Chlorophyll resonance transfer

The probability for resonance transfer of electronic excitation decreases as the distance between the two molecules increases. If chlorophyll molecules were uniformly distributed in three dimensions in the lamellar membranes of chloroplasts (Fig. 1-10), they would have acenter-to-center spacing of approximately 2 nm, an intermolecular distance over which resonance transfer of excitation can readily occur (resonance transfer is effective up to about 10 nm for chlorophyll). Thus both the spectral properties of chlorophyll and its spacing in the lamellar membranes of chloroplasts are conducive to an efficient migration of excitation from molecule to molecule by resonance transfer. 

Figure 5-11. Schematic representation of a group of pigments in a photosystem core that harvests a light quantum (hv) and passes the excitation to a special trap chlorophyll. Short straight lines indicate the inducible dipoles of chlorophyll molecules and the wavy lines indicate resonance transfer. In the reaction center an electron (e ) is transferred from the trap chi to some acceptor (A in the reduced form) and is then replaced by another electron coming from a suitable donor (D+ in the oxidized form).

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. 

Chlorophyll molecules absorb light as individual photons. Each can cause a single photochemical reaction. If there is no direct photochemical reaction, chlorophyll may lose its excitation energy as heat and red-fluorescence, or by resonance transfer. In fluorescence a high energy (short wavelength) photon is absorbed, which promotes an electron. The electron then drops to the lowest... 

The reaction center is a part of the photosynthetic apparatus in chloroplasts where the photochemical process occurs. Reaction centers are a specific pair of chlorophyll molecules in a photosystem that collect light energy absorbed by other chlorophyll molecules in a photosystem and pass it along to an electron acceptor, such as an electron transport chain. The remaining chlorophyll molecules are referred to as antenna molecules of the light-harvesting complexes because they absorb the photons and pass the energy by resonance transfer to the reaction centers. The process occurs on the order of lO seconds. 

Resonance transfer is important for the way chlorophylls function in cells. Only about 1 02 molecule is produced for every 2500 chlorophylls. Instead, most of the chlorophyll molecules act as antenna molecules of the light-harvesting complexes. Antenna molecules absorb photons and the energy is passed by resonance transfer to specific chlorophyll molecules in a relatively few reaction centers. The path the energy takes to arrive at the energy center is random (Figure 17.11). 

The analysis of carotenoid identity, conformation, and binding in vivo should allow further progress to be made in understanding of the functions of these pigments in the photosynthetic machinery. One of the obvious steps toward improvement could be the use of continuously tuneable laser systems in order to obtain more detailed resonance Raman excitation profiles (Sashima et al 2000). This technique will be suitable for the investigation of in vivo systems with more complex carotenoid composition. In addition, this method may be applied for the determination of the energy of forbidden Sj or 2 Ag transition. This is an important parameter, since it allows an assessment of the energy transfer relationship between the carotenoids and chlorophylls within the antenna complex. 

A complete FRET (Forster resonance energy transfer) system based on chlorophyll in the pores of FSM materials was accomplished by Kuroda s group.149,150 They first functionalized the FSM samples with 3-aminopropyl groups to guarantee an ideal position of the macroscopic chlorin units (in the pore center) and prevent their denaturation. Then they ligated chlorophyll derivatives that possess 3-(triethoxy-silyl)-A-methylpropan-l -amine groups to the pore walls. Zinc (for the FRET donor) and copper (for the FRET acceptor) were chosen as the central ions of the chlorins, which made it possible to initiate and record an efficient FRET process (Fig. 3.14). 

Chlorophylls chlorophylls have a variety of functions in photosynthetic systems, including collection of photons, transfer of excitation energy, operation of the primary photoinduced charge separation, and transfer of the resulting photoelectrons. Resonance Raman spectroscopy offers the possibility of selectively observing chlorophylls in their native structures (Lutz, 1984 Koyama et al., 1986 Tasumi and Fujiwara, 1987 Lutz and Robert, 1988 Nozawa et al., 1990 Lutz and Mantele, 1991). Transient Raman spectroscopy is a unique method of revealing the excited state structures of chlorophylls. The T1 and SI states were revealed by nanosecond Raman spectroscopy (Nishizawa et al., 1989 Nishizawa et al., 1991 Nishizawa and Koyama, 1991). 

The excited antenna molecule passes energy to a neighboring chlorophyll molecule (resonance energy transfer), exciting it. 

Carotenoids function in photosynthetic reaction centers (RC) as triplet quenchers of the primary donor chlorophyll or bacteriochlorophyll triplet states. The best studied RCs are those of purple photosynthetic bacteria where atomic models are available based on X-ray crystallography and optical as well as magnetic resonance spectroscopies have yielded a detailed picture of the flow of triplet energy transfer. Good reviews of these topics can be found in (Frank, 1992, 1993 Frank and Cogdell, 1996). 

In a novel experiment, Koyama et al. (57) obtained a spectrum of carotenoid BLM by resonance Raman spectroscopy—a major advance in BLM spectroscopy. For efficient charge transfer, the orientation of chlorophyll molecules at the membrane-solution interface is important. Brasseur et al. (58) developed a procedure for conformation analysis to define the position of chlorophyll in BLM. They found that the porphyrin ring is orientated at an angle of 45 5° to the plane of the BLM, which is in excellent agreement with the value reported previously (44). 

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