Absorption and ADMR studies on bacterial photosynthetic reaction centres with modified pigments (original) (raw)
Related papers
Photochemistry and Photobiology, 1996
The dynamics of triplet energy transfer between the primary donor and the carotenoid were measured on several photosynthetic bacterial reaction center preparations from Rhodobacter sphaeroides: (a) wild-type strain 2.4.1, (b) strain R-26.1, (c) strain R-26.1 exchanged with 132-hydroxy-[Znl-bacteriwhlorophyll at the accessory bacteriochlorophyll (BChl) sites and reconstituted with spheroidene and (a) strain R-26.1 exchanged with [3-vinyl]-19-hydroxybacteriochlorophyll at the accessory BChl sites and reconstituted with spheroidene. The rise and decay times of the primary donor and carotenoid triplet-triplet absorption signals were monitored in the visible wavelength region between 538 and 555 nm as a function of temperature from 4 to 300 K. For the samples containing carotenoids, all of the decay times correspond well to the previously observed times for spheroidene (5 f 2 (IS). The rise times of the m t e n o i d triplets were found in all cases to be biexponential and comprised of a strongly temperature-dependent component and a temperature-independent component. From a comparison of the behavior of the carotenoid-containing samples with that from the reaction center of the carotenoidless mutant Rb. sphaeroides R-26.1, the temperature-independent component has been assigned to the buildup of the primary donor triplet state resulting from charge recombination in the reaction center. Arrhenius plots of the buildup of the carotenoid triplet states were used to determine the activation energies for Mplet energy transfer from the primary donor to the carotenoid. A model for the process of triplet energy transfer that is consistent with the data suggests that the activation barrier is strongly dependent on the triplet state energy of the accessoly BChl pigment, BCM,.
Photosynthesis Research, 1993
Triplet state electron paramagnetic resonance (EPR) experiments have been carried out at X-band on Rb. sphaeroides R-26 reaction centers that have been reconstituted with the carotenoid, spheroidene, and exchanged with 132-OH-Zn-bacteriochlorophyll a and [3-vinyl]-132-OH-bacteriochlorophyll a at the monomeric, 'accessory' bacteriochlorophyll sites BA. B or with pheophytin a at the bacteriopheophytin sites HA B. The primary donor and carotenoid triplet state EPR signals in the temperature range 95 -150 K are coi~pared and contrasted with those from native Rb. sphaeroides wild type and Rb. sphaeroides R-26 reaction centers reconstituted with spheroidene. The temperature dependencies of the EPR signals are strikingly different for the various samples. The data prove that triplet energy transfer from the primary donor to the carotenoid is mediated by the monomeric, BChl B molecule. Furthermore, the data show that triplet energy transfer from the primary donor to the carotenoid is an activated process, the efficiency of which correlates with the estimated triplet state energies of the modified pigments.
Febs Letters, 1990
Monomeric bacteriochlorophylls BA and Ba in photosynthetic reaction centers from Rhodobacter sphueroides R26 were exchanged with (132-hydroxy-)bacteriochlorophylls containing a 3-vinyl-or 3-(a-hydroxyethyl)-substituent instead of the 3-acetyl group. The corresponding binding sites must be tolerant to the introduction of the polar residue at C-13* andmodifications of the 3-acetyl group. According to HPLC analysis, the exchange with both pigments amounts to S$ 50% of the total BChl contained in the complex, corresponding to 6 100% of the monomeric BChl aBA,a. The absorption spectra show significant changes in the Qx and Qv-region of the monomeric bacteriochlorophylls. By contrast, the absorption of the primary donor (P870) and reversible photobleaching is retained. The circular dichroism is also unchanged in the 870 nm region. The positive cd band located at around 800 nm in native reaction centers, shifts with the (blue-shifted) QY absorption(s) of BA and/or Ba, whereas the position of the negative one remains nearly unaffected. The data indicate that the latter is the upper excitonic band of the primary donor, and that there is little interaction of the monomeric BA/Bs with the primary donor.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1989
Reaction centers from the carotenoidless mutant Rb. sphaeroides R26 were treated with sodium borohydride which is known to remove one of the accessory monomeric bacteriochlorophylls (BB). Subsequently, the carotenoid, spheroidene, was incorporated into the modified reaction centers, it is demonstrated by optical absorption and circular dichroism experiments that spheroidene, reconstituted into the sodium borohydride-treated Rb. sphaeroides R26 reaction centers, is bound in a single site, in the same environment and with the same structure as spheroidene reconstituted into untreated (native) Rb. sphaeroides R26 reaction centers. Transient optical and electron spin resonance spectroscopic data indicate that unless the accessory B B is present, the primary donor-to-carotenoid triplet energy transfer reaction is inhibited. These observations provide direct evidence for the involvement of the accessory B n in the triplet energy transfer pathway.
Biochemistry, 1997
Resonance Raman spectroscopy was performed on peripheral light-harvesting proteins from Rhodobacter sphaeroides in which the residue Arg -10 has been modified by site-selected mutagenesis. We show that this residue is indeed involved (as proposed by X-ray crystallographic studies on the LH2 complex from Rhodopseudomonas acidophila), in an H-bond with the acetyl carbonyl of the 800 nmabsorbing BChl in these proteins (B800), and that the presence of such an H-bond induces a ca. 10 nm red shift of the lowest energy transition (Q y ) of this molecule. Moreover, other parameters involved in the fine tuning of the absorption of the B800 molecules may be determined from our experiments, and we propose that the local electromagnetic properties of the B800 binding site may induce an additional 10 nm red shift of this transition. These results constitute the first experimental evidence for the parameters able to modify in ViVo the absorption of "monomeric" BChl molecules, i.e. BChl not involved in strong excitonic interactions, and will be of great help for understanding the absorption properties of such pigments in other light-harvesting systems.
Photochemistry and Photobiology, 1993
,4-dihydrospheroidene a)1,d spheroidene, have been incorporated into the B850 light-harvesting complex of the carotenoidless mutant, photosynthetic bacterium, Rhodobacter sphaeroides R-26.1. The extent of 'If-electron conjugation in these molecules in,creases from 7 to 10 carbon-<:arbon double bonds. Carotenoid-to-bacteriochlorophyll singlet state energy transfer efliciencies were measured using steady-state fluorescence excitation spectroscopy to be 54 :t 2%, 66 :t 4%, 71 :t 611& and 56 :t 3% for the carotenoid series. These results are discussed with respect to the position of the energy levels and the magnitude of spectral overlap between the S, (2'AJ state emission from the isolated carotenoids and the bacteriochlorophyll absorption of the native complex. These studies provide a systematic approach to exploring the efl"ect of excited state energies, spectral overlap and excited state lifetimes on the efficiencies of carotenoid-tobacteriochlorophyll singlet energy transfer in photosynthetic systems.
Triplet energy transfer in bacterial photosynthetic reaction centres
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1998
3-vinyl]-13 2 -OH-bacteriochlorophyll a has been selectively exchanged against native bacteriochlorophyll a in the monomer binding sites at the A-and B-branch of the photosynthetic reaction centre from Rhodobacter sphaeroides. Transient absorption difference measurements were performed on these samples over a temperature range from 4.2 to 300 K with 20 ns time resolution. Specifically the decay of the primary donor triplet state, 3 P 870 , as well as the rise and decay rates of the carotenoid triplet state, 3 Car (spheroidene), were measured. The observed rates revealed a thermally activated carotenoid triplet formation corresponding to the decay of the primary donor triplet state. The activation energies for the triplet energy transfer process were 100( þ 10) cm 31 for reaction centers from wild-type Rhodobacter sphaeroides 2.4.1, with and without exchange of the monomeric bacteriochlorophyll on the electron transfer-active branch, B A . For reaction centers from Rhodobacter sphaeroides R26.1 with both monomers exchanged against [3-vinyl]-13 2 -OH-bacteriochlorophyll a, and subsequent spheroidene reconstitution the activation energy was 460( þ 20) cm 31 . These activation energies correspond to the energy difference between the triplet states of the accessory BChl monomer, B B , and the primary donor when native BChl a or [3-vinyl]-13 2 -OH-BChl a is present in the B B binding site. In all samples the 3 Car formation rates were bi-phasic over a large temperature range. A fast temperature-independent rate was observed on the wavelength of the carotenoid triplett riplet absorption which dominated at very low temperatures. Additionally, a slower temperature-independent 3 Car formation rate was observed at low temperatures which could be explained with the assumption of heterogeneity in the energy barrier ( 3 B B ) and/or the primary donor triplet state ( 3 P 870 ). A tunneling mechanism as proposed earlier by Kolaczkowski (PhD thesis, Brown University, 1989) is not only unnecessary but also incompatible with the available experimental data. ß
The Journal of Physical Chemistry B, 1997
Energy transfer within native and borohydride treated (chemically modified) reaction centers (RCs) isolated from Rhodobacter sphaeroides R26 was investigated, in particular from the excited accessory bacteriochlorophylls (B*) to the primary donor (P). The decay kinetics of chemically modified and native RCs show some similarities as well as distinct differences. In reduced RCs, B* decays with a time constant of 100 fs, both in native and in modified RCs, followed by a partial recovery of the bleaching with a time constant of about 3 ps due to charge separation. In native RCs, however, an induced absorption is observed with a maximum at a delay of 500 fs, which is absent in chemically modified RCs. The initial bleaching in modified and in native RCs is characterized by an anisotropy of 0.4. After the excitation is transferred from B* to P, the anisotropy in modified RCs is decreased to a value of about 0.2. In native RCs the time-resolved anisotropy varies and depends strongly on the wavelength of detection. These observations are analyzed and discussed in terms of the individual contributions of B A and B B to the absorbance kinetics at different wavelengths. In oxidized RCs, in addition to a fast relaxation of about 100 fs in the decay of B*, we observed, a decay component with a time constant of ∼400 fs.
Photochemistry and Photobiology, 1994
The effect of chemical modifications in the side groups of the isocyclic ring V on the formation, optical absorption and circular dichroism of bacteriochlorophyll (Ekhl) dimers was examined in a mixture of formamide and water containing Triton X-100 and variable amounts of pyridine. Substitution of the carbomethoxy group in the C132 position with a hydrogen atom, had no effect on the dimerization constant but increased the shift of the Qy transition by 1000 cm-I with respect to the native Bchl. Substitution of the C13 hydrogen atom with OH decreased the shift of the Q, transition by 400 cm-'. The similarity between the spectra of the modified Bchl dimers and Bchl dimers in vivo indicates that protein binding to the side groups at Bchl dimers may profoundly affect the energy of their Q, transition but have minor effects on the Q, transitions of the monomeric Bchl.