Quenching of the Spheroiden Triplet State with Oxygen in the Reaction Centers of C. sphaeroides in the Temperature Range 0–45°C

Picosecond kinetics of light excitations in photosynthetic purple bacteria in the temperature range of 300-4 K

Triplet energy transfer in bacterial photosynthetic reaction centres

The efficiency of quenching with dissolved oxygen of the triplet state of the carotenoid spheroiden ([T]Car) in the photosynthetic reaction center of purple bacteria Cerebacter (Rhodobacter) sphaeroides in the temperature range from 0°C to +45°C was investigated. For the possibility of [T]Car formation during light excitation of the reaction center, o-phenanthroline (10[−][2] M) was added to the preparations, partially displacing the primary quinone acceptor from the structure of reaction center. The activation energies of the process of [T]Car quenching under normal aerobic conditions and with partial degassing of the studied samples were determined.

The titanium(III) metallocene derivatives [1,3-(Me3C)2C5H3]2TiX, Cp′2TiX, X = Cl, Me, H, OH, are prepared and shown to be monomeric with a d1 electron configuration by solid state magnetic susceptibility studies over the temperature range 5–300 K. Reduction of the chloride by potassium amalgam in an argon atmosphere yields the base-free titanocene Cp′2Ti, which is a spin triplet over the temperature range 5–300 K. In contrast, reduction in a nitrogen atmosphere gives the dimetal metallocene Cp′2Ti(N2)TiCp′2, which shows a singlet–triplet equilibrium over the temperature range 5–300 K, which can be modeled by Heisenberg coupling with −2J = 210 cm−1. The base-free titanocene reacts reversibly with H2 or C2H4 to give Cp′2TiH2 and Cp′2Ti(C2H4), respectively. Both adducts are characterized by X-ray crystallography. The base-free titanocene reacts irreversibly with PhC≡CPh or N2O to give Cp′2Ti(PhC≡CPh) and Cp′4Ti2(μ-O), which are characterized by X-ray crystallography. In contrast Cp′2Ti(C2H4) reacts with N2O to give the bisoxo-bridged dimetal derivative Cp′4Ti2(μ-O)2.

In the cycle of photosynthetic reaction centers, the initially oxidized special pair of bacteriochlorophyll molecules is subsequently reduced by an electron transferred over a chain of four hemes of the complex. Here, we examine the kinetics of electron transfer between the proximal heme c-559 of the chain and the oxidized special pair in the reaction center from Rps. sulfoviridis in the range of temperatures from 294 to 40 K. The experimental data were obtained for three redox states of the reaction center, in which one, two, or three nearest hemes of the chain are reduced prior to special pair oxidation. The experimental kinetic data are analyzed in terms of a Sumi-Marcus-type model developed in our previous paper,1 in which similar measurements were reported on the reaction centers from Rps. viridis. The model allows us to establish a connection between the observed nonexponential electron-transfer kinetics and the local structural relaxation dynamics of the reaction center protein on the microsecond time scale. The activation energy for relaxation dynamics of the protein medium has been found to be around 0.1 eV for all three redox states, which is in contrast to a value around 0.4-0.6 eV in Rps. viridis.1 The possible nature of the difference between the reaction centers from Rps. viridis and Rps. sulfoviridis, which are believed to be very similar, is discussed. The role of the protein glass transition at low temperatures and that of internal water molecules in the process are analyzed.

The EPR method has been used to investigate thermal depolymerization processes of the tetragonal (T) and rhombohedral (R) 2D-polymerized phases of C60 in vacuum and in air. The behavior of paramagnetic centers (PCs) with doublet (S = 1/2) and triplet (S = 1) states during thermal depolymerization of the T and R phases have been analyzed and compared with the behavior of PCs of the monomer C60 phase at temperatures up to 650 K. A complete destruction of the PCs with triplet state on heating in the T and R phases has been observed. The EPR study of the monomer C60 phase shows the appearance of new PCs with triplet state in the 580–650 K temperature range.

Time-resolved spectroscopic studies of recombination of the P(+)HA(-) radical pair in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides give an opportunity to study protein dynamics triggered by light and occurring over the lifetime of P(+)HA(-). The state P(+)HA(-) is formed after the ultrafast light-induced electron transfer from the primary donor pair of bacteriochlorophylls (P) to the acceptor bacteriopheophytin (HA). In order to increase the lifetime of this state, and thus increase the temporal window for the examination of protein dynamics, it is possible to block forward electron transfer from HA(-) to the secondary electron acceptor QA. In this contribution, the dynamics of P(+)HA(-) recombination were compared at a range of temperatures from 77 K to room temperature, electron transfer from HA(-) to QA being blocked either by prereduction of QA or by genetic removal of QA. The observed P(+)HA(-) charge recombination was significantly slower in the QA-deficient RCs, and in both types of complexes, lowering the temperature from RT to 77 K led to a slowing of charge recombination. The effects are explained in the frame of a model in which charge recombination occurs via competing pathways, one of which is thermally activated and includes transient formation of a higher-energy state, P(+)BA(-). An internal electrostatic field supplied by the negative charge on QA increases the free energy levels of the state P(+)HA(-), thus decreasing its energetic distance to the state P(+)BA(-). In addition, the dielectric response of the protein environment to the appearance of the state P(+)HA(-) is accelerated from ∼50-100 ns in the QA-deficient mutant RCs to ∼1-16 ns in WT RCs with a negatively charged QA(-). In both cases, the temperature dependence of the protein dynamics is weak.

π-Pimer, π-Dimer, π-Trimer, and 1D π-Stacks in a Series of Benzene Triimide Radical Anions: Substituent-Modulated π Interactions and Physical Properties in Crystalline State

Transient absorption difference spectroscopy was used to study the temperature dependence of the P798+ decay kinetics in heliobacteria. For membrane samples, two components were obtained from the fitting of kinetic traces in the temperature range of 4-29 degrees C. A 3-9 ms component representing the cytochrome (cyt) c oxidation has an activation energy of 33.0 +/- 2.8 kJ/mol. A 12-22 ms component representing either P798+FX- or P798+FA/B- recombination has an activation energy of 15.3 +/- 2.4 kJ/mol. In isolated reaction centers (RC), only one 14 ms component due to P798+FX- recombination was obtained in this temperature range. The Arrhenius plot shows that the recombination rate of this P798+FX- state is temperature independent in the near room temperature range. For RC in the temperature range of 60-298 K, a 12-15 ms decay was obtained at temperatures greater than 240 K. Biphasic decay traces (12-15 ms and 2-4 ms components) were obtained at temperatures between 170 K and 230 K. Only one 2-4 ms component was found at temperatures lower than 160 K. The gradual switchover from the 12-15 ms to the 2-4 ms component upon cooling may indicate the shift of the P798+FX- recombination state to a state that is prior to P798+FX-, although other interpretations can not be excluded. The absorption difference spectrum (delta A @ 160 K - delta A @ 240 K) in the blue region shows a positive amplitude below 405 nm and a negative amplitude above 405 nm implying that the 2-4 ms decay component may be due to the recombination of P798+A1-, where A1 is a quinone-type acceptor.

The process of electron transfer from the special pair, P, to the primary electron donor, H(A), in quinone-depleted reaction centers (RCs) of Chloroflexus (Cf.) aurantiacus has been investigated over the temperature range from 10 to 295 K using time-resolved pump-probe spectroscopic techniques. The kinetics of the electron transfer reaction, P* → P(+)H(A)(-), was found to be nonexponential, and the degree of nonexponentiality increased strongly as temperature decreased. The temperature-dependent behavior of electron transfer in Cf. aurantiacus RCs was compared with that of the purple bacterium Rhodobacter (Rb.) sphaeroides . Distinct transitions were found in the temperature-dependent kinetics of both Cf. aurantiacus and Rb. sphaeroides RCs, at around 220 and 160 K, respectively. Structural differences between these two RCs, which may be associated with those differences, are discussed. It is suggested that weaker protein-cofactor hydrogen bonding, stronger electrostatic interactions at the protein surface, and larger solvent interactions likely contribute to the higher transition temperature in Cf. aurantiacus RCs temperature-dependent kinetics compared with that of Rb. sphaeroides RCs. The reaction-diffusion model provides an accurate description for the room-temperature electron transfer kinetics in Cf. aurantiacus RCs with no free parameters, using coupling and reorganization energy values previously determined for Rb. sphaeroides , along with an experimental measure of protein conformational diffusion dynamics and an experimental literature value of the free energy gap between P* and P(+)H(A)(-).

Rates of thermoinduced conformational transitions of reaction center (RC) complexes providing effective electron transport were studied in chromatophores and isolated RC preparations of various photosynthesizing purple bacteria using methods of fast freezing and laser-induced temperature jump. Reactions of electron transfer from the primary to secondary quinone acceptors and from the multiheme cytochrome c subunit to photoactive bacteriochlorophyll dimer were used as probes of electron transport efficiency. The thermoinduced transition of the acceptor complex to the conformational state facilitating electron transfer to the secondary quinone acceptor was studied. It was shown that neither the characteristic time of the thermoinduced transition within the temperature range 233-253 K nor the characteristic time of spontaneous decay of this state at 253 K exceeded several tens of milliseconds. In contrast to the quinone complex, the thermoinduced transition of the macromolecular RC complex to the state providing effective electron transport from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer within the temperature range 220-280 K accounts for tens of seconds. This transition is thought to be mediated by large-scale conformational dynamics of the macromolecular RC complex.

Fluorescence and phosphorescence emission and nonradiative relaxation of acridine in a crystalline matrix of 2,3-dimethylnaphthalene

Comparison of atmospheric reactions of NH3 and NH2 with hydroxyl radical on the singlet, doublet and triplet potential energy surfaces, kinetic and mechanistic study

Recently we have introduced magneto-optical difference spectroscopy (MODS) to measure triplet-minus-singlet absorbance difference (T – S) spectra of bacterial photosynthetic reaction centers (RC) over a wide range of temperatures (Hoff et al., 1985; Lous and Hoff, 1986). The MODS technique rests upon the change in yield of the triplet state of the primary donor, 3P, effected by a magnetic field of small amplitude (a few tens of millitesla). The field is modulated at a few hundred hertz and the resulting modulation in absorbance lock-in detected over a wide range of wavelengths. Since the magnetic field BO is a vectorial quantity and the magnetic field effect (MFE) sensitive to the orientation of RC with respect to \( {\vec B_O} \) (see below), one expects that it should be possible to perform a linear dichroic (LD)-MODS experiment, which would result in a LD-(T – S) spectrum. Knowledge of the orientational dependence of the MFE should then allow to extract information on e.g. the magnitude and direction of the dipolar interaction of the primary radical pair (RP) P+I−, where I is the bacteriopheophytin acceptor. The dipolar interaction between P+ and I− plays a significant role in the interpretation of reaction yield detected magnetic resonance (RYDMR) and MFE spectra (Lersch and Michel-Beyerle, 1982; Tang and Norris, 1983; Moehl et al., 1985; Hunter et al., 1987). An independent determination of the dipolar interaction would help in obtaining a reliable value of the isotropic exchange interaction J(P+I−), which is of great interest for understanding photoinduced electron transport (Marcus, 1987; Bixon, 1987).

The electronic structure of Fe2+ in reaction centers from Rhodopseudomonas sphaeroides. I. Static magnetization measurements