Reaction centers photochemistry

The Green Bacteria. II. The Reaction Center Photochemistry and Electron Transport... 

The remarkable efficiency of reaction-center photochemistry has encouraged the design and the study of synthetic models. Most research on artificial photosynthesis has been directed toward mimicry of the natural reaction center (RC). The center functions as a molecular-scale solar photovoltaic device that converts light energy into chemical energy that can be transported and stored for maintenance, growth, and... 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

It has been suggested that P BChl (where BChl is one of the two monomeric or "accessory BChls that are not part of P) is a transient state prior to P "I (14,16,19), although the evidence supporting this view has been criticized (23, 24) Recent subpicosecond studies find no evidence for P "BChl (8,9) These new results do not preclude some involvement of a monomeric BChl in the early photochemistry, only that P BChl apparently is not a kinetically resolved transient state Perhaps P itself contains some charge-transfer character between its component BChls, or between P and one or both of the monomeric BChls (8,9,25-27) One of the two monomeric BChls apparently can be removed by treatment of the reaction center with sodium borohydride (28) and subsequent chromatography, with no impairment of the primary electron transfer reactions (29) Thus, at present it appears that P I is the first resolved radical-pair state, and it forms with a time constant of about 4 ps in Rps sphaeroides ... 

Recent rapid developments in ultrashort pulse laser [1-5] make it possible to probe not only the dynamics of population of the system but also the coherence (or phase) of the system. To treat these problems, the density matrix method is an ideal approach. The main purpose of this paper is to briefly describe the application of the density matrix method in molecular terms and show how to apply it to study the photochemistry and photophysics [6-9]. Ultrafast radiationless transactions taking place in bacterial photosynthetic reaction centers (RCs) are very important examples to which the proposed theoretical approach can be applied. 

In these studies it appeared that low-temperature photoreduction of the bound iron-sulfur centers is not greatly decreased (about 2x) by the removal of phylloquinone (Setif et al, 1987). Low temperature photochemistry was measured by three methods the total amount of iron-sulfur centers A and B reduced by continuous illumination at 77K, the extent of reduction of these centers by saturating laser flashes at 77K, and the flash-induced formation of triplet P-700. These methods show that all the reaction centers are still able to oxidize P-700 and to reduce the iron-sulfur centers. This observation raises serious questions concerning the role of phylloquinone or the significance of low-temperature photochemistry. For the moment we consider that room temperature data are more reliable in indicating that A] is a phylloquinone. 

The key step of the process is the water splitting under absorption of light quanta of relative low energy. Here we will focus mainly on the latter process which appears to be one of the most enigmatic reactions in chemistry and photochemistry and will only briefly consider the light energy conversion reaction centers of PS I and PS II. 

We shall now turn our attention to the specific molecules that act as electron acceptors or donors in chloroplasts. A summary of the characteristics of the most common components of this complex pathway is presented in Table 5-3. Figure 6-4 should also be consulted, if the underlying concept of redox potential is already familiar. We will begin our discussion by considering the photochemistry at the reaction center of Photosystem II and then consider the various substances in the sequence in which they are involved in electron transfer along the pathway from Photosystem II to Photosystem I. We will conclude by considering the fate of the excited electron in Photosystem I. 

Water oxidation and the accompanying O2 evolution follow spontaneously after the photochemistry at the reaction center of Photosystem II has led to Pg80. Thus the required oxidant for water, Pg80 or some intermediate oxidized by it, must have a redox potential more positive than 0.82 V for the electron to move energetically downhill from water to the trap chl+ in the reaction center of Photosystem II. As indicated in Table 5-3, the redox potential of the Peso-Peso couple is about 1.10 V. 

Excitation transfer must be fast enough to deliver excitations to the photochemical reaction center and have them trapped in a time short compared to the excited-state lifetime in the absence of trapping. Excited-state lifetimes of isolated antenna complexes, where the reaction centers have been removed, are typically in the 1 -5 ns range. Observed excited-state lifetimes of systems where antennas are connected to reaction centers are generally on the order of a few tens of picoseconds, which is sufficiently fast to ensure that under physiological conditions that almost all the energy is trapped by photochemistry. 

The photochemistry of the reaction center takes place one electron at a time. However, one of the products of the electron transfer process is a reduced ubiquinone, which has taken up two electrons as well as two protons. To form this species, the reaction center must turn over twice, with electrons entering the complex by donation of cytochrome ci with the oxidized special pair. The electrons accumulate in the quinone acceptors and protons are taken up from the surrounding medium. Finally, a fidly reduced ubiquinol is formed, which is released from the complex into the hydrocarbon portion of the membrane. The quinol is subsequently reoxidized at the cytochrome bc complex (described below). 

Related Reactions. The photochemistry (Patemo-Biichi reactions) of the achiral oxazoline (7) has been studied. The analogous urethane (8), which is chiral by attachment of an apocam-phanoyl group, shows an intriguing stereoselectivity pattern in its reaction with electrophiles. For another case of an oxidative decarboxylation as a key step in the application of the SRSC (selfregeneration of stereogenic centers) principle, see the preparation of the dihydropyrimidone (9) from aspartic acid. ... 

Fig. 2. (A) SDS-PAGE electrophoresis showing the polypeptide subunits of the reaction-center of Rb. sphaeroides R-26 (B) and (C) SDS-PAGE electrophoresis patterns of the LM-complex and the H-subunit obtained by treating the reaction center with SDS/ LDAO followed by centrifugation in a sucrose-density gradient. Figure from Okamura, Steiner and Feher (1974) Characterization of reaction centers from photosynthetic bacteria. I. Subunit structure of the protein mediating the primary photochemistry in Rhodopseudomonas sphaeroides R-26. Biochemistry, 13 137, 138.

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