Porphyrin bacteriopheophytin

Porphyrins and their derivatives play critical roles in many biological functions. Some of the most remarkable examples are protoporphyrin IX and its iron complex that constitutes the heme prosthetic group, and the magnesium complexes of pheophytin a and bacteriopheophytin a that are known as chlorophyll a and bacteriochlorophyll a, respectively. These natural compounds are illustrated in Fig. 1 together with the structure of porphin,... 

Figure 22. (a) A fragment of the photosynthetic reaetion center with the three eomponents Speeial Pair (SP), Accessory Baeteriochlorophyll (BCh), and Bacteriopheophytin (BPh). Photoindueed electron transfer from SP to BPh takes place at a i lobal rate of (3 ps). (b) Prineiple of the modular mimetic approach. D and A are the donor and aeceptor porphyrins, respectively. M is an eleetron transfer mediator whose introduction between D and A can be realized via coordination of a metal center (solid circle) which will also bind the chelating part of the bisporphyrin spacer. 

The determination of the three-dimensional structure naturally solved most of the structural questions. The pigments (6 porphyrins and 2 quinones) are organized in two branches, A and B, within the protein in almost perfect C2 symmetry, see Fig.l. The donor P is a bacteriochlorophyll (BChl) dimer. Da and Db (i.e. BChl moieties closely related to the A or B branch, respectively), and interacts with the two quinone (Q) acceptors, via a BChl monomer, Ba, and a bacteriopheophytin (BPh) monomer, Oa- is the intermediate acceptor I and transfers the electron to Qa, the primary, and via a high-spin iron to Qb, the secondary acceptor, which communicates with a quinone pool in the lipid membrane. P+, in turn, is re-reduced from an associated heme system on a close-by cytochrome protein. Cytochrome and the quinone pool are not depicted in Fig.l. 

It is interesting to note that the use of porphyrinic structures in Nature usually deals with processes involving nonfluorescent species such as iron porphyrins in dioxygen storage, transport, or activation. When photoinduced events are performed, porphyrin structures are rather ignored by Nature, and partially reduced species, such as bacteriochlorin (BC) or bacteriopheophytin, are involved instead. Still, most photochemical and photophysical academic studies or applications inspired by natural photosynthetic processes extensively use porphyrins as chromophores. This deliberate choice may have important consequences. 

---