directly, because of the forbidden nature of the S0 to S1 transition. One method that can be used to determine this energy is two‐photon spectroscopy, in which two photons are absorbed simultaneously, with the sum of their energies equal to the transition energy. The S0 to S1 transition is allowed under these conditions (Krueger et al., 1999).
With both the S2 and S1 states having exceptionally short excited state lifetimes, it is perhaps surprising that carotenoids are able to carry out energy transfer to chlorophylls before they decay to S0, releasing heat. Yet, in many cases, carotenoids are efficient antenna pigments, because the energy transfer process is even faster than the deactivation rate. We will explore some of the details of this energy transfer process in complexes where structural information is available in Chapter 5.
4.6 Bilins
Bilins are linear, open‐chain tetrapyrrole pigments found in the light‐harvesting antenna complexes known as phycobilisomes, which absorb in the spectral region from 550 to 650 nm. Phycobilisomes are well characterized structurally and spectroscopically and are one of the best understood of the various classes of antenna complexes. We will discuss them in more detail in Chapter 5.
Figure 4.13 Structures of two of the most common bilins: phycocyanobilin and phycoerythrobilin.
The bilins resemble a porphyrin that has been split open and twisted into a linear conformation. Indeed, the bilins are actually formed from heme groups in just this fashion, as described below. The structures of the two most commonly found bilins, phycocyanobilin, and phycoerythrobilin, are shown in Fig. 4.13. The bilins are bound to proteins known as biliproteins. The three main classes of biliprotein antenna complexes are allophycocyanin, phycocyanin, and phycoerythrin. Bilins are the only class of photosynthetic pigments that are covalently attached to proteins. They are linked by thioether bonds to specific cysteine amino acid residues. In most cases, a single thioether linkage on ring A is found, although a dual linkage at both ring A and ring D is found on some pigments in phycoerythrin (MacColl, 1998).
The open‐chain tetrapyrrole bilin chromophores are made by a surprisingly complex pathway (Bryant et al., 2020). First, the protoporphyrin IX molecule is synthesized, as described above in the description of chlorophyll biosynthesis. This molecule is converted into a heme by insertion of Fe. The heme is then split open by the action of the enzyme heme oxygenase. Heme oxygenase requires both O2 and NADPH as substrates, producing the molecule biliverdin, which is subsequently reduced, isomerized, and finally ligated to the apoprotein.
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