FADH semiquinone radical

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. 

FADH% flavin adenine dinucleotide, neutral semiquinone radical FAD" , flavin adenine dinucleotide, anion semiquinone radical FMN, flavin mononucleotide ... 

The formation of a blue (neutral) semiquinone in high yield upon irradiation of thioredoxin reductase in the presence of a large excess of EDTA is shown in Fig. 3a. The semiquinone is further reduced to FADH at an even slower rate with maximal semiquinone formation at 4 hr. In contrast to this very slow semiquinone production, enzyme reduced by NADPH in the dark and subsequently exposed to light is rapidly converted to the semiquinone. The rate depends on the amount of NADPH used in the reduction with 0.5 mole NADPH per FAD the half-time is less than 0.5 min, with 2.0 moles NADPH per FAD the half-time is about 2 min. The rate of free radical production (EPR) exactly parallels the rate of increase in absorbance at 580 nm. The exact spectral characteristics of the semiquinone depend on the state of oxidation of the disulfide-dithiol. In the dithiol form the maximum is at 578 nm while in the disulfide form the maximum is at 588 nm. That the spectral properties are determined by the redox state of the disulfide is indicated by three findings. If semiquinone is produced by irradiation following reduction of the enzyme by 0.5 mole/FAD, the maximum is at 588 nm, while if the semiquinone is formed following reduction by 2.0 moles/FAD, the maximum is at 578 nm. Oxidation of enzyme irradiated in the presence of excess EDTA for various lengths of time requires ferricyanide stoichiometric with the ob-... 

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