Metal scavenging ability

Phospholipids are generally considered to have an antioxidant effect in foods, but this activity is confounded by the formation of reducing browning material at elevated temperatures (Chapter 9). Phosphatidylethanolamine appears to have synergistic activity in mixtures with natural tocopherols and synthetic antioxidants. This synergistic activity is often related to the metal scavenging ability of phospholipids. 

Figure 15. The enhanced preservation of cracking acti dty in a DFCC mixture (A) is attributed to the metal scavenger s (MS) ability to irreversibly sorb migrating V (B) thus minimizing direct V-FCC interactions.

On the other hand, polymer-supported task-specific ILs in which the imidazolium cations couple L-proline via the ionic-pair interaction have also been synthesized and applied in metal scavenging and heterogeneous catalysis. The novel materials displayed considerable ability for metal scavenging onto their surface [e.g., Cul, Pd(OAc)2, Pd and IrCh] without the aid of a non-immo-bilized ionic liquid. Moreover, attempts to use these materials in the Cul-cat-alyzed N-arylation of nitrogen-containing heterocycles revealed that these systems are characterized by a much higher activity and recycling ability than... 

It was noted that the efficiency is increased in the presence of amines (Scott, 1993b), so the effectiveness in conjunction with aromatic amine free-radical (CB-D) stabilizers or the amine end groups of nylon would be expected. It is also noted that cuprous salts are often used as the iodide, and the effective alkyl-scavenging ability of iodo compounds has been demonstrated (Henman, 1979). Copper salts are also effective in the melt stabilization of polyesters, but in this case the effectiveness is improved by using a hindered phenol antioxidant. This performance is in contrast to the strong prodegradant effect of copper and other transition metals on the polyolefins as discussed later. 

Polyionic gel beads consist of highly polar microenvironments, suitable for both efficient metal scavenging and active heterogeneous catalyst preparation. They display an efficient metal-soaking ability, which depends on strong non-covalent interactions between metal and ions within the polyionic gel. 

The antioxidant stage of peroxidolytic stabilizers is frequently preceeded by a prooxidant stage which varies in intensity (see Section 19.3.2.2). In the case of metal dithiolates, however, (during both thermal and photooxidation), the prooxidant stage is not observable at the low stabilizer concentrations used normally in polymers. The radical scavenging ability of metal dithiolates may be responsible for the absence of this stage. 

Consequently, the antioxidant activity of GA in biological systems is still an unresolved issue, and therefore it requires a more direct knowledge of the antioxidant capacity of GA that can be obtained by in vitro experiments against different types of oxidant species. The total antioxidant activity of a compound or substance is associated with several processes that include the scavenging of free radical species (eg. HO, ROO ), ability to quench reactive excited states (triplet excited states and/ or oxygen singlet molecular 1O2), and/or sequester of metal ions (Fe2+, Cu2+) to avoid the formation of HO by Fenton type reactions. In the following sections, we will discuss the in vitro antioxidant capacity of GA for some of these processes. 

The detailed mechanism for these Co AlPO-18- and Mn ALPO-18-cata-lyzed oxidations are unknown, but as previously pointed out vide supra) and by analogy to other metal-mediated oxidations a free-radical chain auto-oxidation (a type IIaRH reaction) is anticipated [63], This speculation is supported by several experimental observations that include (1) an induction period for product formation in the oxidation of n-hexane in CoAlPO-36, (2) the reduction of the induction period by the addition of free-radical initiators, (3) the ability to inhibit the reaction with addition of free-radical scavengers, and (4) the direct observation of cyclohexyl hydroperoxide in the oxidation of cyclohexane [62],... 

Other potential health benefits of dietary flavonoids are too numerous to mention here. Suffice it to say that our understanding of the importance of flavonoids in the human diet is continuing to advance rapidly. One suspects that much of the physiological activity associated with flavonoids can be attributed to (i) their proven effectiveness as antioxidants and free radical scavengers, (ii) to their metal complexing capabilities (a capability that drove early advances in absorption spectroscopy and NMR studies), and (iii) to their ability to bind with a high degree of specificity to proteins. 

In in vitro studies the oxidation of LDL by endothelial cells, macrophage and Cu can be inhibited by a wide range of polyphenols and polyphenol-rich extracts [162-164]. Such effects may be due to polyphenols by direct scavenging of the oxidizing species, by regeneration of a-tocopherol in LDL [165], by their ability in binding metal ion and LDL protein [166]. 

Flavonoids have the ability to act as antioxidants by a free radical scavenging mechanism with the formation of less reactive flavonoid phenoxyl radicals [Eq. (1) and (2)]. On the other hand, through then-known chelating ability these compounds may inactivate transition metals ions (iron, copper), thereby suppressing the superoxide-driven Fenton Reaction, Eqs. (3) and (4), which is currently believed to be the most important route to activate oxygen species [51]. 

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