Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 59

The empirical chemical formula for chlorophyll a is C₅₅H₇₂N₄O₅Mg. This simple representation is entirely inadequate to convey the essential properties of this extraordinary molecule. The structural formula for chlorophyll a is shown in Fig. 4.1. It is a squarish planar molecule, about 10 Å on a side. A space‐filling model of chlorophyll a is shown in Fig. 4.2. The Mg atom in the center of the planar portion is coordinated to four nitrogen atoms. The nitrogens are each part of a substructural element of the molecule that is derived from pyrrole, a cyclic organic compound with a nitrogen atom in a five‐membered ring with four carbons. For this reason, chlorophylls and related compounds are often referred to as tetrapyrroles. A fifth ring is formed in the lower right corner, and a long hydrocarbon tail is attached to the lower left (in the standard representation). Chemically, the chlorophylls are related to the porphyrins, which are also tetrapyrroles, but the porphyrins are generally more symmetric molecules.

Figure 4.1 Numbering schemes for chlorophylls and bacteriochlorophylls. Chlorophyll a is shown, although the same basic numbering scheme applies to all chlorophyll‐type pigments. Left: the current IUPAC standard system. Right: the older Fischer numbering system. Hydrogen atoms are not shown.

Figure 4.2 Space filling model of chlorophyll a. Carbon is shown as black, hydrogen as white, oxygen as red, nitrogen as blue, and magnesium as orange.

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.