Non-oxide phases

The PPP has two chemical phases an initial irreversible oxidative phase when G6P is decarboxylated and oxidized to form ribulose-5-phosphate (Ru-5-P) followed by a more complicated but reversible non-oxidative phase involving interconversions of phosphorylated monosaccharides with four, five, six or seven carbon atoms. The... 

The pentose phosphate pathway can be divided in two phases an oxidative phase during which glucose-6-phosphate is converted to ribulose-5-phosphate, and a non-oxidative phase constituting of a series of reversible reactions in which two pentose-phosphate residues are converted to a series of sugar-phosphate molecules of differing lengths (Figure 3-2). 

Transketolase is involved in the non-oxidative phase of pentose phosphate pathway. 

Several perovskite compounds have been proposed as pigments (Table 12.1). They are both oxide and non-oxide phases with crystal symmetry spanning from ideal cubic perovskite to increasingly distorted hexagonal, tetragonal, and orthorhombic types, down to peculiar cases of monoclinic and triclinic lattices. 

Over et al. performed in situ surface X-ray diftiaction experiments (SXRD, see Box 8.1) [19, 20]. The combination of online reaction product analysis with SXRD allowed them to correlate the turn over frequency (TOF—number of reaction products per site per second) for CO oxidation with the structure of the catalytic surface. Figure 8.3b shows that, in the mbar regime, two distinct phases can be present the RUO2 phase and a non-oxidic phase. At tanperatures below 550 K, both phases have a nearly identical activity for temperatures above 550 K, the oxide phase has the higher activity, showing that the oxide is indeed the active phase under these conditions. 

The pentose phosphate pathway can be sectioned into two phases the oxidative phase and the non-oxidative phase. During the oxidative phase (Figure 11.13), glucose 6-phosphate is oxidized by the removal of electrons which are accepted by the coenzyme, NADP, and decarboxylated to yield ribose 5-phosphate. The first reaction involves oxidation by the NADP" -specific enzyme glucose-6-phosphate dehydrogenase with concomitant reduction of NADP to produce 6-phosphoglucono-1,5-lactone. The lactone, although unstable and liable... 

Although the oxidative phase is firmly established in active tissues, the sequence of events during the non-oxidative phase in the liver remains a contentious issue. The pathway will be considered as it occurs within the adipose (fat) tissue (i.e. the F-type pathway). An alternative scheme for the liver (the L-type pathway) has been proposed. 

Tissues which are more active in the synthesis of lipids than nucleotides require NADPH rather than ribose moieties. In such tissues, e.g. adipose tissue, the ribose 5-phosphate enters a series of sugar interconversion reactions which connect the pentose phosphate pathway with glycolysis and gluconeogenesis. These interconversion reactions constitute the non-oxidative phase of the pathway (Figure 11.14) and since oxidation is not involved, NADPH is not produced. Two enzymes catalyse the important reactions transketolase which contains thiamin diphosphate (Figure 12.3a) as its prosthetic group and transaldolase. Both enzymes function in the transfer of carbon units transketolase transfers two-carbon units and transaldolase transfers three-carbon units. The transfer always occurs from a ketose donor to an aldose acceptor. The interconversion sequence requires the oxidative phase to operate three times, i.e. three molecules of glucose 6-phosphate yield three molecules of ribulose 5-phosphate. 

FIGURE 11.14 The non-oxidative phase of the pentose phosphate pathway in adipose tissue... 

Stage 2 Non-oxidative phase 6 Ribulose 5-phosphate- 4 fructose 6-phosphate -1- 2 glyceraldehyde 3-phosphate... 

When the ratio lies markedly in favour of NADPH, the reduced coenzyme competes with NADP for its binding site on the enzyme and inhibits the reaction. Higher NADP concentrations enhance the metabolism of glucose 6-phosphate through the pathway. In addition, ATP acts as a competitive inhibitor (Section 6.3) of the enzyme. The control of the non-oxidative phase has not been elucidated. 

Boulesteix, C. (editor) (1998) Oxides Phase Transitions, Non-Stoichiometry, Superconductors in Key Engineering Materials, vol. 155-156. 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

Analyses of the defect chemistry and thermodynamics of non-stoichiometric phases that are predominately ionic in nature (i.e. halides and oxides) are most often made using quasi-chemical reactions. The concentrations of the point defects are considered to be low, and defect-defect interactions as such are most often disregarded, although defect clusters often are incorporated. The resulting mass action equations give the relationship between the concentrations of point defects and partial pressure or chemical activity of the species involved in the defect reactions. 

The configurational entropy term, given by the degeneracy, gc, is included in AfG but not in AfGc. Let us assume the existence of two compounds with different formal oxidation states for the B atom, ABO3 and ABO2.5. The two compounds have the same (perovskite-type) structure and the non-stoichiometric phase... 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

In situ chemical oxidation using potassium permanganate was also demonstrated to treat dense, non-aqueous-phase liquid (DNAPL) at the Canadian Forces Base Borden in Ontario, Canada, between 1996 and 1997. This application used a series of six injection and five oxidant recovery weUs. The total cost of the project was approximately 45,000 (D18766A, p.l3). 

In an extension of atom-transfer radical reactions to heterocyclic systems, Byers has introduced a novel methodology for the addition of electron-deficient radicals to unprotected pyrroles and indoles in a stannane-fi ee, non-oxidative process <99TL2677>. For exanqrle, photochemical reaction of pyrrole (33) with etl l iodoacetate (34) in presence of thiosulfiite as an iodine reductant, phase transfer catalyst and propylene oxide led to high yields of the 2-alkylated pyrrole 35 <99TL2677>. 

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