Carotenoids fluorescence yield

Theoretical calculations have been made for photosynthetic pigments . An extensive review of models of energy and electron transfer events of synthetic molecules for photosynthesis has been prepared by Wasielewski . Other studies have made on tetraphenylporphyrin-polypeptide pigments , photosensitization of triplet carotenoids , fluorescence yields and lifetimes for bacteriochlorophyll c , triplet yields and ESR of chlorophyll 3 8 and quenching processes of pheophytin 539 ... 

Sin< recognition that antenna carotenoids have a low lying Sj electronic state it has l en generally assumed that EET to chi proceeds from this state. This is based largely on two considerations. 1. Spectral overlap between the carotenoid Si state and the Qy chi absorption transition is more favourable than for the S2 state. This assumption is however not supported by the recent study of Mimuro et al. [204] in which spectral overlap can be shown to be rather similar for a number of mainly Si and mainly Sj emitters. This point may therefore need further clarification. 2. Extremely fast Sj-Si internal conversion. Recent studies based on fluorescence yield measurements and also on femtosecond absorption measurements [200,205,207] indicate in vitro Sj-Si relaxation times of... 

Rhee KH, Morris EP, Zheleva D, Hankamer B, Kiihlbrandt W and Barber J (1997) Two dimensional structure of plant Photosystem II at 8 A resolution. Nature 389 522-526 Rock CD and Zeevaart JAD (1991) The aba mutant of Arabidopsis thaliana is impaired in epoxy-carotenoid biosynthesis. Proc Natl Acad Sci USA 88 7496-7499 Rock CD, Bowlby NR, Hoffmann-Benning S and Zeevaart JAD (1992) The aba mutant of Arabidopsis thaliana (L) Heynh. has reduced chlorophyll fluorescence yields and reduced thylakoid stacking. Plant Physiol 100 1796-1801 Romer S, Humbeck K and Senger H (1990) Relationship between biosynthesis of carotenoids and increasing complexity of Photosystem I in mutant C-6D of Scenedesmus obliquus. Planta 182 216-222... 

These recent experiments demonstrate that it should be possible to use fluorescence to detect resolved, S, - S( emissions in other long carotenoids, although such studies will require samples ofhigh purity and should include a careful analysis of fluorescence excitation spectra to confirm the source of any weak, low energy emissions. The extremely low S, Sp fluorescence yields (<10" formolecies such as 13-carotene) put heavy demands both on the quality of... 

In order to compare our in vitro fluorescence data with carotenoid fluorescence in vivo we have studied the fluorescence of spheroide-ne in the B800-850 complex from R, spheroides. The quantum yield was found to be 2 10, in good agreement with results obtained... 

The iodine-catalyzed photoisomerization of all-trans- a- and (3-carotenes in hexane solutions produced by illumination with 20 W fluorescence light (2000 lux) and monitored by HPLC with diode-array detection yielded a different isomer distribution (Chen et al. 1994). Four cis isomers of [3-carotene (9-cis, 13-cis, 15-cis, and 13,15-cli-r/.v) and three cis isomers of a-carotene (9-cis, 13-cis, and 15-ri.v) were separated and detected. The kinetic data fit into a reversible first-order model. The major isomers formed during the photosensitized reaction of each carotenoid were 13,15-di-d.v- 3-carotene and 13-ds-a-carotene (Chen et al. 1994). 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

Since the initial reports of the C-P-Q triads, a number of other molecules of the D-D -A or D -D-A types have been described. Triad 12, prepared by Wasielewski and coworkers, is a relative of the C-P-Q series in which the secondary donor is an aniline derivative (D), rather than a carotenoid [63]. The bicyclic bridges were introduced in order to add rigidity to the system. The fluorescence lifetime of the porphyrin moiety of 12 was found to be <30ps. This result is consistent with rapid electron transfer to the quinone to yield D-P+-QT. This result was confirmed by transient absorption measurements. The absorption results also revealed that this intermediate charge separated state decays with a rate constant of 1.4 x 1010 s-1 to a final charge separated state D+-P-Qr. Thus, the decay pathways are similar to those shown in Fig. 3 for the C-P-Q triads. This final state has a lifetime of 2.45 ps in butyronitrile (which is similar to that found for 4 in acetonitrile) [44], and is formed with a quantum yield of about 0.71. Thus, the efficiency of the transfer analogous to step 4 in Fig. 3 for this molecule is also about 0.71. 

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. ... 

Excitation of the porphyrin moiety of 2 in dichloromethane solution yields the first excited singlet state, which can decay according to the pathways detailed in Figure 4. As with P-Q dyad 1, photoinitiated electron transfer competes with other decay processes to yield a C-P -Q charge-separated state. Fluorescence decay studies yielded a fluorescence lifetime r of 0.10 ns for 2 [27]. The hydroquinone form of the triad, 3, in which such electron transfer is not possible, has a fluorescence lifetime of 3.4 ns in the same solvent (see Section 111.A.). Application of Eq. (1) yields an electron transfer rate constant fcj in Figure 4 of 9.7 x 10 s", and consequently a quantum yield for this step of essentially unity. Thus, the addition of the carotenoid moiety to the molecule has had little influence upon the initial photodriven electron transfer step. 

Information regarding the solution conformation of 13 was derived from the pyropheophorbide ring current induced shifts in the resonance positions of the carotenoid and quinone moieties. These two species were found to be extended away from the tetrapyrrole, rather than folded back across it. The absorption spectrum of 13 was essentially identical to the sum of the spectra of model compounds. The pyropheophorbide fluorescence, however, was strongly quenched by the addition of the quinone. This implies the formation of a C-Phe -Q state via photoinitiated electron transfer from the pyropheophorbide singlet state, as was observed for C-P-Q triads (see Figure 4). Excitation of the molecule in dichloromethane solution at 207 K with a 590 nm laser pulse led to the observation of a carotenoid radical cation transient absorption. Thus, the C-Phe -Q " state can go on via an electron transfer step analogous to step 4 in Figure 4 to yield a final C -Phe-Q state. This state had a lifetime of 120 ns. The quantum yield at 207 K was 0.04. At ambient temperatures, the lifetime of the carotenoid radical cation dropped to about SO ns, and the quantum yield could not be determined accurately because of the convolution of the decay into the instrument response function. 

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]. 

The low quantum yield of fluorescence in carotenoids is due to an extremely rapid, non-radiative internal conversion l By->2 Ag, followed by the fluorescence decay process 2 Ag->-l Ag. The lifetimes of excited 82(1 By) andSi (2 Ag) states are ofthe order of 0.1 ps and 10 ps, respectively. The 82 lifetime can... 

A maximum cellular content of carotenoids could be estimated since there was a quantitative relationship between total carotenoid content and average fluorescence intensity of a culture. A maximum yield of between 14,600 and 19,(XX) jig g was obtained. However, there was considerable heterogenity in autofluorescence among individual cells which suggests that the yield could be improved. Maximum synthesis could also be improved by directed transport, esterification of astaxanthin, or excretion as described below. 

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