Magnesium tetrapyrroles

The photochemistry of true organomagnesium compounds remains almost completely unexplored. A literature search in preparation of this work found only a few scattered examples of photochemical studies, mostly in relation to Grignard reactions and 1,3-diketonate chelates Similar to the situation with organozinc compounds magnesium tetrapyrrole chelates, i.e. magnesium porphyrins 1, 5,10,15,20-tetraazaporphyrins (por-phyrazines) 2 and phthalocyanines 3 have found more interest. This is primarily related... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

Chlorophyll is a magnesium tetrapyrrole of the chlorin class that has a unique alicyclic ring. Figure 19 shows the structure of chlorophyll, together with parts of the structure of chlorophyll b and bacteriochlorophyll a. The fifth and sixth coordination positions may be filled by solvent water, and also offer the possibility of dimer or higher aggregate formation. 

THE SYNTHESIS OF RADIOACTIVE PHOTOAFFINITY LABELS CONTAINING MAGNESIUM TETRAPYRROLE INTERMEDIATES OF CHLOROPHYLL AND BACTERIOCHLOROPHYLL BIOSYNTHESIS. 

The Synthesis of Radioactive Photoaffinity Labels Containing Magnesium Tetrapyrrole... 

The chlorophylls share a common tetrapyrrole stmcture and in their normal occurrence contain a chelated magnesium ion [28,29]. Structures for some chlorophyll a and b are shown in Figure 13.3. The structural difference is that there is a methyl group in chlorophyll a, whereas there is an aldehyde function in chlorophyll b. The... 

Chlorophylls are macrocyclic tetrapyrrole derivatives containing in their natural form a chelated magnesium ion. The basic structure of chlorophyll pigments is shown in Fig. 2.122. 

The chlorophylls are responsible for the green colour of nearly all plants. The isomers, chlorophyll a and chlorophyll b exist in a 3 1 ratio in plants. Chlorophylls are magnesium-chelated tetrapyrroles with an esterified 20-carbon alcohol, phytol. Acidic conditions can cause the replacement of magnesium for hydrogen and change the chlorophyll into pheophytins. The pheophytins are brown in colour, and are normally undesirable in most foods. 

Similar reactions occur in red algae and cyanobacteria but because the full spectrum of sunlight does not always penetrate very far below the surface of the sea they make use of different pigments. These may be structurally similar to those used by plants, and are therefore classed as bacteriochlorophylls, or belong to the structurally distinct phycobilin class of compounds. Unlike chlorophyll, which has a cyclic tetrapyrrole structure containing magnesium, the phycobilins are acyclic tetrapyrroles with similarities to the breakdown product of the haem ring, bilirubin. 

The chlorophylls are tetrapyrrole pigments in which the porphyrin ring is in the dihydro form and the central metal atom is magnesium. There are two chlorophylls, a and b, which occur together in a ratio of about 1 25. Chlorophyll b differs from chlorophyll a in that the methyl group on carbon 3 is replaced with an aldehyde group. The structural for-... 

The basic structure of Chls is the tetrapyrrolic porphyrin macrocycle with an attached isocyclic ring (Fig. 1). This structure has been termed phytoporphyrin by the lUPAC commission (2). The four nitrogens bind a magnesium ion (Mg +) as central... 

Tetrapyrroles are macrocychc ligands that provide a common skeleton to hemes (that contains iron), chlorophylls (that contains magnesium), corrinoids (that contains cobalt), siroheme (that contains iron), and methanogenesis factor F430 (that contains nickel). 

Figure 19.5. Chlorophyll. Like heme, chlorophyll a is a cyclic tetrapyrrole. One of the pyrrole rings (shown in red) is reduced. A phytol chain (shown in green) is connected by an ester linkage. Magnesium ion binds at the center of the structure.

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