Activity with Branched Substrates

4)GlcNAc units), and bisected di-(GlcNAcT III action) antennary oligosaccharides 

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Bronsted acid (Scheme 2.42) [26-28]. (For experimental details see Chapter 14.9.4). These catalysts mediate the addition of ketones to nitroalkenes at room temperature in the presence of a weak acid co-catalyst, such as benzoic acid or n-butyric acid or acetic acid. The acid additive allows double alkylation to be avoided, and also increases the reaction kinetic. The Jacobsen catalyst 24 showed better enantio- and diastereoselectivity with higher n-alkyl-ethyl ketones or with branched substrates (66 = 86-99% dr = 6/1 to 15/1), and forms preferentially the anti isomer (Scheme 2.42). The selectivity is the consequence of the preferred Z-enamine formation in the transition state the catalyst also activates the acceptor, and orientates in the space. The regioselectively of the alkylation of non-symmetric ketones is the consequence of this orientation. Whilst with small substrates the regioselectivity of the alkylation follows similar patterns (as described in the preceding section), leading to products of thermodynamic control, this selectivity can also be biased by steric factors. 

Interestingly, the arginine switch mechanism was first recognized when it was artificially induced in AAT. When AAT was mutated in six distinct positions, a substantial increase of activity with aromatic substrates was observed. The crystal structure of the engineered enzyme showed that the aromatic side chains could be accommodated at the active site as a result of R292 movement. A similar observation was made on an AAT mutant form whose substrate specificity was broadened using direct evolution techniques, in order to include branched chain and aromatic amino acids. 

Alkylated diphosphines (R,R)-(92) and (93) were used as chiral ligands in the Pt-catalyzed hydroformylations of some alkeneic substrates. These ligands bring about a loss of catalytic activity with respect to the corresponding diphenylphosphine homolog, particularly in the case of the platinum systems. The regioselectivity favors the straight-chain (or less branched) isomer in the case of terminal alkenes with the exception of styrene the enantioselectivity is very low in all cases.320... 

In this context, the most successful will be the branch of pure catalysis of oxidation where the substrate is oxidized in close connection with the catalyst, i.e. the catalyst forms an active intermediate with the substrate or oxidant (or, probably, with both). 

CGS catalyzes the 7-replacement reaction of an activated form of L-homoserine with L-cysteine, leading to cystathionine. 0-Succinyl-L-homoserine (l-OSHS), 0-acetyl-L-homoserine (OAHS), and 0-phospho-L-homoserine (OPHS) are substrates for CGS ftom bacteria, fungi, and plants, respectively. The plant enzyme is also able to convert the microbial substrates, albeit at much higher values. This reaction is the first step in the transsulfuration pathway that converts L-Cys into L-homocysteine, the immediate precursor of L-methionine. The 0-activated L-homoserine substrate is situated at a metabolic branch point between L-Met and L-Thr biosynthesis, and which substrate is used by CGS depends on the species. In analogy with TS, CGS is tightly regulated by SAM concentration in plants. ... 

Additional evidence for two branches in this reaction system is obtained from experiments with pulses of NADH (fig. 6.6). The deviation in G3P is positive but that of 3PG negative. We can explain that by assuming that NADH is an activator or a substrate in the branch with G3P and an inhibitor or product in the branch with 3PG. The sum of the deviations of F1,6BP, G3P, and 3PG is close to zero (see the dotted line in fig. 6.6), which we interpret as follows NADH elicits an overall activation on the two branches leading to 3PG and G3P which increases the sum of the concentrations of these two species. The concentration of the common precursor decreases by the same amount. DHAP follows F1,6BP and therefore DHAP is that precursor. However, the response of DHAP to a pulse of NADH was too small to measure variations in DHAP propagate quickly to F1,6BP, and hence this species shows a negative deviation. F1,6BP and DHAP are close to equilibrium. 

In recent years there has been tremendous interest in porous ceramics because of their applications as filters, membranes, catalytic substrates, thermal insulation, gas-burner media and refractory materials. These are due to their superior properties, such as low bulk density, high permeability, high temperature stability, erosion/corrosion resistance and excellent catalytic activity. One branch of this field is porous SiC ceramics, owing to their low thermal expansion coefficient, high thermal conductivity and excellent mechanical properties. However, it is difficult to sinter SiC ceramics at moderate temperatures due to their covalent nature. In order to realize the low temperature fabrication of porous SiC ceramics, secondary phases may be added to bond SiC. Oxidation bonded porous SiC ceramics have been found to exhibit good thermal shock resistance owing to the microstructure with connected open pores. 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

The functions of ACAD 10 and 11 also remain elusive. Multiple transcripts are made from the genes encoding for ACAD 10 and 11, and some forms of alternative splicing are even absent of exons coding for catalytic domains (He et al. 2011). ACADIO has only limited activity with long branched-chain acyl-CoA substrates, whereas ACAD 11 utilizes unsaturated substrates with 20-26 carbons. ACADIO and 11 are mostly expressed in the foetal and adult brain, respectively. It has been suggested that ACADIO and 11 may serve novel physiological functions in the central nervous system (He et al. 2011). 

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