HAT mechanism

In the HAT mechanism, a hydrogen atom is transferred from the -carbon of the amine to FAD to form a carbinyl radical and FADH semiquinone radical. FADH semiquinone radical abstracts the unpaired electron from the carbinyl radical forming the imine and FADH2 after protonation. 

FIGURE 4.34 Postulated mechanisms for MAO-catalyzed oxidation of primary amines, (A) SET (B) HAT mechanism, and (C) polar mechanism. 

In vitro tests, used in evaluation of antioxidant properties make use of the ability of antioxidants to quench free radicals. Based on this mechanism, the methods are divided into two groups SET - single electron transfer, and HAT - hydrogen atom transfer. Reactions with antioxidants in assays with the DPPH radical, ABTS and the Folin-Ciocalteu reagent both operate according to the SET and HAT mechanism. Due to the kinetics of the reaction, they are included in the... 

Oxidation of the same set of substituted phenols listed in Table 17.3 was also undertaken employing the organic oxidant, cumylperoxyl radical (Cum ). Unlike the metal based oxidation. Fig. 17.11 shows that the phenol oxidation rate constants do not correlate well with the substrate army better than with substrate BDE. This behavior implicates a more synchronous HAT mechanism for the organic radical oxidant. 

Superoxide is believed to react as an effective hydrogen atom acceptor. Thermodynamically, it is one of the weakest oxidants in Table 9.10, but the electron transfer alternative is highly disfavored by the inaccessibility of 022 in aqueous solution Taube estimates that H02- has a pXa value of 21.110 Thus, a HAT mechanism is inferred for the oxidations of [Con(sep)]2 +, [Fen(tacn)2]2 +, [Run(tacn)2]2 +, and [Run(sar)]2 + nl in these reactions the ligands are the hydrogen atom donors, but it is the metal centers that are oxidized. The bimolecular disproportionation of H02 is another likely example of hydrogen atom transfer this mechanism may also apply to the disproportionation via the reaction of H02 with 02, 27 Curiously, the reaction of 02- with the hydrated electron is fast and seems not to be pH dependent. This latter reaction may yield genuine 022- as an intermediate, or it may be a form of PCET where the solvent donates a proton during the addition of the electron. 

This theoretical study [174] revealed that the SET mechanism would operate only in polar media while the HAT mechanism would operate in all the solvents though it would also be more favoured in polar media. The most favoured site of (3-carotene for hydrogen abstraction was found to be C4 of the p-ionone ring (Fig. 3.7a). 

HAT-based assays measure the capability of phenolic antioxidants to quench free radicals (generally, peroxyl radicals considered to be biologically more relevant) by H-atom donation. The HAT mechanisms of antioxidant action in which the hydrogen atom (H) of a phenol (Ar-OH) is transferred to a ROO radical can be summarised by the reaction... 

The Marcus cross relation (and the KSE model) are also valuable as indicators of mechanism. Good agreement between experiment and theory is a strong indication that all of the kinetic components, kxH/v> xh/x> and yh/y are for processes that occur by concerted HAT mechanisms. Of course, Kxh/y must refer to the thermodynamics of the overall H transfer as well. Similar arguments have been made for the application of the cross relation to ET, and Ingold et al. have made this point about the KSE model. For instance, one set... 

The second step is to rotate the seismometer to be north aligned. This can be done digitally, mechanically with a bishop s hat mechanism, or manually with an aligmnent rod. 

Bishop s hat mechanical alignment is a multistep process. A holelock with a bishop s hat mechanism is lowered into the borehole and clamped in place. A north-measuring device is lowered onto the bishop s hat mechanism to determine the orientation of the holelock. The north-measuring device is removed, and the seismometer bishop s hat mate is rotated and locked at the surface. The seismometer is lowered down onto the bishop s hat and is now aligned to north. A bishop s hat mechanism is a device that forces the mating assembly (the seismometer) to a particular orientation. It is called a bishop s hat because it is shaped like a bishop s hat. 

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