Pheophytin, electronic absorption

Figure 7. Electronic absorption spectra of pheophytin-a (sol id) and the g-oxy=deoxo derivative obtained following reduction with borohydride, (dashed) Isolated from DSDP sample 64-479-5-3 ( ).

If the aUomerization reaction takes place in a slightly alkaline medium and under inert atmosphere, there is only solvolysis of the isocyclic ring, and the resulting components are denominated esterified chlorin es and rhodin g-j, depending on whether they come from the a- or fc-series, respectively. Chlorophyll a yields Mg-phytol-chlorin while pheophytin a or pheophorbide a yields phytol-chlorin eg or free chlorin eg, respectively. The same series of transformations can be described for chlorophyll b. The structural difference between these compounds and the purpurins 7 is solely in the substituent of C-15, which, in place of an O, is an H, so that their coloration, polarity, and electron absorption properties are very similar. Other techniques, such as mass spectroscopy (MS), must be used for their differentiation [28]. 

FIGURE 7.6 Electronic absorption spectra of chlorophyll b (—) and pheophytin b (—). 

FIGURE 7.7 Electronic absorption spectra of pheophytin a (—) and Cu-pheophytin a (—). 

The coefficients of extinction in acetone for each pigment are calculated from those given in the bibliography for chlorophylls and pheophytins in ethyl ether [194] by using a pigment solution of known concentration. It is assumed that the coefficients of extinction of compounds with different chemical structure but identical electron absorption spectrum (e.g., chlorophyll/chlorophyllide) do not differ significantly [251]. Thus, the specific coefficient of extinction (e ) can be calculated from a known molar coefficient (st) of another compound (i) with an identical spectrum ... 

Figure 19.10. Electron Chain in the Photosynthetic Bacterial Reaction Center. The absorption of light by the special pair (P960) results in the rapid transfer of an electron from this site to a bacteriopheophytin (BPh), creating a photoinduced charge separation (steps 1 and 2). (The asterisk on P960 stands for excited state.) The possible return of the electron from the pheophytin to the oxidized special pair is suppressed by the "hole" in the special pair being refilled with an electron from the cytochrome subunit and the electron from the pheophytin being transferred to a quinone (Q ) that is farther away from the special pair (steps 3 and 4). The reduction of a quinone (Qg) on the periplasmic side of the membrane results in the uptake of two protons from the periplasmic space (steps 5 and 6). The reduced quinone can move into the quinone pool in the membrane (step 7).

Figure 19.22. Pathway of Electron Flow From H2O to NADP in Photosynthesis. This endergonic reaction is made possible by the absorption of light by photosystem II (P680) and photosystem I (P700). Abbreviations Ph, pheophytin Qa and Qg, plastoquinone-binding proteins Pc, plastocyanin Aq and Aj, acceptors of electrons from P700 Fd, ferredoxin Mn, manganese.

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. 

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. 

Figure 1. Pheophytin-quinone and iron-sulfiir Reaction Centers. The dotted line represents the absorption of light by theprimary electron donor (Chl2 or BChl2).Thelineshows the energy transfers in the Reaction Center, from the PSII tyrosine residue (Yz), through the monomer bacttriochlorophyll (BChl), A) the monomer bacterio-pheophytin (BPhe), or B) pheophytin (Phe) and quinone transfer components, QA and QB, in the pheophytin-quinone type of Reaction Center, and Q through the monomer chlorophyll (Chi), quinone (Q) and F components in the iron-sulfur Reaction Centers.

Reaction Centers from Photosystem II of Green Plants. As a final example we mention the ultrafast research on the photosynthetic reaction center from photosystem II (PSII) of green plants. The reaction center contains six chlorophylls (Chi) and two pheophytins (Pheo). The absorption bands of all these pigments overlap extensively. Therefore, it is even more difficult to assign the different steps in the electron transfer process. Large uncertainty exists about the transfer time from the primary donor to the pheophytin. For instance, the transfer time was reported to be 3 psec. However, a time constant of 21 psec was reported for the same electron transfer event.A difference in the experimental condi-... 

Diagram photosystem II and identify its major components. Describe the roles of P680, pheophytin, and plastoquinone in the absorption of light, separation of charge, and electron transfer in photosystem II. 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

Transient absorption spectroscopy has been used to study isolated Photosystem 2 (PS2) reaction centres stabilised by the use of anaerobic conditions. In the absence of added artificial electron donors and acceptors, the light induced electron transfer properties of the reaction centre are restricted to the formation of the radical pair P680+Pheophytin and charge recombination pathways from this state [1]. This charge recombination has been observed to produce a 23% yield of a chlorophyll triplet state [1]. Attempts to reconstitute these particles with quinone have until now been limited to the observation of a steady state, quinone-mediated photoreduction of the cytochrome b-559 [2]. 

Photosystem II reaction centers (RC) consist of D1 and D2 polypeptides and a bound cytochrome b 559. Isolated RC, named D1/D2 particles, contain 4-6 Chi a. 1 j3-carotene and 1 or 2 cytochrome b 559 per two pheophytin a molecules (1). Polypeptides D1 and D2 exhibit marked homologies with the L and M subunits of the RC of purple bacteria, respectively (2). PS II shares a number of functional similarities with bacterial RCs. In these particles, absorption of a photon results in a rapid charge separation between the primary electron donor. Peso molecule because of the lack of a secondary electron acceptor, relaxation of the D1/D2 reaction centers occurs through the formation of a 900 /is-lived triplet state involving Chi a molecule(s), named P (3). 

The D1/D2/cytochrome b-559 complex contains 4 chlorophylLa molecules, 2 pheophytin-a molecules, 1 cytochrome b-559 and some p-carotene it contains no plastoquinone. In the absence of the secondary acceptors (quinone), electron transfer within this complex Is limited to the formation of the primary radical pair P680+Pheophytin". Absorption spectroscopy of this preparation has indicated the presence of a component decaying with a lifetime of 32-36 ns, corresponding to the lifetime of the primary radical pair (Danielius et al., 1987 Takahashi et al., 1988). More recently, time-resolved fluorescence studies (MImuro et al., 1988 Seibert et al., 1988) have shown that this complex exhibits a lifetime of 25-35 nanoseconds this has also been attributed to charge recombination of the primary radical pair, however, the fluorescence from this component was observed to be less than 2% of the total light emitted. 

The P6W formed by PSII light absorption reduces the oxidized form of its immediate electron acceptor, pheophytin (Pheo) (E o-0.61V), located in the PSII complex. The resulting Pheo" then reduces Q, a plastoquinone (PQ) molecule tightly bound to protein D2 of the reil complex. The receipt of one electron from Pheo" converts Q to Q " this is followed by the receipt of a second, forming Qa At neither the Qa" stage nor the stage are H ions taken up to form the semiquinone or the qninol. The... 

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