Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 66

The chemical structure of bacteriochlorophyll a is shown in Fig. 4.4. It is the principal chlorophyll‐type pigment in the majority of anoxygenic photosynthetic bacteria. The chemical differences between the structures of chlorophyll a and bacteriochlorophyll a are the acetyl group at the C‐3 position and the single bond in ring B between C‐7 and C‐8, instead of the double bond found in chlorophylls. This reduces the degree of conjugation in the macrocycle and also reduces the symmetry of the molecule compared with chlorophylls. These structural changes exert major effects on the spectral properties, which are discussed below.

A few species of the purple photosynthetic bacteria have been found that use zinc (Zn) as the central metal instead of Mg in bacteriochlorophyll a (Wakao et al., 1996). These organisms are found in highly acidic environments where Mg²⁺ is readily displaced by H⁺, whereas Zn is more stable as a central metal. These are the only two metal ions that have been found incorporated into natural chlorophylls, although many other metals can be inserted synthetically into the metal‐free pigments. The reason for this specificity is probably that Mg is very readily available, whereas Zn is a trace element in almost all environments and would therefore often be a limiting nutrient. As discussed below, most other metals are unsuitable for photosynthesis, because pigments with these metals incorporated have a very short excited state lifetime. Zn bacteriochlorophyll has also been found in reaction centers of chloroacidobacteria (Tsukatani et al., 2012).