Reaction center evolution

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

Some authors have described the time evolution of the system by more general methods than time-dependent perturbation theory. For example, War-shel and co-workers have attempted to calculate the evolution of the function /(r, Q, t) defined by Eq. (3) by a semi-classical method [44, 96] the probability for the system to occupy state v]/, is obtained by considering the fluctuations of the energy gap between and 11, which are induced by the trajectories of all the atoms of the system. These trajectories are generated through molecular dynamics models based on classical equations of motion. This method was in particular applied to simulate the kinetics of the primary electron transfer process in the bacterial reaction center [97]. Mikkelsen and Ratner have recently proposed a very different approach to the electron transfer problem, in which the time evolution of the system is described by a time-dependent statistical density operator [98, 99]. 

Experimental determination of Ay for a reaction requires the rate constant k to be determined at different pressures, k is obtained as a fit parameter by the reproduction of the experimental kinetic data with a suitable model. The data are the concentration of the reactants or of the products, or any other coordinate representing their concentration, as a function of time. The choice of a kinetic model for a solid-state chemical reaction is not trivial because many steps, having comparable rates, may be involved in making the kinetic law the superposition of the kinetics of all the different, and often unknown, processes. The evolution of the reaction should be analyzed considering all the fundamental aspects of condensed phase reactions and, in particular, beside the strictly chemical transformations, also the diffusion (transport of matter to and from the reaction center) and the nucleation processes. 

Evolution Requires the Accumulation of Four Oxidizing Equivalents in the Reaction Center of Photosystem II... 

The amount of 02 released when a suspension of algae (Chlorella pyrenoidosa) was excited with a train of short flashes of light, as a function of (a) the time between the flashes and (b) the intensity of the flashes. Each flash lasted about 10 /as. The total amount of 02 released was measured after several thousand flashes and was divided by the number of flashes and by the amount of chlorophyll in the suspension. (What happens on the first few flashes is discussed later.) The measurements in (a) were made with flashes comparable to the strongest flashes used in (b). For the measurements shown in (b), the flashes were spaced 20 ms apart. Note that the maximum amount of 02 released per flash was only one molecule of 02 per several thousand molecules of chlorophyll. With saturating flashes, the amount of 02 evolution is limited by the concentration of reaction centers, whereas most of the chlorophyll in the algae is part of the antenna system. 

If the photosystem II reaction center transfers only one electron at a time, how does it assemble the four oxidizing equivalents needed for oxidation of H20 to 02 One possibility is that several different photosystem II reaction centers cooperate, but this seems not to happen. Instead, each reaction center progresses independently through a series of oxidation states, advancing to the next state each time it absorbs a photon. In this event 02 evolution would occur only when a reaction center has accumulated four oxidizing equivalents. Support for this conclusion comes from measurements made by Pierre Joliot of the amount of 02 evolved on each flash when chloroplasts are excited with a series of short flashes after a period of darkness. No 02 is released on the first or second flashes (fig. 15.21), but on the third flash, there is a burst of 02. After this, the amount of 02 released on each flash oscillates, going through a maximum every fourth flash. 

For each photon absorbed by any of the accessory pigments or Chi s whose excitations are funneled into a reaction center, one electron can be removed from its trap chi. Because four electrons are involved per 02 derived from water, the evolution of this molecule of 02 requires the absorption of four photons by Photosystem II or the light-harvesting antennae feeding into it (see Eq. 5.8 and Fig. 5-15). An additional four photons whose excitations arrive at the trap chi of Photosystem I are required for the reduction of the two molecules of NADP+ to NADPH necessary for the subsequent reduction of one C02 molecule (Eq. 5.9 Figs. 5-1 and 5-15). Hence eight photons are needed for the evolution of one molecule of 02 and the fixation of one molecule of C02. (In Chapter 6, Section 6.3D, we will consider how many photons are used to provide the ATP s required per C02 fixed.) The series representation (Fig. 5-15) proposed by Hill and Fay... 

The chlorophyll-protein complexes are oriented in the lamellar membranes in such a way that the electron transfer steps at the reaction centers lead to an outward movement of electrons. For instance, the electron donated by Photosystem II moves from the lumen side to the stromal side of a thylakoid (see Figs. 1-10 and 5-19). The electron that is donated back to the trap chi (Pgg0) comes from H20, leading to the evolution of 02 by Photosystem II (Eq. 5.8). The 02 and the H+ from this reaction are released inside the thylakoid (Fig. 5-19). Because 02 is a small neutral molecule, it readily diffuses out across the lamellar membranes into the chloroplast stroma. However, the proton (H+) carries a charge and hence has a low partition coefficient (Chapter 1, Section 1.4A) for the membrane, so it does not readily move out of the thylakoid lumen. 

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. 

Woodbury, N. W., Lin, S., LIN, X. M., Peloquin, J. M., Taguchi, A. M. W., Williams, J. C., and Allen, J. P, 1995, The role of reaction-center excited-state evolution during charge separation in a Rhodobacter sphaeroides mutant with an initial electron-donor midpoint potential 260mV above wild-type. Chem. Phys., 197 4059421. 

Typical values of Ro between chlorophylls and bacteri-ochlorophylls are 60-100 A (assuming = 1). Excitation transfer is therefore a longer range process than is electron transfer (see below). In fact, the structures of anteima complexes have undoubtedly been fine tuned by evolution to minimize excited-state electron transfer processes, while at the same time efficiently delivering energy to the reaction center. 

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