Molecular Mechanisms of Photosynthesis. Robert E. Blankenship
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a Purple a or b, but not both in the same species.
b Most cyanobacteria contain Chl a as their only chlorophyll‐type pigment. Prochlorophytes contain in addition Chl b. Some types also contain Chl c, d, or f.
An important variant of chlorophyll a is chlorophyll a′. This pigment differs from chlorophyll a only in the stereochemistry at the C‐132 position. It is found in small but reproducible amounts in photosystem I complexes, where one molecule forms half of P700, the special pair of pigments that is the primary electron donor (see Chapter 7). The spectral and redox properties of chlorophyll a′ are very similar to those of chlorophyll a. Current evidence suggests that chlorophyll a′ is made from pre‐existing chlorophyll a, although the putative C‐132 invertase enzyme has not been identified.
4.1.2 Chlorophyll b
Chlorophyll b is identical to chlorophyll a except at the C‐7 position, where a formyl group replaces the methyl group. This change shifts the maximum absorption to shorter wavelengths. Chlorophyll b is the major accessory light‐absorbing pigment in light‐harvesting complexes in the majority of eukaryotic photosynthetic organisms, including plants and green algae, and is not found in reaction center complexes. In photosynthetic prokaryotes, it is found only in the prochlorophytes, in some cases as divinyl chlorophyll b.
4.1.3 Chlorophyll c
Chlorophyll c is perhaps the most unusual of all the chlorophylls, in that it does not have an isoprenoid tail and also does not have ring D reduced. It is therefore chemically classified as a porphyrin, and not as a chlorin. Chlorophyll c is found exclusively in various groups of marine algae, such as diatoms and dinoflagellates. It functions as an accessory light‐harvesting pigment in pigment–protein complexes similar to those involving chlorophyll b in plants and green algae. There are several structural variants of chlorophyll c, which vary in some of the peripheral ring substituents, as shown in Fig. 4.3.
4.1.4 Chlorophyll d
Chlorophyll d is different from chlorophyll a in only one respect: the substituent at the C‐3 position is a formyl group in chlorophyll d, instead of the vinyl group found in chlorophyll a.