**3. References**

130 Artificial Photosynthesis

c. Enhanced post-illumination respiration (EPIR) was first reported by Falkowski et al. (1985) and reexamined by Beardall et al. (1994). However, since that phenomenon that by steeply increasing respiration losses reduces net production and yields is dose

d. Thermal dissipation, also termed NPQ (nonphotochemical quenching [of fluorescence]) in variable fluorescence-based studies, may account for the loss of from one half to nearly all of the absorbed light energy. Thermal dissipation, measured directly by photoacoustics, depends on the light history of the organism, with losses increasing under high light, and any stress may account for losses of 30-80% of absorbed light

e. The xanthophyll cycle allows the fine tuning of the photosynthetic apparatus to ambient light by switching between two states of the pigment couples constituting the xanthophyll cycle. When exposed to low light, most of its energy is used in the photochemical photolysis of water and the subsequent reduction of CO2 to high energy photosynthate. Under high light, the xanthophyll cycle pigments undergo epoxidation and now divert light energy to harmless heat rather than damaging excess light (Adams et al., 1999; Demmig-Adams, 1998). Again, the relatively slow activation of the cycle, or its 'switching' to the high light state, prevents, or at least minimizes, its beneficial, albeit

f. RuBisCO - A dynamic model for photosynthesis was developed to elucidate the effect of flashing light that enhances the efficiency of photosynthesis. One particular feature of the model is that discrete RuBP particles circulate in the Calvin cycle and their speeds in the cycle are determined by the amount of ATP generated in the photon reception process. This can realize the light saturation under continuous light and the flashing light effect under fluctuating illumination. Laboratory experiments were conducted on

a. Short duration periods of intense light are too brief to result in damage to the photosynthetic apparatus leading to photoinhibition of photosynthetic rates. b. Short light periods do not allow sufficient time for the full activation of the xanthophylldriven thermal energy dissipation of absorbed light, which would reduce the energy

c. Short light periods reduce the biomass losses incurred due to enhanced post-

d. Dark intervals allow the regeneration and reoxidation of intermediate electron and CO2

In summary, we can conclude that present developments allow better understanding of the mechanisms involved in the long-known enhancement of photosynthesis under fluctuating and flashing light. Future research will focus on defining the boundaries of the process in terms of irradiance levels, frequencies, and duty cycle. Regarding the mechanisms, these are likely to be explored in detail in order to understand their relative contribution under different conditions, and with more careful examination of the taxonomic differences in both

This study was supported by NATO SfP 981883. We thank Sharon Victor for the English

dependent, the brevity of the high light exposure minimizes its effects.

energy (Dubinsky et al., 1998; Pinchasov et al., 2005).

costly, activity under fluctuating light.

*Chaetoceros calcitrans* (Yoshimoto et al., 2005). All of the above lead us to the following conclusions:

acceptors in the quinone pool and the Calvin cycle.

allocated to photochemistry.

illumination respiration.

characteristics and mechanisms.

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**7** 

*Japan* 

**An Approach Based on Synthetic Organic** 

Photosynthesis has driven the development of life which is powered by the efficient capture and conversion of sunlight. Carotenoids are naturally occurring pigments that absorb sunlight in the spectral region in which the sun irradiates maximally. These molecules transfer the absorbed energy to chlorophylls and the primary photochemical events of photosynthesis are initiated. More than a half of photosynthesis is performed in the ocean, although the oceanic photosynthesis is relatively less studied. Marine carotenoid, peridinin, has been known as the main light-harvesting pigment in photosynthesis in the sea and forms the unique water soluble peridinin-chlorophyll a (Chl a)–protein (PCP) complex. The crystal structure of the main form of the PCP trimer from *Amphidinium carterae* was determined by X-ray crystallography as shown in Fig. 1 (A) (Hoffman et al., 1996). Each of the polypeptides binds eight peridinin molecules and two Chl a molecules, and the allene function of peridinin exsists in the center of the PCP. In this complex, a so-called antenna pigment, peridinin exhibits exceptionally high (> 95%) energy transfer efficiencies to Chl a (Song et al., 1976; Mimuro et al., 1993). This energy transfer efficiency is thought to be related to the unique structure of peridinin, which possesses allene and ylidenebutenolide functions and the unusual C37 carbon skeleton referred to as a 'nor-carotenoid' (Fig. 1 (B)) (Stain et al., 1971). There are, however, no studies on the relationship between the structural

features of peridinin and its super ability for the energy transfer in the PCP complex.

In order to clear this efficient energy transfer mechanism, there are many and hot discussions in spectroscopic fields. In particular, the presence of an intramolecular charge transfer (ICT) excited state of peridinin has been proposed. It has been anticipated that the highly efficient energy transfer is caused through this key energy level, and a detailed discussion on this is described in the later chapters. This particular excited state is thought to be related to the intricate structure of peridinin. However, the precise nature of the ICT excited state and its role in light-harvesting have not yet been entirely clear, and there are no studies on the relation between the structural features of peridinin and its super ability for the energy transfer in the PCP complex. This is because the synthesis of various kinds of desired peridinin derivatives are not easy. Then, we started the research work to clear the

**1. Introduction** 

**Chemistry Toward Elucidation of** 

**Highly Efficient Energy Transfer** 

Takayuki Kajikawa and Shigeo Katsumura

*Kwansei Gakuin University* 

**Ability of Peridinin in Photosynthesis** 

