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The chlorophylls are a structurally and functionally distinct group of macrocyclic tetrapyrrole pigments that may occur at the porphyrin, chlorin or bacteriochlorin oxidation levels. Biosynthetically, they are derived from protoporphyrin IX. Structurally, they are characterized by the presence of a fifth ring, isocyclic ring E, which confers the prefix ‘phyto’ to the porphyrin and chlorin type molecules. Chlorophylls generally have Mg as the central metal and a long-chain esterifying alcohol at C-17.

Chlorophylls (Chls) and bacteriochlorophylls (BChls) are complex tetrapyrroles that continue to attract the attention of synthetic chemists. While Chl reactivity is similar to that of porphyrins, the reduced pyrrole rings which lower their stability and the complex substituent pattern which is observed in nature, makes their syntheses challenging. This chapter reviews the literature from 1990 to 2001 and highlights selected new developments in the organic and structural chemistry of Chl and BChl derivatives with the basic five-ring phytochlorin macrocycle. Structural studies have concentrated on elucidating the influence of different metals and peripheral substituents on the conformational flexibility of the underlying macrocycle. In synthetic chemistry, no significant efforts were made with respect to novel total syntheses of Chls; however, significant advances were made with partial syntheses of numerous BChls with phytochlorin structure, the preparation of divinyl-pheophorbides (Pheides), and the modification of the 3-vinyl group and ring E in Pheides.

Chlorophyll (Chl) pigments are found in nine Divisions of aquatic chromophyte algae, co-occurring with Chl and carotenoids in chloroplast thylakoids, and in two Divisions of photosynthetic prokaryotes. Chls differ from Chls , and in being Mg-phytoporphyrins rather than Mg-chlorins. In addition to Chls , and , many new Chl -like pigments have recently been isolated and characterized in parallel with advances in separation techniques and spectroscopic methods. Since 1990, the number of known Chls increased from seven (Chls , , , Chl c, a Chl -like pigment from , [DV]-PChlide (i.e. MgDVP) and a nonpolar Chl -like pigment) to eleven compounds. Novel Chls include [MV]-Chl from the haptophyte , a Chl -like pigment from the dinoflagellate and two Chl pigments esterified with monogalactosyl diacylglycerides (MGDG) which have been identified as Chl -MGDG (18:4/14:0) from and Chl -MGDG (14:0/14:0) from , respectively. In addition, one nonpolar Chl -like pigment has been isolated from . Chl is the major Chl pigment found in all Divisions of chromophyte algae, either alone or with significant quantities of Chls and/or . Chl diversity is highest in the Haptophyta.

Primary charge separation in photosynthesis is initiated by a few specialized chlorophyll (Chl) or bacteriochlorophyll (BChl) molecules in the reaction centers. Excitation of the primary donors leads to reduction of the primary and secondary electron acceptors, which often contain structurally distinct Chl and BChl derivatives: such specially-tailored Chls include ‘prime-type,’ ‘metal-free’ and ‘Zn-containing’ Chls. The functions of many such pigments have long remained unclear, but recently the roles of some have been elucidated. Here, a short overview is given on the minor but specially-tailored Chls that function as key components in photosynthesis.

This chapter discusses the occurrence, properties and relevance of chlorophyll (Chl) degradation products that are formed in vivo in heavy metal-stressed plants or by digestion of algae in marine invertebrates, that are formed during extraction or processing of dead plant material. The in vivo substitution of the central Mg ion of chlorophyll by heavy metals constitutes an important part of the damage occurring in metalstressed plants. In , this reaction varies with light intensity. In low irradiance combined with a dark phase, the light-harvesting complex II (LHC II) is the main target, while in high irradiance the LHC II is inaccessible to substitution of Mg. Instead, an insertion of heavy metals into the pheophytin (Phe) of the Photosystem II reaction center (PS II RC) has been proposed. In algae with different light harvesting proteins, this light-dependent difference is absent. In brown algae, Chl in the Chl LHC is always accessible to substitution of Mg by heavy metals.

Analysis by a combination of absorption-, fluorescence-, circular dichroism-, mass- and nuclear magnetic resonance- spectrometry is often used to investigate the structure of chlorophylls. Here, we show several examples of spectroscopic determination of molecular structure of the recently-discovered chlorophylls and compare them with the well-known chlorophylls, chlorophyll and bacteriochlorophyll .

Chlorophyll (Chl) assays derive their importance from the essential role of Chls in the harvesting of solar energy and its transduction to biologically useful chemical energy (ATP) and reducing power (NADPH or NADH) during photosynthesis in higher plants, marine and aquatic algae, and in photosynthetic bacteria. Accurate determination of Chl and concentrations and of Chl ratios has been an essential tool in photosynthesis research in higher plants and green algae. Spectrophotometric and spectrofluorimetric assays, relying on the characteristic absorption and fluorescence properties of the chlorophylls, will be described and accurate data presented for spectrophotometric assays in a wide variety of solvents.

The improvements in high performance liquid chromatography (HPLC) analysis of chlorophylls (Chls) and bacteriochlorophylls (BChls) during the last decade rely mainly on the application of newly developed stationary phases combined with new mobile phases developed with special regard to the nature of the ionpairing agents employed for achieving the retention of free acid forms, which is especially important for the Chls c group. The application of mass spectrometry (MS) as a detection technique coupled on-line with liquid chromatography (LC) has provided important structural information. All these tools have contributed to the discovery of the biosynthetic pathways of higher plant Chls, to the study of the composition and distribution of new Chls in algae, and to the characterization of complex pools of BChls. Faster, more sensitive and more specific HPLC methods are expected in the very near future, with the development of new columns and detection techniques, e.g. monolithic stationary phases and coupled liquid chromatography-nuclear magnetic resonance (LC-NMR) systems.

Convenient laboratory techniques for large-scale preparations of chlorophylls (Chls) using dioxane precipitation and column chromatography are described. Partially purified Chls are obtained using a precipitation procedure that involves the selective interaction of Chls with dioxane in the presence of water. This procedure can be used to separate Chls from most of the carotenoids and some lipids present in the initial extract. Simple techniques of open-column chromatography are described using diethylaminoethyl [DEAE]-Toyopearl and Sepharose CL-6B in combination, DEAE-cellulose and powdered sucrose. These chromatographic techniques rely on the use of the partially purified Chls obtained by the dioxane precipitation procedure as a starting material, which makes preparation easier and very effective. An application of these techniques for the isolation of bacteriochlorophyll is also described. Despite the simple procedures, and without elaborate facilities or expensive reagents, contamination of the Chl preparations is low as judged by high-performance liquid chromatography. The methods described are rapid and give good yields of Chls without causing chemical modifications during manipulations.

This introductory chapter collates and highlights important issues in the chlorophyll biosynthetic and degradative pathways, which are described and discussed in the following seven chapters. These pathways occur in a large and very diverse range of photosynthetic organisms including photosynthetic bacteria, algae and higher plants.