Antenna molecules, chlorophyll pigments

Light excites an antenna molecule (chlorophyll or iccessory pigment), raising an electron to a higher energy level. 

The two photochemical reactions are performed by two photosystems. Each photosystem consists of a so-called reaction centre, where the primary energy conversion takes place, associated with a few hundred pigment molecules (chlorophylls and carotenoids see Fig. 2) serving as light-harvesting antennas, which transfer the absorbed energy as electronic excitation energy to the reaction centres. 

It is now known that in the bacterial systems the donor is a pair of chlorophyll molecules, the special pair", P, and the initial acceptors is either another chlorophyll molecule, an auxiliary chlorophyll B, or a bacteriopheophytin, H, a chlorophyll molecule in which the central magnesium ion has been replaced by a pair of protons. Although many pigments are present acting as antenna molecules to gather the light, additional chlorophylls are most common. How can the same molecule have such different roles In spite of the fact that the antenna and the RC pair molecules are the same, light is transferred from the antenna to the RC with unit efficiency. 

The first part of the process occurs in light-harvesting complexes. Each multisubunit protein complex contains multiple antenna pigment molecules, chlorophylls and some accessory pigments, and two chlorophyll molecules that act as the reaction center. The reaction center traps energy quanta excited by the absorption of light. 

Schematic diagram of the surface of a photosystem in the thylakoid membrane. It contains a patch-like mosaic of several hundred chlorophyll and carotenoid antenna molecules oriented in the membrane. An exciton absorbed by an antenna molecule quickly migrates via the pigment molecules to the reaction centre, P700 its path is shown by the coloured arrows. Although all the antenna molecules can absorb light, only the reaction centre molecule can convert the excitation energy into electron flow.

The water oxidation half-reaction, introduced by Equation (1.1), is the most challenging obstacle for solar hydrogen production, since it requires a four-electron transfer process coupled to the removal of four protons from water molecules to form the oxygen oxygen bond. In Nature, this process is driven by solar light captured by chlorophyll pigments embedded in the protein antennas... 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

Such a process can naturally be expected to play a certain part in the mechanism of directed energy transport in biological systems, in particular, in the transfer of absorbed energy from the antenna chlorophyll molecules to the reactive center in the photosynthetic system of plants. In Ref. [30], energy exchange between molecules of the photosynthetic pigments chlorophyll a and pheophytin a was studied experimentally with pigments introduced into the polar matrix. 

Figure 10.16 Solar energy transfer from accessory pigments to the reaction centre, (a) The photon absorption by a component of the antenna complex transfers to a reaction centre chlorophyll, or, less frequently, is reemitted as fluorescence, (b) The electron ends up on the reaction centre chlorophyll because its lowest excited state has a lower energy than that of the other antenna pigment molecules. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

The photosynthetic apparatus is found in and on membrane structures, which, in plant cells and algae, are located in chloroplasts and are called thylakoids. In bacteria the photosynthetic membrane is derived by complex invagination of the cytoplasmic membrane. The photosynthetic apparatus is made up of antennae, which contain light-harvesting pigment molecules (usually chlorophylls or bacteriochlorophylls) and photochemical reaction centres, which also contain pigments, together with the necessary enzymes and coenzymes. 

The unique water-soluble peridinin- Chi a-protein (PCP) complexes are found in many dynoflagellates in addition to intrinsic membrane complexes. [64] It contains Chi a and the unusual carotenoid peridinin in stoichiometric ratio of 1 4. Unlike other families of antennas, the main light-harvesting pigments are carotenoids, not chlorophylls. The structure of the PCP consists of a protein that folds into four domains, each of which embeds four peridinin molecules and a single Chi a. The protein then forms trimers, suggested to be located in the lumen [64] in contact with both LHCI and LHCII [66], allowing efficient EET to occur. 

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