Magnesium porphyrin complexes

Magnesium porphyrin complexes called chlorophyll (Chi, 12) play important roles in photosynthesis in green plants and some bacteria. In green plants the chloroplasts of the thylakoid membrane contain macromolecular anisotropically oriented protein complexes with photosystems II and I (Fig. 2-26). 

Preeminent in importance among the macro-cyclic complexes of Group 2 elements are the chlorophylls, which are modified porphyrin complexes of Mg. These compounds are vital to the process of photosynthesis in green plants (see Panel). Magnesium and Ca are also intimately... 

A prime example of this is the crucial ease of such oxidation in the magnesium(II) complex of the chlorin system present in chlorophyll during photosynthesis. Table 8 shows the half-wave potentials for the first ring oxidation of some metalloporphyrins, the examples chosen being based on 5,10,15,20-tetraphenyl porphyrin, TPP (2) and on 1,2,7,8,12,13,17,18-octaethylporphyrin, OEP (16). 

Porphyrins 21 are the backbone of major players in life cycles—cytochromes (Scheme 8). There are three types of cytochromes, classified by their color, or more precisely by their long-wavelength absorption band, as a (600 mn), b (563 nm), and c (550 nm). They are protein conjugates of a porphyrin complex with iron(II), which is a coenzyme called heme (22). In plants, porphyrins form a complex with magnesium-(II) chlorophylls a and b (23), vital in photosynthesis. Porphyrin derivatives are used in photodynamic therapy for dermatological diseases such as psoriasis, and for skin or subcutaneous cancer.5c-e... 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

Storm, C. B., A. H. Corwin, R. Arellano, M. Marts, and R. Weintraub Stability Constants of Magnesium-Porphyrin-P3fridine Complexes Solvent and Sub-stitutent Effects. J.. m. Chem. Soc. 88, 2525 (1966). 

The biosynthetic chain of chlorophyll begins with the small building blocks, acetate and glycine molecules, which are part of the basic metabolic milieu. These small molecules are condensed in a series of n steps to form the complex molecule protoporphyrin. From protoporphyrin two classes of compounds are formed namely, the iron porphyrins or hemes and the magnesium porphyrins which give rise eventually to chlorophyll. According to this scheme, heme and chlorophyll are related to each other biochemically, since both arise from the same precursor molecule, protoporphyrin. 

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