Tetrapyrrole core

A comparison of different metal-containing and metal-free macrocycles showed that the metal centers seem to stabilize the tetrapyrrole core [104, 128, 144]. As a result, the second decomposition step is shifted to much higher temperatures compared to the metal-free porphyrin and the overall mass loss is much smaller, as it becomes apparent from Fig. 16.14. Furthermore, the mass fragments related to the decomposition of the tetrapyrrole core (HCN) are detected at significantly higher temperatures in the coupled mass spectrometer for metal-containing macrocycles compared to the metal-free ones [104, 112, 128,144],... 

Regarding non-pyrolyzed macrocycles, hydrogen peroxide formed in the reduction cycle can lead to broken bonds between the substituents and the tetrapyrrole core. Changes in the electronic structure connected therewith cause a decrease of the reduction activity. In acidic conditions as given in PEM-FC, demetallation can also cause an activity decline [71, 87, 98, 102, 210, 211]. Furthermore, carbon monoxide is able to bind irreversibly to the metal centers and thereby deactivating them [212]. With respect to Eq. 16.7, carbon monoxide might be formed by carbon oxidation during FC operation. 

Vitamin B12 (cobalamine) is one of the most complex low-molecular-weight substances occurring in nature. The core of the molecule consists of a tetrapyrrol system (corrin), with cobalt as the central atom (see p. 108). The vitamin is exclusively synthesized by microorganisms. It is abundant in liver, meat, eggs, and milk, but not in plant products. As the intestinal flora synthesize vitamin B12, strict vegetarians usually also have an adequate supply of the vitamin. 

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 view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the onter periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B. Two potential application of corroles are in tumor detection and their use in photovoltaic devices. Selective snbstitntion of corroles via nitration, hydroformylation, and chlorosulfonation for the gallinm were studied in detail and the respective mechanistic pathways and spectroscopic data were reported, (an example is shown in Fignre 27). Overall, over 139 varions corroles were synthesized and the effect of various metal complexation pertaining to their selective reactivity examined. ... 

Figure 5, based originally on a figure by Eschenmoser [13] and reproduced many times since then, shows the relationship between the macrocycle cores of these naturally occurring tetrapyrroles by indicating with heavy lines the de-... 

The shared structural core of tetrapyrroles intuitively implies a common biosynthetic pathway. This is indeed the case. All tetrapyrroles derive from a common precursor molecule, 5-aminolevulinic acid (ALA), and the following biosynthetic steps up to uroporphyrinogen III are all conserved. The major branching point occurs... 

The abbreviations for porphyrins and other tetrapyrroles, axial ligands and other species that will be used in this article are listed in Sect. E. A generalized description of the porphine nucleus is given in Fig. 1. The generalized notation for the atoms of the porphine nucleus shown was originally suggested by Hoard Structural parameters of interest include bond distances and angles, the size of the central hole (Ct"-N), the position of the metal atom with respect to the mean plane of the core and a conformational description of the core itself. 

---