Oxidative activation steric considerations

These data demonstrate that both GSH and GSSG have profound effects on Na/K ATPase activity and may act in concert to modify enzyme activity during oxidant stress. However, it should be recognized that the steric conformation of an isolated enzyme preparation in a chemically buffered solution may be considerably different to the native enzyme located in a dynamic lipid bilayer. For this reason, these investigations have been extended to include a variety of preparations in which the Na/K pump is in its native environment. 

Phosphinite pincer iridium systems have also been shown to have a lower tendency to oxidatively add TEE to give (vinyl)(hydride) complexes similar to 3 [18]. While this has been identified as one of the major catalyst deactivation processes in phosphine pincer iridium catalysis, apparently with complexes such as 5, only olefin coordination can occur. However, this is a considerably weaker bonding and is less detrimental to catalyst activity. Eased on steric arguments, product olefin coordination (e.g. COE) is favored over TEE coordination, and therefore at a high TON and high product concentrations the phosphinite catalysts 5 are markedly less active than the phosphine analogues 1. 

As compared to conventional petrochemicals, the significant hindrance of carbohydrates induces many diffusional limitations and activity of solid catalysts is obviously strictly governed by the accessibility of the catalytic sites. In this context, the porosity of commonly used siliceous-based catalysts or metal oxides is not crucial since, because of the steric hindrance of carbohydrates, the catalytic reaction mainly takes place on the catalyst surface. In the case of organic polymers, utilization of flexible polymeric chains considerably improves the accessibility of the catalytic sites. 

A current hypothesis, which is receiving considerable attention, is that one can indeed produce a surface which actively repels proteins and other macromolecules123 124, 133). The basic idea is presented in Fig. 25, which shows that a neutral hydrophilic polymer, which exhibits considerable mobility or dynamics in the aqueous phase, can actively repel macromolecules from the interface by steric exclusion and interface entropy methods. This method has been well-known and applied in the field of colloid stability for many years 120). The most effective polymer appears to be polyethylene oxide, probably because of its very high chain mobility and only modest hydrogen bonding tendencies 121 123>. 

Since cis-1 is considerably more stable than trans-1 (Table 1), a solution of 1 would contain almost exclusively c/ -l and hence presumably overwhelmingly form cis-2 upon oxidative addition of H2. However, trans-1 is more stable than cis-2 by = 10 kcal/mol (Table 3), and it is thus possible that the dihydrogen addition bypasses cis-2 altogether. Interconversion of TBP and SQP complexes often proceeds with low activation energies (33,48), and we have located the transition state for conversion of cis-2 to the thermodynamically favored product trans-2. When M = Rh (cis-2a — trans-2a), the transition state is about 10 kcal/ mol above cis-2a. The transition state for the SQP TBP interconversion when M = Ir (cis-2b —> trans-2h) is also about 10 kcal/mol above the SQP conformer (cis-2b), so with both metals the rearrangement should be facile at ambient temperatures. The activation energy for cis-2 - trans-2 interconversion should be even less with sterically bulky phosphines that selectively destabilize cis-2. 

Optimum protection of the substrate towards oxidation can be achieved when both 2- and 6-positions of the phenol are substituted with tertiary butyl groups and the 4-position is substituted with an n-alkyl group [28]. Replacement of a tertiary butyl group by a methyl group in the ortho position reduces the antioxidant activity considerably. Table 4.1. Steric hindrance in both ortho positions beneficially prevents hydrogen abstraction by the phenoxy radical. Reaction (4.34) ... 

Optical activity in biopolymers has been known and studied well before this phenomenon was observed in synthetic polymers. Homopolymerization of vinyl monomers does not result in structures with asymmetric centers (The role of the end groups is generally negligible). Polymers can be formed and will exhibit optical activity, however, that will contain centers of asymmetry in the backbones [73]. This can be a result of optical activity in the monomers. This activity becomes incorporated into the polymer backbone in the process of chain growth. It can also be a result of polymerization that involves asymmetric induction [74, 75]. These processes in polymer formation are explained in subsequent chapters. An example of inclusion of an optically active monomer into the polymer chain is the polymerization of optically active propylene oxide. (See Chap. 5 for additional discussion). The process of chain growth is such that the monomer addition is sterically controlled by the asymmetric portion of the monomer. Several factors appear important in order to produce measurable optical activity in copolymers [76]. These are (1) Selection of comonomer must be such that the induced asymmetric center in the polymer backbone remains a center of asymmetry. (2) The four substituents on the originally inducing center on the center portion must differ considerably in size. (3) The location... 

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