Methene groups

Figure 3.13 Side-chain elongation of amino acids. In analogy to the conversion of valine to leucine, the methene group is introduced to various other amino acids, which subsequently serve as precursors of glucosinolates.

In the same way, the cyclic ketal of 3-ionone leads to a methenic ketal which regenerates 40 on hydrolysis, trans - cis isomerization gives rise to a semicyclic stem in which the transfer of methyl hydrogen brings about the formation of a methene group through a six-center transition state. 

In normal blood 0.2-0.5% HbCO is present, due to CO formation from the a-methene groups of the hemoglobin molecule during physiological decomposition (M2, S2). In smokers up to 3% HbCO has been found. 

The chemical structure of sulfhemogiobin (SHb), a green hemoglobin derivative, has not yet been completely elucidated. The central iron atom is in the ferrous state. Combination of SHb with CO to SHbCO has been encountered (V2). The structure proposed by Nijveld (N2) for the a-methene group in the porphyrin ring,... 

COH and in the latter the methene group is replaced by >0 and an adjacent carbon atom on a pyrrole ring carries an OH group. 

Thus, studies show that thermooxidative transformation of copolymers VA and DMB associated with oxidative reactions of methene groups, as well as the collapse of the VA and DMB - links to difficult - etheric relations. A distinctive feature of the thermooxidative processes is their intensification with the increase of the content of DMB in copolymer. 

The NMR spectrum is consistent with structure 46. The single vinyl proton appears as a triplet at 4.74, 4.61, and 4.48 r and the corresponding splitting is found in the doublet at 6.88 and 6.75 t assigned to the methylene group a to the carboxyl group. The methene protons and those of the g m-dimethyl group appear at 5.33 and 8.90 t, respectively. No resonance characteristic of cyclopropane structures is observed. 

Porphyrin ligands are dianionic ligands containing four nitrogen donors. The core of the porphyrin ring, shown in Figure 4.7, consists of four pyrrole groups linked by methene... 

Haem catabolism occurs in both the liver and gut. Microsomal haem oxygenase of the reticulo-endothelial cells and hepatic parenchymal cells convert haem to biliverdin, and thence rapidly to bilirubin via a reductase. In the gut reduction of methene and vinyl groups of bilirubin results in the formation of colourless urobilinogens which on oxidation produce orange stercobilins that appear in the faeces. All these pigments contain the 4 nitrogen atoms which were initially derived from glycine via haem. 

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