Chlorophyll, donor/acceptor

CFP-YFP donor-acceptor pair, YFP is several times brighter than CFP [62]. Lastly, for studying dynamic protein associations in plants, the presence of chlorophyll pigments in leaf and stem cells is an additional limitation. These pigments directly absorb the fluorescence, which decreases blue fluorescence intensity for BFP and CFP donors that can be erroneously interpreted as reduced donor fluorescence quantum yield caused by FRET [18]. If sensitized emission or FSPIM is the only available method for quantifying FRET, then it is very important to restrict measurements to chlorophyll free areas within the cells. 

V = 10 cm and R = 1.9 nm for ET in the quinone-chlorophyll system. These estimates of donor-acceptor separation are in good agreement with the results of magnetic measurements (17). 

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... 

The protein complex of T. elongatus consists of 12 subunits that contain 96 Chi a and 22 carotenoid molecules, 3 [4Fe4S] centres and 2 phylloquinone (vitamin K,) molecules (for molecular structures see Fig. 2). The cofactors of the ET chain are arranged in two branches as pairs of molecules related by a pseudo-C2 axis. After light excitation an electron is donated from the primary donor P700, a pair of chlorophylls, to monomeric chlorophyll a (acceptor A0), phylloquinone (A() and the 3 iron-sulfur centres (F , Oa and B). It has been controversially discussed in the literature whether both highly symmetric pigment branches are... 

Although covalently linked donor-acceptor systems of small organic chromo-phores have been studied for some time in order to uncover the basic principles of electron and energy transfer, the first covalently linked cyclic tetrapyrroles were reported by Gouterman, Dolphin and coworkers in 1972 [27]. In 1976 the first dimeric chlorophyll-based models were reported. Structure la, based upon pyropheophorbide-a, was prepared by Boxer and Closs [28], whereas the pheo-phorbide-a derivative lb was reported by Wasielewski, Studier and Katz [29]. 

Using this principle, light induced charge separation has been achieved for a number of donor/acceptor pairs. Noteworthy are the examples where pyrene served as an electron acceptor and dimethyl aniline as a donor as well as the photoinduced reduction of duroquinone by chlorophyll-a. Experimental data obtained for the latter system are presented in Fig. 4.6. The upper two traces represent absorption versus time curves from micellar solutions containing chl-a alone. They reflect formation and decay of chl-a triplet states. The time course of both the 685 and 465 nm absorption is drastically affected when DQ is co-solubilized with chl-a in the micelles. In particular, a long-term bleaching of chl-a absorption is noted which can be attributed to chl-a+ cation radical formation via ... 

Hunter CN, ArtymiukPJ and van Amerongen H (1994b) Many chlorophylls make light work. Curr Biol 4 344-346 Jeevarajan JA and Kispert LD(1996) Electrochemical oxidation of carotenoids containing donor/acceptor substituents. J Electroanal Chem 411 57-66... 

An approach, similar to that of the chelated heme model systems, has been adopted by many researchers to prepare porphyrins with appended quinone groups. Such systems have stimulated interest as possible models for the primary electron transfer event of photosynthesis, where photoinduced charge transfer occurs from excited singlet state chlorophyll donors to nearby quinone acceptors. 

Because of its ionic nature magnesium forms very few coordination compounds. Donor-acceptor species in aqueous solutions are short-lived, but magnesium bromide and iodide have sufficient acceptor properties to dissolve in donor solvents such as alcohols and ketones. Magnesium appears to be an essential element for life, occurring in chlorophyll. 

The majority of studies on the distance dependence in electron transfer systems have involved modified enzymatic substrates/" " potential models of chlorophyll photosystems/ and donor-acceptor pairs separated by phospholipid bilayers/ There has been considerable progress in these areas despite the formidable synthetic problems, the difficulty in systematically varying only one parameter in a series of related studies, and the difficulty in evaluating k u in such complicated systems. Some reviews of this work have appeared. Electrochemical studies, using organic bridging groups attached to metal surfaces, should also be noted. ... 

For examples see a) Wiederrecht, G.P., M.R Niemczyk, W.A. Svec, and M.R. Wasielewski (1996). Ultrafast photoinduced electron transfer in a chlorophyll-based triad VibrationaUy hot ion pair intermediates and dynamic solvent effects. J. Am. Chem. Soc. 118(1), 81-88 and b) Shiratori, H., T. Ohno, K. Nozaki, I. Yamazaki, Y. Nishimura, and A. Osuka (1998). Coordination control of intramolecular electron transfer in boronate ester-bridged donor-acceptor molecules. Chem. Commun. (15), 1539-1540. 

The photosystem 11 reaction centre complex isolated in Triton X-100 (1,2) does not retain the intrinsic quinone acceptors, and Q, but does show electron transfer to the artificial electron acceptor, silicomolybdate (3). In the presence of this acceptor the light minus dark difference spectrum, which we attribute to oxidation of P680, includes a distinct shoulder at about 670 nm (4). Reaction centres isolated in Triton X-100 and investigated under aerobic conditions have been found to be very susceptible to photodamage (4,5). Here we have used maltoside-exchanged reaction centres and anaerobic conditions to investigate in more detail the photoaccumulation of oxidised chlorophyll donors and their absorption characteristics. 

There has been much speculation that the A state is important in carotenoid to chlorophyll energy transfer. Depending upon the size of the A-B state energy gap and the relative widths of the A and B state emissions, energy transfer to chlorophyll from the energetically lower carotenoid A state might have more favorable donor-acceptor spectral overlap than transfer from the carotenoid B state. Also, since B to A internal conversion occurs within 100 fs following initial population of the optically accessible B state [4, 5, 6], transfer solely from the B state would have to be extremely rapid. There has, however, never been direct evidence of the involvement of a carotenoid A state in carotenoid to chlorophyll... 

Parson, W. W. (1991) Reaction centers, in H. Scheer (ed.). Chlorophylls, CRC Press, p. 1153. Platt, J. R. (1959) Carotene-donor-acceptor complexes in photosynthesis. Science 129,372. 

The immediate electron acceptor for P700 is a special molecule of chlorophyll. This unique Chi a (Aq) rapidly passes the electron to a specialized quinone (Aj), which in turn passes the e to the first in a series of membrane-bound ferredoxins (Fd, Chapter 21). This Fd series ends with a soluble form of ferredoxin, Fd, which serves as the immediate electron donor to the fiavo-protein (Fp) that catalyzes NADP reduction, namely, ferredoxin NADP reductase. 

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