Plant type pheophytins

BACTERIAL REACTION CENTERS WITH PLANT-TYPE PHEOPHYTINS... 

Three plant-type pheophytins were tested i) [3-acetyl]-Phe a, which differs from BPhe a only by the unsaturation of ring II. ii) Phe a, which has... 

The absorption spectra of Phe a and 13 -hydroxy-Phe a are essentially identical (Fig. 1). In comparison to BPhe a, there are two characteristic blue-shifted Q -bands for the two plant-type pheophytins, and the Q -bands are blue-shifted by 70-80 nm, too. Unlike Phe a, [3-acetyl]-Phe a shows a split Soret-band as do the bacteriopheophytins. Providing the same assignments, they are red-shifted by 24 nm (B )and 28 nm (B ). The main peak of the Soret-band of Phe a is red-shifted by 52 nm compared to B of BPhe a. 

Table 1. Exchange rates of some plant-type pheophytins and bacterial-type pheophytins...

The absorption spectra of the RC modified with plant-type pheophytins... 

At the g-sites, differential exchange kinetics were observed, with B exchanging more rapidly that B. A similar difference was seen in earlier BPhe exchange experiments. The present results with plant-type pheophytins indicate no obvious difference with these pigments, but due to band-overlap in the Q -region this result needs further studies, e.g. at low temperature. ... 

Bacterial Reaction Centers with Plant-type Pheophytins... 

Photosynthetic bacteria have relatively simple phototransduction machinery, with one of two general types of reaction center. One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg2+ ion) to a quinone. The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center. Cyanobacteria and plants have two photosystems (PSI, PSII), one of each type, acting in tandem. Biochemical and biophysical... 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

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