Energy-transfer reactions, carotenoids

The lack of energy transfer in 24 is in marked contrast to the results for a variety of other bichromophoric molecules where singlet energy transfer occurs over many tens of angstroms via the Forster dipole-dipole mechanism. Since the effidency of Forster energy transfer depends upon the fluorescence quantum yield of the donor, we postulated that the lack of energy transfer in 24 was due to the very low fluorescence quantum yield of the carotenoid, and further concluded that energy transfer from carotenoid polyenes to chlorophyll in photosynthetic reaction centers could therefore not occur by the dipole-dipole mechanism [72]. 

Fig. 15. (A) Absorption, fluorescence and phosphorescence spectra of BChl a in vitro at 77 K spectra scaled for convenient presentation also note break of horizontal scale (B) Phosphorescence spectrum of quinone-depleted (-Q) and quinone-containing (+Q) Rb. sphaeroides reaction centers in polyvinyl-alcohol film at 22 K (C) Energy diagram for the components involved in triplet-triplet energy transfer with carotenoids. (A) and (B) and numerical values for the triplet-state energies of BChls a and b and the primary-donors of Rb. sphaeroides and Rp. viridis, i.e., [BChl a and [BChl bjj, respectively, are taken from Takiff and Boxer (1987) Phosphorescence spectra ofbacteriochlorophylls. J Am Chem Soc 110 4425.

Wu Y, Piekara-Sady L and Kispert LD(1991) Photochemically generated carotenoid radicals on Nafion film and silica gel An EPR and ENDOR study. Chem Phys Lett 180 573-577 Yeates TO, Komiya H, Chirino A, Rees DC, Allen JP and Feher G (1988) Structure of the reaction center from Rhodobacter sphaeroides R-26 and 2.4.1 Protein-cofactor (bacterio-chlorophyll, bacteriopheophytin, and carotenoid) interactions. Proc Natl Acad Sci USA 85 7993-7997 Young AJ (1991) The photoprotective role of carotenoids in higher plants. Physiol Plant 83 702-708 Young AJ and Frank HA (1996) Energy transfer reactions involving carotenoids Quenchingofchlorophyll fluorescence. J Photochem Photobiol B Biol 36 3-15... 

A, Absorption chi, chlorophyll car, carotenoid EET, excitonic energy transfer EF, exoplasmic fracture face EM, electron microscopy FWHM, full width at half maximum lEF, Isoelectric Focusing, LD, linear dichroism LHC, light harvesting complex PAGE, polyacrylamide gel electophoresis PF, protoplasmic fracture face PS, photosystem RC, reaction centre SDS, sodium dodecyl sulphate SSTT, single step transfer time. 

Energy transfer studies of 11 and related molecules demonstrated that the carotenoid moiety is active in antenna function (singlet-singlet energy transfer) and photoprotection (triplet-triplet energy transfer) in these molecules, just as it is in natural reaction centers [51, 62]. 

Cogdell, R.J., Parson, W.W. and Kerr, M.A. 1976. The type, amount, location and energy transfer properties of the carotenoid in reaction centers from Rhodopseudomonas sphaeroides. Biochim. Biophys. Acta, 430. 83-93. 

Light harvesting and energy transfer by the chlorophyll (Chi) and carotenoid molecules of the antenna (A) complexes to the reaction center (RC) of PSII. 

Miscellaneous Physical Chemistry. A kinetic study has been made of the electrochemical reduction of /8-carotene. The photoelectron quantum yield spectrum and photoelectron microscopy of /3-carotene have been described. Second-order rate constants for electron-transfer reactions of radical cations and anions of six carotenoids have been determined. Electronic energy transfer from O2 to carotenoids, e.g. canthaxanthin [/8,/3-carotene-4,4 -dione (192)], has been demonstrated. Several aspects of the physical chemistry of retinal and related compounds have been reported, including studies of electrochemical reduction, the properties of symmetric and asymmetric retinal bilayers, retinal as a source of 02, and the fluorescence lifetimes of retinal. Calculations have been made of photoisomerization quantum yields for 11-cis-retinal and analogues and of the conversion of even-7r-orbital into odd-TT-orbital systems related to retinylidene Schiff bases. ... 

Miscellaneous Physical Chemistry. Various aspects of the physical chemistry of /3-carotene and related carotenoids have been reported, including several theoretical calculations related to spectroscopic properties,investigations of carotenoid triplet states and triplet energies,studies of carotenoid radical ions, and examination of electron-transfer reactions between carotenoids and chlorophyll Two reviews offer brief surveys of the year s literature on the photochemistry of... 

Fig. 1 (B) illustrates the second major function of carotenoids, namely, photoprotection of chlorophyll, either by direct triplet-triplet energy transfer [labeled T-T j from Chl to the carotenoid or by indirect transfer via the formation of singlet oxygen. The reaction steps for each of the two mechanisms are shown in Fig. 1, bottom, right. Further details on both the light harvesting by carotenoids and photoprotective role of carotenoids will be described in the following sections.

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