Magnesium porphyrin, complex with

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... 

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). 

One of the important properties of porphyrins is that they complex with divalent metals, the pyrrole nitrogens being ideally spaced to allow this. Of vital importance to life processes are the porphyrin derivatives chlorophyll and haem. Chlorophyll (actually a mixture of structurally similar porphyrins chlorophyll a is shown) contains magnesium, and is, of course, the light-gathering pigment in plants that permits photosynthesis. 

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. 

The porphyrins are cyclic compounds formed from four pyrrole rings linked by methane bridges in a ring system (White et al., 1959 Harper, 1963). A characteristic property of porphyrins is the formation of complexes with metal ions such as iron and magnesium. 

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. 

When ferrous iron is inserted into protoporphyrin, the ubiquitous iron porphyrin or heme is formed. Because of the resonating structure, an electron donor or acceptor molecule need not come in contact with the iron atom directly to oxidize or reduce the iron atom it is probably sufficient that contact be made with any portion of the resonating molecule for the iron atom to be oxidized or reduced. The oxidative properties of the iron atom in heme are modified by the iron being held in this ring and are further modified by the heme being attached to specific proteins. In nature the other metal that complexes with protoporphyrin is magnesium Mg protoporphyrin is an intermediate compound in the biosynthetic chain of chlorophyll synthesis. The movements of the ir electrons in the porphyrin are undoubtedly intimately connected with the functioning of the heme and chlorophyll structures, but of this we know very little. 

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