Chlorophyll dimer

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Figure 12.7 The photosynthetic unit, in which an antenna chlorophyll molecule is excited by photon absorption and the energy is transferred to the chlorophyll dimer at the reaction centre...

In reaction centers, this energy drives an electron transfer reaction, which in turn initiates a series of slower chemical reactions. Energy is saved as redox energy,29,30 inducing a charge separation in a chlorophyll dimer called the special... 

A clear understanding of why j depends on R is offered by the computational results for a chlorophyll dimer of LHCII as function of a systematic increase in the chlorophyll-a center-to-center separation, as reported in Fig. 2.4. 

Natural photosynthesis provides the most dramatic demonstration of the potential hidden in this basic photoreaction. In (bacterial) photosynthesis a chlorophyll-dimer (BC)2—the special pair —receives the radiation energy and thereby gains the energy required to enable it to transfer an electron to a pheophytin moiety (BP), an act occurring within 2-3 picoseconds (Martin et al. 1986) even at very low temperatures. Subsequently the electron is transferred to a quinone acceptor (MQ), which once again occurs (Holten et al. 1978) on a very short time scale of about 230 ps. 

Figure 2.9. Dependence of maximum rate constant of ET on the edge-to-edge distance in photosynthetic RCs of bacteria and plant PSI 1, Ao - A, 2, IT - QA 3, A,-Fx H - P+ 5, C559 -P+ 6, Fx - FA 7, QA - P 8, P - Bel 9, P700 - Ao 10, Qa - QB P700 is the chlorophyll dimer, Ao is chlorophyll, A, is phylloquinone and Fx and FA are the 4 Fe-4S clusters. The straight line is related to the dependence of the attenuation parameter for spin exchange (Yse) in homogeneous non-conducting media. Filled circles correspond to a regular dependence, open circles to a deviation (Likhtenshtein, 1996). Reproduced with permission.

The time scale of different steps of electron transfer along the PS I cascade system is also similar to those ofthe electron jump in bacterial RCs. The primary transfer from the excited chlorophyll dimer, primary donor P, to A0 takes place with a time constant of about 25 ps. The next step from A0 to a secondary acceptor occurs in 200-600 ps. The recombination time constants ofP+ with reduced intermediate acceptors increase as the electron moves along the chain, and range from nanoseconds for transition A0 — P+ to millscconds for transition reduce FX to P+ (Shuvalov and Krasnovsky, 1981 Schloder et al., 1998 Shmidt et al., 2000 Shmidt et al., Guergova-Kuras et. al., 2001 Setif et al., 2001 Vassiliev et all., 2001 Gobets et al., 2001 and references therein). Kinetic and spectral inhomogenity of samples of PS I has been reported (Shmidt et al., 2000 ... 

The concept of chlorophyll serving as the primary electron acceptor was amended hy Kamen" to include a model consisting of a chlorophyll dimer. The energy of a (red) photon removes an electron from one Chi ofthe Chl Chl pair and places it on the other Chi to form ChC Chr. Kamen also envisioned that spectroscopic differentiation of Chl and Chl would he difficult because the bridge-carbon resonance would be hindered, and the formation of either charged species would be associated with an absorbance deaease. Kamen s original concept was quite prophetic and became pertinent to all types of photosynthetic reaction centers to come under investigation later. 

Fig. 1. Structures of model systems and the photosynthetic unit, (a) Rhodamine B adsorbed on the ab-plane of the anthracene single crystal (1), (b) steroid skeleton with biphenyl as donor and acceptor A (8), (c) donor and acceptor molecules bonded by methylene chain (28), (d) molecule with completely rigid skeleton (13), (e) methylviologen-capped porphyrin (9), (f) reaction center of Rps. viridis (29). D Chlorophyll dimer, M Chlorophyll monomer, PrPheophytin, QrQuinone.

D. Excited Singlet State Properties of Chlorophyll Dimers.611... 

The pheophytin alcohol-chlorophyll dimer complex reproduces faithfully most of the features associated with the primary reactions of photosynthetic bacteria. The absorption of a photon results quickly (< 10 ps) in the formation of a state that involves a cation of the photoactive dimer and an anion of pheophytin. The state is not fluorescent and decays in 10-20 ns when further photochemistry is blocked. These observations are in contrast to a number of complexes that have been recently synthesized by direct linkage of pyrochlorophyll or pyropheophytin monomer to the pyrochlorophyll dimer. Although both these complexes are somewhat simpler to work with, since they can be prepared free of any other components, they show only a limited amount of fluorescence quenching. This is somewhat disappointing, but indicates how important the orientation of the dimer with respect to the electron acceptor must be to insure a rapid electron transfer reaction in the excited state. Once the structures of the several dimer-acceptor complexes have been determined, it is hoped that we will better understand the conditions necessary for effective electron-transfer reactions. 

Fig. 4. High-field (left side) and X-band (right side) EPR spectra on two transient radicals created within the photocycle in photosynthetic proteins, a) P ) chlorophyll dimer cation radical of deuterated cyanobacteria, measured at 140 and at 9 GHz. b) Q semiquinone radical of bacterial reaction centres of Rhodobacter sphaeroides R26 measured at 95 and at 9 GHz,...

A nice example is the primary photosynthesis process. Here, light energy which is absorbed by the antenna chlorophyll molecules is conducted to a chlorophyll dimer in the reaction centre. Only there do the chemical reactions take place which lead to charge separation and finally to photosynthesis. For more details, cf [M6], Sect. 20.7. 

Syntheses of oligoporphyrins have targeted the construction of model systems to miinic the assembly of chlorophylls in photosynthesis of green plants and bacteria. In earlier times, a chlorophyll dimer was considered the key unit of antenna systems which gather sunlight for production of carbohydrate. Many model systems that elucidate the mechanism of photoinduced electron-transfer reaction of photosynthesis have been reported and well documented in many reviews. Syntheses of covalently linked electron donors and acceptors have been extended to model compounds appended to successive electron acceptors such as quinone derivatives. 

The first cyclic porphyrin dimer (cyclophane porphyrin) linked with two ester groups 15 was synthesized as a model of antenna chlorophyll dimer by condensation of a 2,12-dipropionate porphyrin and a 2,12-bis-(3-hydroxypropyl) porphyrin in high dilution in 1977." In order to clarify the mechanism of electron-transfer reactions in biological systems, a variety of porphyrin dimers have been reported as model systems of parts of the photosynthetic apparatus in the last two decades. The first synthesis of cyclic porphyrin oligomers was reported by Hamilton, Lehn and Sessler in 1986. ... 

The photochemical charge separation in photosystem I (PS I)(see (1) for a recent review), starting from the excited primary donor, P700 (a chlorophyll dimer), involves five membrane-bound electron acceptors the primary acceptor Aq (chlorophyll a), a secondary acceptor (presumably vitamin Kj ) and three iron sulfur centers, F, Fg and F. ... 

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