In 1997, the first edition of Phytoplankton Pigments in Oceanography was published (Jeffrey et al., 1997) in which a chapter was presented by Porra et al. (1997) on photosynthetic pigments, namely, chlorophylls (Chls), phycobilins and carotenoids. In this current volume, the phycobilins and carotenoids are discussed elsewhere (Chapters 9 and 3, this volume). The earlier presentation (Porra et al., 1997) only briefly described the Chl biosynthetic pathway while addressing the functions and locations of Chls in protein complexes of both the light-harvesting antenna complexes and reaction centres of the two photosystems present in the chloroplasts of higher plants and green algae. A comprehensive survey of Chl biodegradation was also described in the earlier presentation (Porra et al., 1997) but more recent information is now available (Kräutler and Hörtensteiner, 2006). In this current chapter, a more comprehensive account of the Chl biosynthetic pathway is presented together with the structures of many of the naturally occurring Chls, with a special focus on recently discovered Chls and their possible syntheses.
Structures of chlorophylls
The structures and properties of naturally occurring Chls have been extensively reviewed (Scheer, 1991, 2003, 2006). The Chls are mostly magnesium coordination complexes, but also rarely Zn-coordination complexes (see Sections 2.2.3 and 184.108.40.206), of cyclic tetrapyrroles which contain a fifth isocyclic (cyclopentanone) ring E constructed enzymically from the 13-propionate side chain of Mg-protoporphyrin IX (see Section 2.4.3): for the IUPAC-IUB tetrapyrrole atom numbering and ring labelling systems, see Figure 2.1B. All Chls possess a 131-oxo group and mostly, but not always, a 132 methylcarboxylate substituent while the 17-propionate side chain is usually, but not always, esterified with a long-chain isoprenoid alcohol such as phytol, farnesol or geranyl-geraniol.