The Primary Donor

A coherent interpretation for many experimental results was provided by the concept of a PS I reaction centre. This centre has now been isolated, albeit perhaps not in a definitely pure state. It is made up of a few hydrophobic polypeptides, the primary donor (P-700), several electron acceptors (Fig. 2), and about 50 molecules of pigment (chlorophyll a and /3-carotene). This composition is analogous to that of other types of reaction centres. 

BChl a in solution ( 660 + 10 mV).66 ENDOR/TRIPLE further showed that the spin is completely localized on one BChl half (Fig. 3). The pigment analysis showed that the RC contains 3 BChl and 3 BPh, whereas 4 BChl and two BPh are found in wild type. On the basis of earlier work5467 it is therefore assumed that in these mutants, a BChl-BPh heterodimer is the primary donor. Since BPh has a higher redox potential than BChl,66 only the BChl half is oxidized and carries the spin and positive charge. This situation is energetically less favorable than delocalization in a dimeric species, which leads to the observed increase of P 865. For the heterodimer mutants, a lower quantum yield and reduced ET rates have been observed. The experiments clearly show that dimer formation alone significantly lowers and, thereby, adjusts the donor s oxidation potential.68 

A profound effect on the electronic asymmetry of the dimer has been detected for site-directed mutants of R. sphaeroides in which hydrogen bonds to the carbonyl groups of the BChl are either broken or formed.69 71 The H-bond effect can be traced back to the electrostatic interaction between the oriented dipole of 

Very thorough theoretical work on the dimer problem in photosynthetic RCs has been published in a series of papers by Reimers and Hush, see references 64,75. Very recently the authors gave a unified description of the electrochemical, charge distribution and spectroscopic properties of P,+ in bRCs.76 

The work by Allen and coworkers69,78 has shown that the redox potential of the 

In the BChl g containing heliobacteria Heliobacillus mobilis and Heliobacterium chlorum symmetric dimers for the primary donor radical cation PgJ5 have been found based on EPR and ENDOR data.85 This symmetric dimer is consistent with the homodimeric structure of the RC. The same reason was invoked to explain the high symmetry of the donor radical-cation Pgg5 in green sulfur bacteria, which is made up from a BChl a dimer.86 For a review see reference 87. Note that these RCs belong to the type I RCs. 

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Bratt, P. J., M. Rohrer et al. (1997). Submillimeter high-field EPR studies of the primary donor in plant photosystem IP700+. J. Phys. Chem. B 101 9686-9689. 

Burghaus, O., M. Plato et al. (1991). 3mm EPR investigation of the primary donor cation radical in single crystals of Rhodobacter sphaeroides R-26 reaction centers. Chem. Phys. Lett. 185 381-386. 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

Photochemical electron transfer occurs from the excited singlet state of the primary donor. 

The primary donor in Photosystem I P700 is thought to be a special pair of chlorophyll a molecules. Katz and Hindman (18) have reviewed a number of systems designed to mimic the properties of P700 ranging from chlorophyll a in certain solvents under special conditions where dimers form spontaneously (19) to covalently linked chlorophylls (20). Using these models it has been possible to mimic many of the optical, EPR and redox properties of the in vivo P700 entity. 

Figure 4.60 Geometry and the primary donor-acceptor interactions of [Pt(PH3)2 (H)(H2)]+ (a) from the H2 c-bond into the Pt—H ct antibond and (b) from the dpt lone pair into the cthh antibond.

Perhaps the most Important effect of conformational variations In electron transfer reactions would be to alter the distances and the relative orientations of donors and acceptors. In photosynthetic RC s, where the primary donors and acceptors lie within 4-5A of each other ( ), small structural displacements (, 5A) may significantly affect rates of back reactions. If they occur rapidly (24), (Conformational movements on a picosecond time scale are not Inconsistent with resonance Raman data on photo-dlssoclated heme-CO complexes (25)), On a longer time scale, protein rearrangements triggered by and propagating from the chromophores may also help subsequent reactions such as the transport of protons that Is Initiated by the primary photochemical event In the R,C, (26),... 

Figure 3 Special TRIPLE resonance spectra of the primary donor radical-cation P in the bRC of R. sphaeroides wild type and mutant HE(M202) (His - Glu) and of monomeric BChl a " in organic solvents, all spectra in isotropic solution. The isotropic hfcs are directly obtained from the special TRIPLE frequency nSJ = Ais,J2.H The oxidation potential of the primary donor is also given (vs. NHE). Adapted from reference 68.

The primary donor triplet state 3P The triplet state of the primary donor is formed by recombination of the primary radical pair P,+4>X of prereduced bRCs in which the ET to the quinones is blocked ... 

The main result of the early EPR and ODMR work was a reduction of the ZFS parameters D( and when comparing monomeric 3BChl and the 3P state of the primary donor in bRCs (for a collection of data see reference 22). This has been interpreted as resulting from a delocalization of the triplet exciton in the BChl-dimer. However, due to the complexity of the electronic system, that could also involve charge transfer states, a final quantitative conclusion has been difficult. From a determination of the triplet axes in bRC single crystals104 107 it was concluded that in R. sphaeroides the triplet is indeed delocalized whereas in B. viridis it seems to be located on one monomeric BChl b half. 

Further work using time-resolved EPR and magnetophotoselection (MPS), using plane-polarized light to excite the triplet state, gave information on the orientation of the optical transition dipole axes relative to the principal axes of the triplet state. By this technique the transition moments of the primary donor"6, the carotenoid in the bRC"7 and the bacteriopheophytin in the inactive B branch 4>0"8 were determined. 

The protein complex of T. elongatus consists of 12 subunits that contain 96 Chi a and 22 carotenoid molecules, 3 [4Fe4S] centres and 2 phylloquinone (vitamin K,) molecules (for molecular structures see Fig. 2). The cofactors of the ET chain are arranged in two branches as pairs of molecules related by a pseudo-C2 axis. After light excitation an electron is donated from the primary donor P700, a pair of chlorophylls, to monomeric chlorophyll a (acceptor A0), phylloquinone (A() and the 3 iron-sulfur centres (F , Oa and B). It has been controversially discussed in the literature whether both highly symmetric pigment branches are... 

This view has been challenged by Mac et al. who detected only four sets of nitrogen couplings and assigned them to one monomeric Chi a + forming the primary donor.209 In their model the observed changes of spin densities are interpreted within the hybrid orbital model.203 The data have recently been critically reviewed.187... 

The impact on the electronic structure affecting the oxidation potential and the electron spin distribution has been explained within a simple molecular orbital (MO) model.189 199 As in the bRC the formation of a dimer leads to a decrease of the oxidation potential of the primary donor ( see section 2.1). 

The Primary Donor Triplet State iP7a0 If in the charge separation process electron-transfer in PS I beyond the first acceptor A0 is blocked by treatment with sodium dithionite at high pH and illumination, which reduces the iron-sulfur centres (F) and the quinone (A, the triplet state of the donor, 3P7ao, is obtained via radical-pair recombination from the triplet RP according to ... 

The obtained data clearly show that the g-anisotropy of the triplet states is larger than that of the respective cation-radical. A similar effect has been observed for the triplet states of the primary donors in PS II231 and in the bacterial RC.111112114 This can be explained by the fact that the triplet electrons probe the spin distribution in two different orbitals (HOMO and LUMO), and the latter has a rather large spin density at the nitrogens and the central magnesium (cf. references 216, 218), by which the spin-orbit coupling and the g-anisotropy is increased. 

The Primary Donor. - The radical-cation P+ In the bRC of purple bacteria and also in PS I the primary electron donors have been identified as (B)Chl dimers and EPR/ENDOR clearly showed that the unpaired electron and the positive charge - is (asymmetrically) distributed in a supermolecular orbital extending over both dimer halves (see sections 2.1,3.1). Dimer formation has the important consequence of charge delocalization and this stabilization of the primary donor radical-cation leads to a decrease of the oxidation potential. A fine tuning of the potential is possible through interactions with the environment, e.g. via H-bonds. 

Fig. 1. A. Noise level expressed in milli optical density, obtained after 1 minute of data acquisition. B. Time dependent absorption change of the keto group of the primary donor of the bacterial reaction center, at 1685 cm 1 and 1715 cm 1 upon excitation at 600 nm, noise level 30 pOD, measured in the Lissajous scanner. The solid line through the data points is a fit with = 3.8 ps, t2 = 16 ps, t3 = 4 ns and t5 = oc. The time scale is linear up to 3 ps and logarithmic thereafter.

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

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