Primary Charge Separation Events

Boxer has reviewed the theoretical and experimental evidence bearing on the proposed contributions of the monomeric bacteriochlorophyll to super-exhange coupling, in the primary electron transfer step, between the electronically excited special pair of bacteriochlorophylls and the pheophytin acceptor of the reaction centers in photosynthetic bacteria.Sugawara et al have used a density matrix theoretical approach to show that the protein influences the primary charge separation event in the photosynthetic reaction center. 

In photosynthetic proteins, the primary charge separation and the sequence of electron transfer reactions can be utilized in the photosignal generation in electrochemical cells. The signal is the result of a combination of photophysical, photochemical and electrochemical events. The first is related to elearonic excitation followed by charge separation, the second deals with leaaions of excited molecules and finally, the electrochemical step involves a charge transfer at the interface between the electrolyte and the elearode. 

The photochemical activity of PS II reaction centers associated with primary charge separation has been documented by direct measurements of electron transfer by substrate donors (including water) or acceptors as well as by spectroscopic methods involving optical absorption, fluorescence and EPR. On the picosecond time scale it appears that the sequence of events and even the kinetics associated with the earliest steps are very similar between PS II and the purple bacterial reaction centers (15). Nevertheless, other aspects of this similarity remain to be demonstrated whether the primary electron donor of PS II consists of a special pair of chlorophylls, whether the PS II reaction center possesses a structural two-fold symmetry together with a functional asymmetry and whether there is a portion of the PS II complex that corresponds to the H polypeptide. 

It was earlier reported that photoinhibition in intact thylakoid membranes is a multistep event and that inhibition of electron transport is followed by a slow degradation of the reaction center proteins D1 and D2 (3). From the spectroscopic data presented above we conclude that the primary damage to the reaction center proteins is caused by the primary charge separation or donor side reactions in centers were the function of ( A has been impair. To further test the hypothesis that the protein degradation is initiated by reactions on the oxidizing side of PSII, we... 

Strand cleavage studies have provided relative rate constants for hole transport versus the rate constant for the initial chemical event leading to strand cleavage [18-20]. However, they do not provide absolute rate constants for hole transport processes. Several years ago we introduced a method based on femtosecond time-resolved transient-absorption spectroscopy for investigating the dynamics of charge separation and charge recombination in synthetic DNA hairpins [21, 22]. Recently, we have found that extensions of this method into the nanosecond and microsecond time domains permit investigation of the dynamics of hole transport from a primary hole... 

It is clear that charge separation is the primary event, followed by electron bombardment of dinitrogen in the air near the charged (presumably new) surface. The emission is accompanied by the expected radio signals, and electron and positive ion release (12), and thus will be called "lightning" throughout this paper. 

Fig. 20. Model for the primary event in vision. Isomerization of the 11.12 bond leads to charge separation at Ihe Schiff base site. This process, as shown, can possibly be followed by proton transfer, the latter resulting from the charge separation. In rhodopsin, the second negative charge responsible for wavelength regulation is shown close to the 11,12 bond of the polyene chain. This model assumes that hypsorhodopsin is the unprotonated form of the Schiff base, and that it is formed possibly by proton transfer from the Schiff base nitrogen in some pigments. From Honig ct al. [207].

Many of the PRC ET reactions exhibit only modest variations with temperature. The rate of the primary photochemical event increases at cryogenic temperatiue. Several other reaction rates decrease by only small factors when temperatures are lowered. For charge separation, this behavior can be attributed to driving-force-optimized reactions. 

The primary photochemical charge-separation process, i.e., P870-t-A -> P870 +A in purple photosynthetic bacteria requiresthat there is a reaction partner to accept the electron released by the primary donor. Again, using D-[P-A] to represent the core composition of the bacterial reaction center, we can write the following sequence of events ... 

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