Light-gathering antennae

Figure 12.1. Schematic diagram of a photosynthesis reaction center. Light is absorbed by pigments in the light-gathering antenna and absorbed energy is transferred to a photochemically active site P, where it is utilized to initiate photosynthetic reactions.

As indicated in Figure I, wild-type bacterial reaction centers also contain a carotenoid polyene. This polyene is not involved as a donor or acceptor in the normal electron transfer sequence, although carotenoid radical cations have been observed spectroscopically in photosynthetic preparations under certain conditions [18,19]. In many of the artificial photosynthetic systems which will be discussed below, the carotenoid is used as a convenient secondary electron donor. Carotenoids do perform two important functions in photosynthesis. They provide photoprotection from singlet oxygen damage, and act as light-gathering antennas for the special pair (see Sections III and IV). 

Light, circularly polaridzed, 41, 139-44. 154, 158, 162-63 elUplically polarized, 139-43 linearly polarized, 1-3, 5, 38-41, 139-41 Light-gathering antennae, 473 Linear momentum operator. 22. 24, 56, 145 Line-shape function, 156 Liquid crystals, 272 Localized orbital model, 115-16... 

However, each of these states plays a role in gathering light. The operation of the light-harvesting antenna of purple bacteria is based on an energy funnel to focus excitation energy to the reaction center. There are two obvious ways to construct such a funnel (1) Select different chemical species that absorb at the... 

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an auxiliary chlorophyll B, or a bacteriopheophytin, H, a chlorophyll molecule in which the central magnesium ion has been replaced by a pair of protons.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. 

Figure 25 An artificial antenna-reaction-center complex that mimics the early stages of photosynthesis. The central hexaphenylbenzene core provides structure and rigidity for the surrounding wheel of five bis(phenylethynyl)anthracene antennas that gather light at 430 nm. The energy is transferred to a porph) complex in 1-lOps (green arrows), whra-e it excites an electron that is transferred to the fullerene acceptor in 80 ps the resulting charge-separated state has a lifetime of 15 ns. Complexes such as the one shown provide the first steps in artificial photosynthesis. They have the potential to drive furtha- chanical reactions, such as the oxidation of water to produce H2 or the reduction of CO2 to CH4, alcohol, or other fuels. (Adapted with permission from Ref. 69. American Chemical Society, 2006.)...

---