Photosystem mimic

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. 

Huang, R Kurz, R Styring, S. 2007. EPR investigations of synthetic manganese complexes as bio-mimics of the water oxidation complex in photosystem II. Appl. Magn. Reson. 31 301-320. 

In an important step to mimic the natural photosystem, tyrosine residues tethered to a Rubpy sensitizer as in 15 have been shown to reduce the Rum center obtained after oxidative quenching with methylviologen or [Co(NH3)5C1]2+.187 Formation of the resulting tyrosyl radical is a proton coupled process and it has been shown to be a concerted process in which the reorganization energy associated with deprotonation can be tuned by H-bonding and pH.188191 Similar results are observed for tyrosyl residues tethered to Re(I)diimine based chromophores.192... 

An idealised complete cycle is one which employs the acceptor QH (or QH2) of cycle C1 as the donor of the C2 cycle. C T and C2 cycles, thus, roughly mimic photosystems II and I of photosynthesis respectively. 

In searching for a suitable electron acceptor, it seems reasonable to just mimic what is known about the initial acceptors in vivo. In photosynthetic bacteria it has been well established that bacteriopheophytin a is one of the first electron acceptors. In photosystem I of green plants a chlorophyll a dimer or monomer have been proposed as the acceptor. Either chlorophyll a or pheophytin a would be excellent choices as electron acceptors. However, the singlet lifetime of the pyrochlorophyll a dimer in toluene is only 4 ns. " For a diffusion-controlled electron transfer reaction between the dimer and one of the in vivo acceptors to take place in a few nanoseconds would require a 10 to 10 molar concentration of pheophytin a or chlorophyll a. The molar extinction coefficient of these molecules is on the order of 40,000. At a concentration of 10 M, the absorption of pheophytin a (or the chlorophyll a monomer) would be much too high. The solution of this dilemma is to link an electron acceptor such as pheophytin or chlorophyll to the dimer. Linking the dimer to an electron acceptor not only solves the diffusion problem, but also begins to mimic the photosynthetic reaction center. 

As was presented in Section III, chlorophyll a adducts with ethanol have been prepared that successfully mimic the optical and ESR properties of photosystem I reaction center chlorophyll. These chlorophyll a special pair systems are assembled from the two monomer units by cooling a mixture of chlorophyll a in the presence of water or toluene to 100 K. The formation of the desired structure depends not only on the chlorophyll a concentration, but also on the mole ratio of chlorophyll a to nucleophiles in solution, the solvent, the rate of cooling, etc. There is reason to suppose that mixtures of various species of poorly defined structure are present in even the best of these preparations. The uncertainties in composition and structure and the experimental problems and restrictions imposed by working at low temperature in organic glasses have limited the information that can be derived from such model systems. The solution to this problem is to provide a mode of physical attachment between two chlorophyll molecules so that the magnitude of the entropy of dimerization is lowered. 

Covalently linked dimers of both chlorophyll a and pyrochlorophyll a have been prepared, which mimic the spectroscopic and redox properties of P700. The two chlorophylls are joined in each case at their propionic acid side chains via an ethylene glycol diester linkage. The orientation of the chlorophyll macrocycles with respect to one another (Figs. 11 and 12) and consequently their electronic properties depend strongly on the solvent. The structure of most interest is the folded one (Fig. 11) because of its similarity to the photoactive dimer in photosystem I. 

The X-ray structural studies of the photosystem II reaction center of Rhodo-pseudomonas viridis and related aspects of plant membrane structure have been reviewed/ Photoinduced charge separation in synthetic, porphyrin-based molecules which mimic aspects of the natural photosystems have also been reviwed/ ... 

Fig. 1. Simulations of the k -weighted k-space Fe EXAFS data from the Photosystem I core protein containing Fx (solid line -experimental dotted line - simulation). After background removal and weighting by k, data from k=3 to k=12 A-i were Fourier filtered with window limits at R =0.5 and R =3.3 A. The similation shown was performed by the method of Teo and Lee using two shells. The parameters for simulation (a) minic a [4Fe-4S] center and employ 4 S atoms at 2.27 A with a Debye-Wdler disorder parameter of 0.075 A and 3 Fe neighbors at 2.7 A with a disorder parameter of 0.1 A. The parameters for simulation (b) mimic a [2Fe-2S] center and employ 4 S atoms at 2.26 A with a Debye-Wdler disorder parameter of 0.08 A and 1 Fe neighbor at 2.7 A with a disorder parameter of 0.07 A.

A further extension of the previous approach consists of coupling water oxidation catalysts to photochemical units creating complex photoelec-trochemical nanostructures that mimic biologic proton-coupled electron transfer in Photosystem II. 

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