Chlorophyll deexcitations

Figure 4-9. Energy level diagram indicating the principal electronic states and some of the transitions of chlorophyll. Straight vertical lines represent the absorption of light wavy lines indicate radiationless transitions, for which the energy is eventually released as heat and broken lines indicate those deexcitations accompanied by radiation. In the literature, for chlorophyll is also referred to as S0, as Slt S H as S2, and T sfj as Tt (similar symbols occur for carotenoids).

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. 

This second molecule thereby becomes excited, indicated by S2(W)W-), and the molecule that absorbed the photon becomes deexcited and is returned to its ground state. Such transfer of electronic excitation from molecule to molecule underlies the energy migration among the pigments involved in photosynthesis (see Chapter 5, Sections 5.3 and 5.4). We will assume that Equation 4.8 represents a first-order reaction, as it does for the excitation exchanges between chlorophyll molecules in vivo (in certain cases, Eq. 4.8 can represent a second-order reaction, i.e., dS /dt then equals fc S j). 

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... 

Figure 5-7. Energy level diagram including vibrational sublevels, indicating the principal electronic states and some of the transitions for carotenoids. The three straight vertical lines represent the three absorption bands observed in absorption spectra, the wavy lines indicate possible radiationless transitions, and the broad arrows indicate deexcitation processes (see Fig. 4-9 for an analogous diagram for chlorophyll).

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. 

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. 

As we have just calculated, each chlorophyll molecule in an unshaded chloroplast can absorb a photon about once every 0.1 s. When there are 250 chlorophylls per reaction center, 12.5 of these molecules are excited every 5 ms (250 chlorophylls x 10 excitations per chlorophyll/1 s x 0.005 s). However, because the average processing time per reaction center is about 5 ms, only one of these 12.5 excitations can be used photochemically — the others are dissipated by nonphotochemical deexcitation reactions. Consequently, although the biochemical reactions leading to CO2 fixation operate at their maximum rates under such conditions of high PPF, over 90% of the electronic excitations caused by light absorption are not used for photosynthesis (Fig. 5-12). 

Energy transfer between chlorophyll and carotenoids occurs by two pathways a static route involving no intramolecular motion, and a dynamic mechanism which involves movement of the two chromophores. Sufficient carotenoid molecules are normally present to deexcite the chl and O2 present, but if this concentration is reduced, for example, by inhibition of carotenoid biosynthesis, then 2 accumulates, and rapid destruction of membranes occurs by peroxidation. " ... 

---