Chlorophyll, electron transfer

This symmetry is important in bringing the two chlorophyll molecules of the "special pair" into close contact, giving them their unique function in initiating electron transfer. They are bound in a hydrophobic pocket close to the symmetry axis between the D and E transmembrane a helices of both... 

One apparent discrepancy between the spectroscopic data and the crystal structure is that no spectroscopic signal has been measured for participation of the accessory chlorophyll molecule Ba in the electron transfer process. However, as seen in Figure 12.15, this chlorophyll molecule is between the special pair and the pheophytin molecule and provides an obvious link for electron transfer in two steps from the special pair through Ba to the pheophytin. This discrepancy has prompted recent, very rapid measurements of the electron transfer steps, still without any signal from Ba- This means either... 

Lafferty, J., Truscott, T.C., and Land, E.J., Electron transfer reactions involving chlorophylls a and b and carotenoids, J. Chem. Soc. Farad. Trans., lA, 2760, 1978. Burri, B.J., Clifford, A.J., and Dixon, Z.R., Beta-carotene depletion and oxidative damage in women, in Natural Antioxidants and Anticarcinogens in Nutrition, Health and Disease, Kumulainen, J.T. and Salonen, J.T., Eds., Royal Society of Chemistry, Stockholm, 1999, 231. 

Fugimori, E. and M. Talva. 1966. Light-induced electron transfer between chlorophyll and hydroquinone and the effect of oxygen and beta-carotene. Photochem. Photobiol. 5 877-887. 

Haem, Fe Chlorophyll, Mg Coenzyme B12, Co Factor F-430, Ni Electron transfer in membranes and elsewhere Light capture and transduction in membranes Transfer of methyl, rearrangements of substrates Activation of carbon monoxide... 

Butler, W. L. Chlorophyll fluorescence A probe for electron transfer and energy transfer. 

Hynninen and coworkers <99JCS(PT1)2403> used a similar approach to prepare phytochlorin-C6o diad 38 (Scheme 11). The protocol employed the pyrolysis of the natural chlorophyll a molecule 35, followed by transesterification and demetallation to furnish derivative 36. Subsequent oxidation of 36 with OsCU and NaI04 has allowed the synthesis of the formyl derivative 37, which was further used as precursor of the azomethinic ylide intermediate in the 1,3-DC reaction with Cm leading to the formation of diad 38. Photochemical studies revealed that this diad underwent a fast intramolecular photoinduced electron transfer in polar solvents such a benzonitrile <99JACS9378>. 

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

A.B.P. Lever, York University Most model photocatalysts undergo electron transfer reactions via their spin triplet states. You point out that there are advantages to using the spin singlet state for electron transfer as exemplified by chlorophyll. What kind of structural or electronic features should be built into model photocatalysts to favour use of their spin singlet states for electron transfer quenching ... 

The chlorophyll molecule (309) is involved in initiating photosynthesis in green plants and contains magnesium coordinated to a partially reduced porphyrin (namely, a chlorin derivative). Life relies ultimately on the unique redox and electron transfer abilities of the chlorophylls which are necessary for the conversion of light to chemical energy. Chlorophyll mainly absorbs light from the far red region of the spectrum... 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

Natural photosynthesis applies electron transfer systems, where a relay of electron-transfer reactions evolves among chlorophyll and quinone moieties embed-... 

Consequences of Electron Transfer in Chlorophylls, Chlorins, and Porphyrins... 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

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