Covalently bound nitrogen

The nature of the covalently bound nitrogen in ammonium lignin sulfonates or the associated carbohydrates has not yet been reported, but if it is present as a primary or secondary amine, the reactivity toward formaldehyde would likely be increased (5) and the crosslink density of the resultant adhesive thereby augmented. 

Biotin is involved in carboxylation and decarboxylation reactions. It is covalently bound to its enzyme. In the carboxylase reaction, C02 is first attached to biotin at the ureido nitrogen, opposite the side chain in an ATP-dependent reaction. The activated C02 is then transferred from carboxybiotin to the substrate. The four enzymes of the intermediary metabolism requiring biotin as a prosthetic group are pyruvate carboxylase (pyruvate oxaloacetate), propionyl-CoA-carboxylase (propionyl-CoA methylmalonyl-CoA), 3-methylcroto-nyl-CoA-carboxylase (metabolism of leucine), and actyl-CoA-carboxylase (acetyl-CoA malonyl-CoA) [1]. 

Pyridoxal phosphate mainly serves as coenzyme in the amino acid metabolism and is covalently bound to its enzyme via a Schiff base. In the enzymatic reaction, the amino group of the substrate and the aldehyde group of PLP form a Schiff base, too. The subsequent reactions can take place at the a-, (3-, or y-carbon of the respective substrate. Common types of reactions are decarboxylations (formation of biogenic amines), transaminations (transfer of the amino nitrogen of one amino acid to the keto analog of another amino acid), and eliminations. 

Hydrogen bound covalently to nitrogen, oxygen or fluorine may bond with another atom of one of these elements by a unique linkage known as the hydrogen bond . Since the hydrogen nucleus cannot control more than two electrons this is not a covalent bond but a powerful dipole-dipole interaction, traditionally indicated by dotted lines. It occurs in water, hydrofluoric acid, undissociated ammonium hydroxide and many hydrates, e.g. ... 

The first time the body realizes that arsenic has been incorporated is when the redox activity (as above) proceeds at potentials when nitrogen or phosphorus are inert. By the time we detect the arsenic poisoning (i.e. we feel unwell), it is generally too late, since atoms of arsenic are covalently bound within body tissue and cannot just be flushed out or treated with an antidote. The arsenic sequesters electrons that might otherwise be involved in other relay cycles, which is a concurrent kinetic process see Figure 8.16. 

Adsorption of CD on palladium showed distinct differences from that on platinum, which is already apparent from the spectra compared in Fig. 33. The ATR spectra did not show any indication of the presence of a species covalently bound by Pt C ci-bond (a-hydrogen abstraction, species 2 in the case of platinum). The strongly adsorbed 7i-bonded flat species (1 and T) were found to be more favored on platinum, whereas on palladium the tilted, nitrogen lone-pair-bound species (3 ) were dominant. Furthermore, the comparative study showed that CD is more strongly adsorbed on platinum than on palladium, indicated by the more prominent shift of the vibrational modes with respect to the free molecule on platinum than on palladium. 

Representatives of all kinds have been explored for synthetic applications while mechanistic investigations were mainly focussed on the distinct FruA enzymes isolated from rabbit muscle [196] and yeast [197,198]. For mechanistic reasons, all DHAP aldolases appear to be highly specific for the donor component DHAP [199], and only a few isosteric replacements of the ester oxygen for sulfur (46), nitrogen (47), or methylene carbon (48) were found to be tolerable in preparative experiments (Fig. 7) [200,201], Earlier assay results [202] that had indicated activity also for a racemic methyl-branched DHAP analog 53 are now considered to be artefactual [203]. Dihydroxyacetone sulfate 50 has been shown to be covalently bound via Schiff base formation, but apparently no a-deprotonation occurred as neither H/D-exchange nor C-C... 

The H NMR spectra of ferricytochromes d are typical of high spin iron(III). The iron atom is pentacoordinated, with four ligands provided by the nitrogen atoms of a porphyrin (see Fig. 5.7) and the fifth ligand being a histidine residue exposed to the solvent. Being a cytochrome of c type, the heme moiety is covalently bound to two cysteinyl residues by means of thioether links (Fig. 5.11). 

Activation reactions catalyzed by serine proteases (including kallikreins) are an example of limited proteolysis in which the hydrolysis is limited to one or two particular peptide bonds. Hydrolysis of peptide bonds starts with the oxygen atom of the hydroxyl group of the serine residue that attacks the carbonyl carbon atom of the susceptible peptide bond. At the same time, the serine transfers a proton first to the histidine residue of the catalytic triad and then to the nitrogen atom of the susceptible peptide bond, which is then cleaved and released. The other part of the substrate is now covalently bound to the serine by an ester bond. The charge that develops at this stage is partially neutralized by the third (asparate) residue of the catalytic triad. This process is followed by deacylation, in which the histidine draws a... 

With the exception of the silicones, macromolecules built of covalently bound atoms other than carbon, nitrogen and oxygen are rare. 

The nitryl halides (C2 symmetry) are planar in analogy to the isoelectronic COs. The gas phase and solid-state structures for CINO2 both show covalently bound molecules (see Table 40). In contrast to the nitrosyl hahdes, the halogen-nitrogen bond distances in the gas phase and in the solid state are similar. The nitryl halides NO2F and NO2CI are the mixed anhydrides of nitric acid and hydrofluoric or hydrochloric acid, respectively. 

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