Nitrogen biological roles

Ni2 + bound to deprotonated peptide nitrogens in tri- and tetrapeptides can be oxidized to Ni3+ [331-333]. The presence of Ni3+ has been reported in bacterial hydrogenases and dehydrogenases [334, 335], which suggests a biological role for the Ni3 + /Ni2+ redox couple. 

The biological role of the isolated polysaccharides appears to be essentially that of haptenes. They are able to react with immune serum in a dilution of the order 1 1,000,000 but cannot elicit antibody formation. In considering these results it should be noted that the fractions still contain traces of nitrogen, and that traces of protein, whether in combination or admixture, may modify biological properties. 

The elements of groups 13—16 fall into three categories (Fig. 1.3), the metalloids, the other metals, and the nonmetals. The important biological role of some of the nonmetals, oxygen, nitrogen, phosphorus, sulfur, and selenium together with the halogens, chlorine and iodine, will be discussed in Chapter 18. 

Despite the above important common denominators, and the similar size and chemical properties of chemically equivalent W and Mo species, important differences have evolved in the biological roles of these two metals. Two particular points highlight these differences (i) the significance of these metals for nitrogen fixation (ii) the prevalence of Mo over W as the catalytic centre of oxo-transfer enzymes. 

Pyrrolizidine alkaloids (PA) are complex molecules named for their inclusion of a pyrrolizidine nucleus a pair of linked pyrrole rings. Each pyrrole can be drawn as a five-sided structure with four carbons and one nitrogen forming the ring [21]. Pyrroles are incorporated into the chlorophyll molecule the biological role of PAs in plants remains unknown. 

We Cannot Live Without Malignant Air - The Biological Role of Nitrogen 1987... 

The compounds of this article, ie, ftve-membered heterocycles containing two adjacent nitrogen atoms, can best be discussed according to the number of double bonds present. Pyrazoles contain two double bonds within the nucleus, imparting an aromatic character to these molecules. They are stable compounds and can display the isomeric forms, (1) and (2), when properly substituted. Pyrazoles are scarce ia nature when compared to the imidazoles (3), which are widespread and have a central role ia many biological processes. 

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