The five rings in chlorophylls are lettered A through E, and the substituent positions on the macrocycle are numbered clockwise, beginning in ring A, as shown in Fig. 4.1, according to the officially recognized International Union of Pure and Applied Chemistry (IUPAC) nomenclature. An older nomenclature known as the Fischer system is also shown in Fig. 4.1. All of the older literature uses the Fischer nomenclature, so it is necessary to be conversant with both systems. In this book, the IUPAC system will be used exclusively.
By convention, the y molecular axis of all chlorophylls is defined as passing through the N atoms of rings A and C, with the x axis passing through the N atoms in rings B and D. The z axis is perpendicular to the plane of the macrocycle. An extensive delocalized π electron system extends over most of the molecule, with the exception of ring D, in which the C‐17–C‐18 double bond is reduced to a single bond. The tail is formed by condensation of four five‐carbon isoprene units and is then esterified to ring D. It is often called the phytyl tail, after the polyisoprenoid alcohol precursor phytol that is attached during biosynthesis. It is also sometimes called the isoprenoid tail.
Most of the chlorophylls are classified chemically as chlorins rather than porphyrins, by virtue of the reduced ring D. Most of the bacteriochlorophylls are similarly called bacteriochlorins, because of the reduction of both rings B and D. All chlorophylls and bacteriochlorophylls contain the extra ring E, which is called the isocyclic ring.
Most chlorophyll‐type pigments contain three chiral carbon atoms, C‐132, C‐17, and C‐18. Bacteriochlorophyll a contains two additional chiral centers, C‐7 and C‐8. In all cases, the stereochemical fidelity of the biosynthetic enzymes is extremely high, so the compounds found in cells are a single species and not mixtures of diastereomers (except as noted below).
Figure 4.3 Chemical structures of chlorophylls a, b, c, d, and f. R1, R2, etc. refer to ring substituents. In some cases, more than one possible group can be found at some positions.
The structures of all major chlorophylls and bacteriochlorophylls are shown in Figs. 4.3 and 4.4. The distribution of photosynthetic pigments in different classes of photosynthetic organisms is given in Table 4.1.
Figure 4.4 Chemical structures of bacteriochlorophylls a, b, c, d, e, f, and g.
4.1.1 Chlorophyll a
Chlorophyll a is found in all known eukaryotic photosynthetic organisms. Among prokaryotes, it is found in large quantities only in the cyanobacteria (including the prochlorophytes), although traces of chlorophyll a or minor variants are found in some anoxygenic bacteria, where it is thought to have an important function as an intermediate in the electron transport chain. Some prochlorophytes contain divinyl chlorophyll a, in which the substituent at the C‐8 position on ring B is vinyl instead of ethyl.
Table 4.1 Distribution of chlorophylls and bacteriochlorophylls
| Type of organism | Chl a | Chl b | Chl c | Chl d,f | BChl a | BChl b | BChl c,d,e | BChl g | Carotenoids | Bilins |
|---|---|---|---|---|---|---|---|---|---|---|
| Purple bacteria | +a | +a | + | |||||||
| Green sulfur bacteria | + | + | + | |||||||
| Filamentous anoxygenic phototrophs | + | + | + | |||||||
| Heliobacteria | + | + | ||||||||
| Cyanobacteria | + | +b | +b | +b | + | + | ||||
| Green algae | + | + | + | |||||||
| Diatoms | + | + | + | |||||||
| Brown algae | + | + |
|
