Trapping, loss and annihilation of excitations in a photosynthetic system. I. Theoretical aspects

In this paper are discussed a few theoretical aspects of the transfer, trapping, loss and annihilation of excitations as they occur in a photosynthetic system after a picosecond light pulse. A random-walk model is introduced to describe the dynamical behavior of the excitations in a domain and is used to calculate the parameter that determines the shape of the total fluorescence yield vs. pulse intensity curve in the case in which the reaction centers are all in the closed state (Paillotin, G., Swenberg, C.E., Breton, J. and Geacintov, N.E. (1979) Biophys. J. 25, 513–533). It is shown that this parameter depends critically on the number, λ, of connected photosynthetic units in a domain. A master equation is postulated to describe the decay of the excitations in the case where the transition of the reaction center from the open to the closed state, induced by the capture of an excitation, is included. The trapping and loss of excitation in a mixture of open and closed reaction centers, generated in the course of the transfer process, is assumed to be described by an equation that is the equivalent for a single domain of the Vredenberg-Duysens relation (Vredenberg, W.J. and Duysens, L.N.M. (1963) Nature 197, 355–357). The master equation is used to find the total probability of loss per excitation, U_λ(z), and the total fraction of reaction centers closed, V_λ(z), as a function of the average number of excitations z created in a domain when the reaction centers are all in the open state before the pulse. It is shown that, for most photosynthetic systems, an increase of U_λ(z) with z can occur only if λ 3. It is further concluded that the combined measurement of U_λ(z) and V_λ(z) can give detailed information about λ and the parameters involved in the transfer process.     

Quinone and pheophytin in the photosynthetic reaction center II from spinach chloroplasts

Prenylquinones and pheophytin a in a preparation of photosynthetic reaction center II from spinach chloroplasts were chemically determined. Each reaction center II had two molecules, each of plastoquinone-9 and pheophytin a, but practically no phylloquinone, α-tocopherylquinone or α-tocopherol.     

High-resolution optical absorption-difference spectra of the triplet state of the primary donor in isolated reaction centers of the photosynthetic bacteria Rhodopseudomonas sphaeroides R-26 and Rhodopseudomonas viridis measured with optically detected magnetic resonance at 1.2 K

We have recorded triplet optical absorption-difference spectra of the reaction center triplet state of isolated reaction centers from Rhodopseudomonas sphaeroides R-26 and Rps. viridis with optical absorption-detected electron spin resonance in zero magnetic field (ADMR) at 1.2 K. This technique is one to two orders of magnitude more sensitive than conventional flash absorption spectroscopy, and consequently allows a much higher spectral resolution. Besides the relatively broad bleachings and appearances found previously (see, e.g., Shuvalov V.A. and Parson W.W. (1981) Biochim. Biophys. Acta 638, 50–59) we have found strong, sharp oscillations in the wavelength regions 790–830 nm (Rps. sphaeroides) and 810–890 nm (Rps. viridis). For Rps. viridis these features are resolved into two band shifts (a blue shift at about 830 nm and a red shift at about 855 nm) and a strong, narrow absorption band at 838 nm. For Rps. sphaeroides R-26 the features are resolved into a red shift at about 810 nm and a strong absorption band at 807 nm. We conclude that the appearance of the absorption bands at 807 and 838 nm, respectively, is due to monomeric bacteriochlorophyll. Apparently, the exciton interaction between the pigments constituting the primary donor is much weaker in the triplet state than in the singlet state, and at low temperature the triplet is localized on one of the bacteriochlorophylls on an optical time scale. The fact that for Rps. sphaeroides the strong band shift and the monomeric band found at 1.2 K are absent at 293 K and very weak at 77 K indicates that these features are strongly temperature dependent. It seems, therefore, premature to ascribe the temperature dependence between 293 and 77 K of the intensity of the triplet absorption-difference spectrum at 810 nm (solely) to a delocalization of the triplet state on one of the accessory bacteriochlorophyll pigments.     

Structural aspects of vectorial electron transfer in photosynthetic reaction centers

Structural aspects of photosynthetic reaction centers in bacteria and plants are discussed in relation with the ability of these structures to perform a photoinduced electron transfer from one side of the membrane to the other. A comparison is made with recently synthesized artificial models. Functional similarities between the acceptor sides of bacterial and of Photosystem-II centers are utilized to hypothesize on their structure.This review corresponds to a lecture delivered at the 3rd European Bioenergetics Conference, Hannover, September 1984.     

Stopped-flow kinetic study of the regeneration reaction of tocopheroxyl radical by reduced ubiquinone-10 in solution

Kinetic study of the reaction between tocopheroxyl (vitamin E radical) and reduced ubiquinone, n = 10) has been performed. The rates of reaction of ubiquinol with α-tocopheroxyl 1 and seven kinds of alkyl substituted tocopheroxyl radicals 2–8 in solution have been determined spectrophotometrically, using a stopped-flow technique. The result shows that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of tocopheroxyls increases. For the tocopheroxyls with two alkyl substituents at ortho positions (C-5 and C-7), the second-order rate constants, k₁, obtained vary i n the order of 10², and decrease predominantly, as the size of two ortho-alkyl groups (methyl, ethyl, isopropyl and tert-buty) in tocopheroxyl increases. On the other hand, the reaction between tocopheroxyl and ubiquinone-10 (oxidized ubiquinone) has not been observed. The result indicates that ubiquinol-10 regenerates tocopherol by donating a hydrogen atom of the 1-OH and/or 4-OH group to the tocopheroxyl radical. For instance, the k₁ values obtained for α-tocopheroxyl are 3.74 · 10⁵ M^?1 · s^?1 and 2.15 · ⁵ M^?1 · s^?1 in benzene and ethanol solution at 25°C, respectively. The above reaction rates, k₁, obtained were compared with those of vitamin C with α-tocopheroxyl reported by Packer et al. (k₂ = 1.55 · 10⁶ M^?1 · s^?1) and Scarpa et al. (K₂ = 2 · 10⁵ 10⁵ M^?1 · s^?1), which is well known as a usual regeneration reaction of tocopheroxyl in biomembrane systems. The result suggests that ubiquinol-10 also regenerates the tocopheroxyl to tocopherol and prevents lipid peroxidation in various tissues and mitochondria.     

Efficient photochemical activity and strong dichroism of single crystals of reaction centers from Rhodopseudomonas viridis

Crystallized reaction centers from Rhodopseudomonas viridis (i) are photochemically active with electron transfer from the special pair to the quinones, (ii) show dichroism giving valuable information on the orientation of the different chromophores and (iii) allow chemical treatment in the crystalline phase.     

Abdominal pigmentation and growth temperature in Indian Drosophila melanogaster: Evidence for genotype-environment interaction

Phenotypic variability for abdominal pigmentation in females of an Indian natural population ofDrosophila melanogaster was studied using isofemale lines and by rearing the larvae and pupae at 4 different temperatures ranging from 20–30°C. The dark pigmented area was found to increase in all the three segments when the growth temperature decreases. A significant positive correlation was detected for the occurrence of dark pigmentation in the 5th and 6th segments in each growth temperature but for other comparisons the correlation was not regular. Analysis of variance (ANOVA) was carried out both for individual segments over different growth temperatures and also for each temperature over the three abdominal segments and in all cases found to be statistically significant. The results are quite different from the earlier observation in FrenchDrosophila melanogaster and suggest that genes controlling pigmentation are temperature dependent; temperature could affect post-transitional events involved in pigmentation. The present findings also clearly indicate that significant genotype-environment interaction exists, responsible for the production of desired phenotype at the opportune moment during the life span of a species.     

The effect of ambient redox potential on the transient electron spin echo signals observed in chloroplasts and photosynthetic algae

Time-resolved electron spin echo (ESE) studies were carried out at room temperature on chloroplast preparations and whole cells of photosynthetic algae. The signals observed exhibit the unexpected special ESE signal which we have proposed to be the result of transient interactions between P⁺-700 and an early electron acceptor of Photosystem I (Thurnauer, M.C. and Norris, J.R. (1980) Chem. Phys. Lett. 76, 557–561). The intensity of the special ESE signal decreases with the chemical reduction of the Center A-Center B complex. The results suggest that in the untreated photosynthetic systems we are initially observing P⁺-700 as it interacts with the reduced acceptor which precedes the Center A-Center B complex. Then the decay of the special ESE signal (approx. 170 ns) gives the lifetime of this reduced acceptor as it participates in forward electron transport.     

Formation of band-type electric patterns in Characean cells

It is observed by experiments that band patterns of alternating acid and alkaline zones are formed on the surface of Characean cells under illumination. In order to understand theoretically such pattern formation, we employ a reaction-diffusion equation model with activator-inhibitor interaction. We study the existence problem of band patterns and hysteresis phenomena such as patterns appearing and disappearing with changing light intensity by using singular perturbation methods.