Chlorophyll orientation

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

Modeling of the reaction center inside the hole of LHl shows that the primary photon acceptor—the special pair of chlorophyll molecules—is located at the same level in the membrane, about 10 A from the periplasmic side, as the 850-nm chlorophyll molecules in LH2, and by analogy the 875-nm chlorophyll molecules of LHl. Furthermore, the orientation of these chlorophyll molecules is such that very rapid energy transfer can take place within a plane parallel to the membrane surface. The position and orientation of the chlorophyll molecules in these rings are thus optimal for efficient energy transfer to the reaction center. 

While phototactic action spectra measured in some Phormidium species indicate that chlorophyll a is not involved in the absorption of phototactically active light (see below), the phototactic action spectrum of Anabaena variabilis106) shows slight activity around 440 nm and a distinct peak at around 670 nm, both indicating chlorophyll a. Since blockers of the photosynthetic electron transport, such as DCMU andDBMIB, (see below) do not affect phototactic orientation, the active light seems not to be utilized via the photosynthetic electron transport chain (for further information see below). 

The action spectmm of positive and negative phototaxis of Anabaena variabilis was measured recently106). This species contains no C-phycoerythrin. Accordingly, maximum activity is found at around 615 nm (Fig. 7). In addition, in this form a second maximum occurs at around 675 nm, and a third small, but distinct, one at 440 nm, both indicating that chlorophyll a is also involved in the active light absorption (see above). The utilization via photosynthesis, however, could be excluded in this case, since the trichomes oriented themselves perfectly well to the light direction in the presence of photosynthetic inhibitors, such as DCMU and DBMIB, at concentrations in which the photosynthetic oxygen evolution was almost completely inhibited. 

Yang, C.M., Lee, C. N. and Chou, C.H. (2002). Effects of three allelopathic phenolics on chlorophyll accumulation of rice (Oryza sativa) seedlings I. Inhibition of supply orientation. Botanical Bulletin of Academia Sinica 43 299-304. 

Fig. 1. The structure of the LHCII monomer as derived from electron crystallography [51], A proposed topography of the polypeptide in the photosynthetic membrane. Letters A, B and C indicate the three hydrophobic ix-helices spanning the membrane. Chlorophyll molecules are arranged into two rings roughly parallel to the membrane plane. B Approximate position of the chlorophyll in the upper level (left) and lower level (right) on the membrane plane. Dashed lines outline a-helices A, B and C. Chlorophyll molecules are oriented perpendicular to the membrane plane and are thus represented as black bars. Chlorophylls numbered as 6,7 and 8 are closer to those belonging to the lower layer than the other pigment molecules...

We examine here possible structural effects that may result from or accompany the generation of the primary photoproducts, and speculate about the consequences of concomitant changes in distances,conformations, relative orientations and charges on the electronic profiles of and interactions between the BChls, BPheos and their radicals. Because the primary events in green plant photosynthesis also involve a series of chlorophyll donors and acceptors ( ), similar trends should therefore prevail for chlorophyll radicals as well. Furthermore, radicals of porphyrins and hydroporphyrins (saturated porphyrins such as chlorins and isobacteriochlorins) have been... 

At room temperature, Stehlik and coworkers228 reported a reduction of for 3P700 whereas D remained the same as found at low temperatures. The authors interpreted their findings as resulting from triplet delocalization over two co-planar Chi a molecules at elevated temperatures. The magnetic z-axes of the two chlorophylls are parallel and their magnetic x (and y) axes form an angle of approximately 55°. In addition, the orientation dependence of the triplet state showed that the plane of the Chi, which carries the triplet at low temperature, is oriented perpendicular to the membrane,229 i.e. the triplet state must be located on PA or PB. 

An intermediate-level description of the proteins that orient chlorophyll molecules in chloroplasts. 

Since chlorophyll can be removed readily from chloroplasts by mild solvent extraction, it might appear that it is simply dissolved in the lipid portion of the membranes. However, from measurements of dichroism (Gregory,226 p. Ill) it was concluded that the chlorophyll molecules within the membranes have a definite orientation with respect to the planes of the thylakoids and are probably bound to fixed structures. The absorption spectrum of chlorophyll in leaves has bands that are shifted to the red by up to 900 cm-1 from the position of chlorophyll a in acetone. Most green plants contain at least four major chlorophyll bands at 662, 670, 677, and 683 nm as well as other minor bands264 (Fig. 23-21). This fact suggested that... 

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