Chlorophyll lifetime

The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

Holub, O., Seufferheld, M. J., Gohlke, C., Govindjee, G. J., Heiss, G. J. and Clegg, R. M. (2007). Fluorescence lifetime imaging microscopy of Chlamydomonas reinhardtii Non-photochemical quenching mutants and the effect of photosynthetic inhibitors on the slow chlorophyll fluorescence transients. J. Microsc. 226, 90-120. 

A potentially more serious problem than that of low efficiency is the requirement that the photochemical absorber operate without any significant side reactions. For example, if the quantum yield for side reactions were 1%, then after only 100 cycles the concentration of the absorber would have decreased to 37% of its original concentration. It is interesting that in photosynthesis each chlorophyll molecule processes at least 10 photons in its lifetime in a leaf. This means that the quantum yield for reactions leading to the degradation of chlorophyll must be less than 10-5 ... 

Photon The amount of chlorophyll a (Mr 892) in a spinach leaf is about 20 pg/cm2 of leaf. In noonday sunlight (average energy 5.4 J/cm2 min), the leaf absorbs about 50% of the radiation. How often does a single chlorophyll molecule absorb a photon Given that the average lifetime of an excited chlorophyll molecule in vivo is 1 ns, what fraction of the chlorophyll molecules is excited at any one time ... 

A major thrust of recent work on FTMS of large biomolecules has dealt with questions concerning the lifetime of molecular ions formed by high-energy particle bombardment. Chait and Field have reported that a large fraction of the molecular ions of chlorophyll A formed by 252Cf fission fragment ionization decomposes with lifetimes of less than a few... 

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. 

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

The rather long lifetime of the radical-pair in the reaction center core (30ns) led us to examine its lifetime in other PS-II preparations. It was hypothesized that an equilibrium was established between the radical-pair and the singlet excited state of P-680 or of chlorophyll a in the antenna (we assume that the antenna is made of n chlorophyll molecules) ... 

The primary processes of photochemistry involve the light absorption event, which we have already discussed, together with the subsequent deexcitation reactions. We can portray such transitions on an energy level diagram, as in Figure 4-9 for chlorophyll. In this section we discuss the various deexcitation processes, including a consideration of their rate constants and lifetimes. 

To illustrate the use of Equation 4.16, let us consider the quantum yield for chlorophyll fluorescence, Of. The fluorescence lifetime tf of the lower excited singlet state of chlorophyll is 1.5 x 10-8 s, and the observed lifetime r for deexcitation of this excited state in ether is 0.5 x 10-8 s. By Equation... 

In Chapter 4 (Section 4.3B) we noted that an upper time limit within which processes involving excited singlet states must occur is provided by the kinetics of fluorescence deexcitation. The lifetime for chlorophyll fluorescence from the lower excited singlet state is about 1.5 x 10 8 s. Time is therefore sufficient for approximately 10,000 transfers of excitation among the Chi a molecules—each transfer requiring 1 or 2 x 10 12 s—before the loss of the excitation by the emission of fluorescence. The number of excitation transfers among Chi a molecules is actually much less than this for reasons that will shortly become clear. 

Because one of the trap chi s is present per approximately 230 chlorophylls (Table 5-1), on average only a few hundred transfers are necessary to get an excitation from Chi a to P680 or P70o- Thus the calculated 10,000 transfers of excitation from one Chi a to another possible within the fluorescence lifetime do not occur. Because each excitation transfer takes 1 or... 

The characteristics of fluorescence also provide information on the lifetime of the excited singlet state of chlorophyll in vivo and thus on the time available for migration of excitations. Specifically, approximately 1 to... 

When the excitation migrates to a trap such as P680 or P70o> this special Chi a dimer goes to an excited singlet state, as would any other Chi a. Because the trap chi cannot readily excite other chlorophylls by resonance transfer, it might become deexcited by the emission of fluorescence. However, very little fluorescence from the trap chi s is observed in vivo. This is explained by the occurrence of a relatively rapid photochemical event (see Eq. 5.5 trap chi + A — trap chl+ + A ) the donation within 10-10 s of an electron to an acceptor prevents the deexcitation of the trap chi s by fluorescence, which has a longer lifetime. 

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