Chlorophylls and carotenoids

The most important functional parts of a photosynthetic antenna are the chromophores. All photosynthetic organisms use either chlorophylls (mainly Chl a and Chl b, but also a few other Chls) or bacteriochlorophylls (e.g. BChl a and BChl b) as pigments. The chromophore moiety of the Chls and BChls are magnesium-containing cyclical tetrapyrroles, the so-called chlorins and bacteriochlorins. Figure 1 . 4 ~sh ows the structures of the most common Chls and BChls. All Chls also contain a long ester side chain, which help in anchoring them in their surrounding protein by hydrophobic interactions. In addition all photosynthetic antennae contain carotenoids, which play a dual function as energy donors and photoprotective pigments (for an overview, see Smith, 1991).
 

Because the pigments in antenna systems must be held at a certain range of optimal distances and orientations to each other and to the RCs, all known antenna systems (with one notable exception-the chlorosome structures discussed below) are pigment-protein complexes. The proteins both hold the pigments in a defined position and provide the possibility of fine-tuning their absorption properties by specific pigment-protein interactions. All Chls and BChls, as well as the carotenoids, are attached non-covalently to the proteins by way of either weak ligand-metal interactions of the central Mg of the Chls to amino acid side chains (often histidine), or by other non-covalent interactions such as hydrogen bonding, En-interactions or hydrophobic interactions with their long ester side chains (Smith, 1991).

A few families of photosynthetic organisms make use of additional chromophores and/or other interaction principles in their antennae. A notable, and indeed in all respects exotic, antenna system is employed by the green sulphur bacteria and green gliding bacteria in their chlorosomal antennae. As briefly explained in Chapter 1 and shown in Fig. 1.8, these are extramembranous antennae, located on the inner surface of the cytoplasma membrane, which do not contain proteins in their central part. Instead these chlorosomal antennae consist of huge (several tens to hundred thousands of Chls each) pigment aggregates formed by self-organisation of the special bacteriochlorophylls BChl c, d, e. As Fig. 2.3 shows, these special BChls of green bacteria are characterised by their 3'-OH groups. These are not found in any of the usual Chls or BChls and they enable the molecules to interact closely with each other. Actually these 'bacteriochlorophyl1s'-as they are traditionally called-are chlorophylls, rather than true bacteriochlorophylls in terms of their electronic structures (Smith, 199 1).
 

Another class of photosynthetic pigments is found in the phycobiliproteins of cyanobacteria, red algae and cryptomonads. These are open-chain tetrapymoles which are covalently bound to their apoproteins, typically via S-cysteine linkages (Stanier, 1974; Wildman and Bowen, 1974; Gantt, 1975; Zuber, 1978; Scheer, 1981; Holzwarth, 1986; Holzwarth, 1991; Schaffner et al., 1991; MacColl and Guard-Friar, 1987).