Ultrafast internal conversion in 132-OHNi-bacteriochlorophyll in reaction centers of Rhodobacter sphaeroides R26 (original) (raw)
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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.
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.
Bacteriochlorophyll a with Ni 2+ replacing the central Mg 2+ ion was used as an ultrafast excitation energy dissipation center in reconstituted bacterial LH1 complexes. B870, a carotenoid-less LH1 complex, and B880, an LH1 complex containing spheroidene, were obtained via reconstitution from the subunits isolated from chromatophores of Rhodospirillum rubrum. Ni-substituted bacteriochlorophyll a added to the reconstitution mixture partially substituted the native pigment in both forms of LH1. The excited-state dynamics of the reconstituted LH1 complexes were probed by femtosecond pump−probe transient absorption spectroscopy in the visible and near-infrared spectral region. Spheroidene-binding B880 containing no excitation dissipation centers displayed complex dynamics in the time range of 0.1−10 ps, reflecting internal conversion and intersystem crossing in the carotenoid, exciton relaxation in BChl complement, and energy transfer from carotenoid to the latter. In B870, some aggregation-induced excitation energy quenching was present. The binding of Ni-BChl a to both B870 and B880 resulted in strong quenching of the excited states with main deexcitation lifetime of ca. 2 ps. The LH1 excited-state lifetime could be modeled with an intrinsic decay time constant in Ni-substituted bacteriochlorophyll a of 160 fs. The presence of carotenoid in LH1 did not influence the kinetics of energy trapping by Ni-BChl unless the carotenoid was directly excited, in which case the kinetics was limited by a slower carotenoid S 1 to bacteriochlorophyll energy transfer.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1985
Bacteriochiorophyll (BChl) luminescence lifetimes (~) were measured in purple bacteria Rhodospirillum rubrum and Rhodopseudomonas sphaeroides at low-excitation pulse energy with the use of a picosecond luminescence spectrochronograph of high sensitivity and high time-resolution. Average high-frequency excitation light density was changed from about 1 • 10 t3 photons • cm -2 • s -I up to 1 • 1017 photons ° cm -2 • s-i. Maximal energy density in a single pulse was in the range 10-14-10-10 J/cm 2, which completely rules out nonlinear exciton interactions. In this range ~ increased as a function of excitation light density from about 60 ps to 210 ps. Luminescence yield (~b) for the bacteria investigated measured under continuous or picosecond excitation changed in a similar manner as ~. The luminescence increase was shown to accompany the conversion of the reaction centers to the closed, photooxidized state. Luminescence decay of R. rubrum and Rps. sphaeroides chromatophores without any chemical additions was well approximated by a single exponential component both at low and at saturating intensities of exciting light. The time necessary for the primary charge separation to occur was shown to be 60 + 10 ps. The pairwise jump-time of excitation-energy transfer, as well as excitation-diffusion characteristics were estimated from these data. On the basis of life-time measurements in the state of active photosynthesis, the quantum yield of the primary charge separation in the reaction centers was estimated to be equal to 0.95 _+ 0.02. In intact cells as well as in chromatophores in the presence of reducing agents, a nanosecond component of emission decay was also observed. The relative amplitude of this component, being several percent of the picosecond one at low-excitation intensity levels, increased (2-3)-times with excitation density. Its life-time was estimated to be 3 _+ 1 ns. The nanosecond component appeared only under conditions when a part of the reaction centers were converted to the closed state PQ-.
Proceedings of the National Academy of Sciences, 1999
A pathway of electron transfer is described that operates in the wild-type reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroides . The pathway does not involve the excited state of the special pair dimer of bacteriochlorophylls (P*), but instead is driven by the excited state of the monomeric bacteriochlorophyll (B A *) present in the active branch of pigments along which electron transfer occurs. Pump-probe experiments were performed at 77 K on membrane-bound RCs by using different excitation wavelengths, to investigate the formation of the charge separated state P + H A − . In experiments in which P or B A was selectively excited at 880 nm or 796 nm, respectively, the formation of P + H A − was associated with similar time constants of 1.5 ps and 1.7 ps. However, the spectral changes associated with the two time constants are very different. Global analysis of the transient spectra shows that a mixture of P + B A − and P* is formed in parallel from B A * on ...
Biochemistry, 2001
Replacement of the central Mg in chlorophylls by Ni opens an ultrafast (tens of femtoseconds time range) radiationless de-excitation path, while the principal ground-state absorption and coordination properties of the pigment are retained. A method has been developed for substituting the native bacteriochlorophyll a by Ni-bacteriochlorophyll a ([Ni]-BChl) in the light harvesting antenna of the core complex (LH1) from the purple bacterium, Rhodobacter (Rb.) sphaeroides, to investigate its unit size and excited state properties. The components of the complex have been extracted with an organic solvent from freeze-dried membranes of an LH1-only strain of Rb. sphaeroides and transferred into the micelles of n-octyl--glucopyranoside (OG). Reconstitution was achieved by solubilization in 3.4% OG, followed by dilution, yielding a complex nearly identical to the native one, in terms of absorption, fluorescence, and circular dichroism spectra as well as energy transfer efficiency from carotenoid to bacteriochlorophyll. By adding increasing amounts of [Ni]-BChl to the reconstitution mixture, a series of LH1 complexes was obtained that contain increasing levels of this efficient excitation trap. In contrast to the nearly unchanged absorption, the presence of [Ni]-BChl in LH1 markedly affects the emission properties. Incorporation of only 3.2 and 20% [Ni]-BChl reduces the emission by 50% and nearly 100%, respectively. The subnanosecond fluorescence kinetics of the complexes were monoexponential, with the lifetime identical to that of the native complex, and its amplitude decreasing in parallel with the steady-state fluorescence yield. Quantitative analysis of the data, based on a Poisson distribution of the modified pigment in the reconstituted complex, suggests that the presence of a single excitation trap per LH1 unit suffices for efficient emission quenching and that this unit contains 20 ( 1 BChl molecules.
Journal of Luminescence, 1997
A series of transmetalated bacteriochlorophylls, where the central magnesium has been replaced by Pd, Ni, Zn, Cd, Cu, have been investigated by linear and non-linear laser spectroscopic methods. A strong dependence of the fluorescence on the central metal was obtained. Differences in fluorescence lifetimes and quantum yields are caused mainly by different efficiencies of intersystem crossing. Strong excited state absorptions were found, in particular, the Pd-compound has a strong transient absorption in the 550-670 nm range. The results are of interest with respect to a possible oxygen-free sensitizing action of Pd-BPhe a for photodynamic tumor therapy.
Ultrafast Photophysics and Photochemistry of [Ni]-Bacteriochlorophyll a
The Journal of Physical Chemistry B, 1999
The ultrafast photodynamics of bacteriochlorophyll with the central Mg 2+-ion replaced by Ni 2+ ([Ni]-BChl) in toluene and pyridine have been studied by femtosecond time-resolved fluorescence and absorption spectroscopy with a time resolution of 100 fs in the spectral range of 470-900 nm. Excitation of tetracoordinated [Ni]-BChl in toluene in its red-most absorption band leads to four transient states which decay single-exponentially with lifetimes of 100 fs, 450 fs, 4 ps, 25 ps. Except for the 4 ps component these kinetics arise from an internal conversion cascade conserving the overall multiplicity of the singlet configuration. The spectral characteristics of the intermediates indicate a deactivation process via several πand metalcentered excited states. The 4 ps kinetic results from a relaxation within the metal d-states caused by a change of size of the central nickel ion upon electronic excitation. A different deactivation pattern is observed for [Ni]-BChl in pyridine, where the central metal is additionally coordinated by two axial solvent molecules. On the basis of their dynamics and their spectral positions, three different intermediates have been identified. The fastest decay with 150 fs reflects an internal conversion process. This is followed by a process that can be approximately described by time constants of 6 and 90 ps. This decay pattern, concomitant with the spectral changes, is attributed to the ejection of the axial solvent ligands accompanied by intersystem crossing. The slowest component has a characteristic time of >2 ns and is attributed to the rebinding of the pyridine ligands.
Photosynthesis Research, 2015
Submillisecond dark-light changes of the yield (induction) and anisotropy of fluorescence under laser diode excitation were measured in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides. Narrow band (1-2 nm) laser diodes emitting at 808 and 865 nm were used to selectively excite the accessory bacteriochlorophyll (B, 800 nm) or the upper excitonic state of the bacteriochlorophyll dimer (P-, 810 nm) and the lower excitonic state of the dimer (P+, 865 nm), respectively. The fluorescence spectrum of the wild type showed two bands centered at 850 nm (B) and 910 nm (P-). While the monotonous decay of the fluorescence yield at 910 nm tracked the light-induced oxidation of the dimer, the kinetics of the fluorescence yield at 850 nm showed an initial rise before a decrease. The anisotropy of the fluorescence excited at 865 nm (P-) was very close to the limiting value (0.4) across the whole spectral range. The excitation of both B and Pat 808 nm resulted in wavelength-dependent depolarization of the fluorescence from 0.35 to 0.24 in the wild type and from 0.30 to 0.24 in the reaction center of triple mutant (L131LH-M160LH-M197FH). The additivity law of the anisotropies of the fluorescence species accounts for the wavelength dependence of the anisotropy. The measured fluorescence yields and anisotropies are interpreted in terms of very fast energy transfer from 1B* to 1P-(either directly or indirectly by internal conversion from 1P+) and to the oxidized dimer.