**1. Introduction**

134 Artificial Photosynthesis

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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

An Approach Based on Synthetic Organic Chemistry Toward Elucidation

AcO OH •

Allene function

AcO OH •

AcO OH •

> S O O S N

S O O S N

> S O O S N

> S O O S N

**13**: C17-Olefin Segment

**12**: C17-Acetylene Segment

AcO OH •

Fig. 2. Structure of peridinin and its derivatives

•

derivatives **2**-**9** by this efficient strategy.

AcO

AcO

AcO

O

**2**: Acetylene Derivative

**3**: Olefin Derivative

**4**: Diolefin Derivative

AcO OH

AcO OH

O

•

O

AcO

AcO

Fig. 3. Synthetic strategy

O

of Highly Efficient Energy Transfer Ability of Peridinin in Photosynthesis 137

half segments such as **10** and **11** with the suitable -ylidenebutenolide half segments such as **14** and **15**, respectively. Thus, a coupling between C15-allenic segment **11** and C20 ylidenebutenolide segment **14** would produce C35 peridinin derivative **6**. Meanwhile, applying the same method to the coupling between C17-allenic segment **10** and C22 ylidenebutenolide segment **15** might produce the desired C39 peridinin derivative **7**, if the coupling product would be enough stable to be handled. On the other hand, we planned to synthesize acetylene ester derivative **8** and olefin ester derivative **9** by a coupling between the C17-allenic half-segment **10** and the corresponding ylidenebutenolide-modified halfsegments **16** and **17**. We have really achieved the synthesis of these complex peridinin

OH

O

OH

Ylidenebutenolide ring

OH

OH

CO2Me

**8**: Acetylene Ester Derivative

**9**: Olefin Ester Derivative

OH

OH

OH

OH

O

O

O

O

O O

CO2Me

CO2Me

**16**: C20-Acetylene Methyl Ester Segment

**17**: C20-Olefin Methyl Ester Segment

<sup>O</sup> <sup>O</sup>

O

CO2Me

O

O

OH

OH

O O

O

O

O O

**10**: C17-Allenic Segment **14**: C20-Ylidenebutenolide Segment

Allene modified half-segment Ylidenebutenolide modified half-segment

**11**: C15-Allenic Segment **15**: C22-Ylidenebutenolide Segment

O H

> O H

> > H

O H O

C37-Peridinin (**1**)

**5**: C33-Peridinin Derivative

**6**: C35-Peridinin Derivative

**7**: C39-Peridinin Derivative

Allene-modified Derivatives Polyene chain-modified Derivatives Ylidenebutenolide-modified Derivatives

O O

> O O

subjects of why peridinin possesses a unique allene group, a ylidenebutneolide ring and an irregular C37 skeleton, and how these functions play a role in the exceptionally high energy transfer and in the special excited state, ICT state.

Fig. 1. (A) Crystal structure of PCP complex and (B) the structure of peridinin
