Tetrapyrrole classes

The substances from the tetrapyrrole class are remarkable both through the very deep colour and through the extreme stability to chemical and physical agents, which naturally go together according to the resonance theory. To this class belong the coloured constituent of the blood haemin and of plants chlorophyll, all derivatives of porphyrin and also the artificial pigments discovered by Linstead, the phthalocya-... 

As a class, metal-ion derivatives of tetrapyrrole macrocyclic rings, such as the corrins or porphyrins (see Chapter 1 for the parent ring structures), are of major biological importance. 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

A third class of accessory pigment of more limited distribution are the open tetrapyrroles, sometimes called "plant bile pigments" because of their relationship to the pigments of animal bile (Fig. 24-24). 

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. 

Porphyrin analogs, homologs, and porphyrinoids - The class of tetrapyrrole chromophores can be enlargened by alteration of the numbers of carbon atoms linking the pyrrole rings [101]. A beautiful porphyrin isomer, porphycene, has been prepared by Vogel and coworkers [102], This porphyrin isomer, H2(Pyc),... 

Corroles and corrinoids - For results on the noble metal complexes of this interesting class of tetrapyrroles, see the article by S. Licoccia in this volume [105]. 

Phthalocyanines - Pthalocyanine complexes form a very large class among tetrapyrrole complexes. Their chemistry, physics, and applications for novel materials are being reviewed in a multi volume series [106]. Usually they are made by reductive tetramerization of phthalodinitrile with a metal salt (see below), but metallations of free phthalocyanine, H2(Pc), are also documented. Work in the last two decades has been concentrated on phthalocyanine complexes of Ru, Os, Rh, Pd, Pt, and Ag Berezin gives references on IrCl(Pc) and AuCl(Pc) [107]. 

Corroles form an interesting class of compounds to be investigated in order to study how variations in symmetry and the degree of conjugation affect the chemistry of tetrapyrroles. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

On the other hand, photosubstitutions of tetrapyrrole complexes are rare processes. The main reason for such distinctions between tetrapyrrole complexes and common inorganic compounds lies in the electronic structure of the two mentioned classes of compounds in their low-lying photoreactive excited states. 

The third class (c) of photoredox processes, namely electron transfer from the coordinated tetrapyrrole ring (its jr-system) to the central atom or vice versa, comprises a few cases, represented in Table 4 by the neutral complex Sn(Pc)2... 

The third group (class 3 of the above classification) of the tetrapyrrole ring localized photoreactions can be exemplified [231] by the formation of oxonia-chlorins from chlorophyll derivatives (X = Cl, CF3COO)... 

Another interesting class of anion receptors based upon protonated nitrogen atoms are the expanded porphyrin macrocycles such as 4.17 (diprotonated sapphyrin) and compound 4.18. The tetrapyrrole porphyrin macrocycles are excellent hosts for metal cations such as Fe2+ and Mg2+ (e.g. haemoglobin and chlorophylls, Sections 2.3-2.5) however, their cavity dimensions are too small to accommodate anions. Conversely, expanded porphyrins such as 4.17 comprising five or more pyrrole residues present a rigid macrocyclic cavity about 5.5A in diameter, in which (particularly when protonated) the Nff... 

The nomenclature of porphyrins, which belong to the larger class of tetrapyrrole compounds, is sometimes obscured by historical remnants (e.g. chlorin which does not contain any chlorine substituent, see H22c in fig. 10, or 2,4-di(o -methoxyethyl)-deuteroporphyrin for TMb, see fig. 11 below). IUPAC has published nomenclature rules in 19863 and the numbering adopted for the ring is given in fig. 10. The 5, 10, 15, and 20 positions are commonly referred to as meso positions the roman number after a name (I though IV) denotes the relative positions of substituents a and b. 

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. 

Interestingly, only two types of pigments appear to be involved in all known photochemical reactions in plants and algae. These are the carotenoids and the tetrapyrroles, the latter class including the chlorophylls, the phycobilins, and phytochrome. The maximum absorption coefficients for the most intense absorption bands are slightly over 104 m2 mol-1 in each case, with 7 to 12 double bonds in the main conjugated system. Cytochromes, which are involved in the electron transport reactions in chloroplasts and mitochondria, are also tetrapyrroles (considered later in this chapter). Table 5-1 summarizes the relative frequency of the main types of photosynthetic pigments. 

Hundreds of distinct pigments are found in various photosynthetic organisms. They can be grouped into three main classes chlorophylls (including bacteriochloro-phylls), carotenoids, and bilins. The chlorophylls are cyclic tetrapyrroles with a central Mg. They are found in both antenna and reaction center complexes of all types of photosynthetic... 

Although the different sequences and folds of proteins provide the most disparate metal binding sites (some examples of which are provided in Fig. 3), Nature has evolved to select other organic or inorganic ligands for metal ions in proteins, which we call special metal cofactors. These cofactors can be grouped into two broad classes tetrapyrroles and metaUoclusters. 

Another class of nitrogen-bridged expanded porphyrins was first reported in 1993 by Dolphin and coworkers. Referred to as porphocyanines, the first of these tetrapyrrolic expanded porphyrins to be reported was system 9.110. It was prepared via the oxidative dimerization (with loss of ammonia) of the bis(amino-methyl)dipyrrylmethane derivative 9.109, a precursor derived, in turn, from the bis(-cyano)-substituted dipyrrylmethane 9.108 (Scheme 9.2.2). ... 

The first class of tetrad molecules discussed in the previous section focused on electron transfer between quinone moieties, whereas the second featured electron transfer involving two tetrapyrrole moieties. Phenomena of both types are observed in the natural photosynthetic apparatus. Conceptually, both lines of research can be fused via construction and study of C-P-P-Q-Q pentad molecules. The first of these, 22, has recently been synthesized [66]. The carotenodiporphyrin portion of the molecule resembles that of tetrad 18, but the porphyrin bearing the carotenoid contains a zinc ion. The diquinone is related to that of 15, but it is joined to the porphyrin by a different linkage. 

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