotropy of about 0.1 after 2 ms. When the concentration of eosin-F-actin was varied in the range from 7 to 28 microM, the transient absorption anisotropy curves obtained were almost indistinguishable from each other. These results show that the anisotropy decay arises from internal motion of eosin-F-actin. Analysis of the transient absorption anisotropy curves indicates that the internal motion detected by the decay in anisotropy is primarily a twisting of actin protomers in the F-actin helix; bending of the actin filament makes a minor contribution only to the measured decay. The torsional rigidity calculated from the transient absorption anisotropy is 0.2 X 10(-17) dyn cm2 at 20 degrees C, which is about an order of magnitude smaller than the flexural rigidity determined from previous studies. Thus, we conclude that F-actin is more flexible in twisting than in bending. The calculated root-mean-square fluctuation of the torsional angle between adjacent actin protomers in the actin helix is about 4 degrees at 20 degrees C. We also found that the torsional rigidity is approximately constant in the temperature range from 5 to approximately 35 degrees C, and that the binding of phalloidin does not appreciably affect the torsional motion of F-actin.     

Light-induced red shift of the Qx absorption band of light-harvesting bacteriochlorophyll in Rhodopseudomonas capsulata and Rhodopseudomonas sphaeroides

In chromatophores from Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata, the Q_x band(s) of the light-harvesting bacteriochlorophyll (BChl) (λ_max ~590 nm) shifts to the red in response to a light-induced membrane potential, as indicated by the characteristics of the light-minus-dark difference spectrum. In green strains, containing light-harvesting complexes I and II, and one or more of neurosporene, methoxyneurosporene, and hydroxyneurosporene as carotenoids, the absorption changes due to the BChl and carotenoid responses to membrane potential in the spectral region 540–610 nm are comparable in magnitude and overlap with cytochrome and reaction center absorption changes in coupled chromatophores. In strains lacking carotenoid and light-harvesting complex II, the BChl shift absorption change is relatively smaller, due in part to the lower BChl/reaction center ratio.In the carotenoid-containing strains, the peak-to-trough absorption change in the BChl difference spectrum is 5–8% of the peak-to-trough change due to the shift of the longest-wavelength carotenoid band, although the absorption of the BChl band is 25–40% of that of the carotenoid band. The responding BChl band(s) does not appear to be significantly red-shifted in the dark in comparison to the total BChl Q_x band absorption.     

A study of the content of pigments in the light-harvesting antenna of the green bacterium from the new family Oscillochloridaceae

The properties of the light-harvesting superantenna of the photosynthesizing bacteria from the new family of green filamentous bacteria Oscillochloridaceae were investigated by optical spectroscopy. The antenna of Oscillochloris trichoides consists of peripheral chlorosomal and membrane subantennas. A method of isolation of Osc. trichoides chlorosomal antenna was developed using the chaothropic agent sodium thiocyanate, which simultaneously acts to stabilize chlorosomal activity. An analysis of the second derivatives of the absorption spectra of isolated chlorosomes and their acetone-methanol extracts suggested that BChl c was a predominant light-harvesting pigment in Osc. trichoides chlorosomes. Besides, it was found that, in addition to the BChl c-antenna, chlorosomes contain a minor BChl a-antenna. It was shown that the membrane BChl a-subantenna is a light-harvesting complex with absorption maxima in the near infrared region at 805 and 860 nm. Analysis of the spectral data obtained suggested that the Osc. trichoides chlorosomal antenna resembles those from Chlorobiaceae species, whereas the membrane B805-860 BChl a antenna of Osc. trichoides is close to the membrane B808-866 BChl a antenna of Chloroflexaceae species.