Flavin radicals color

Neutral flavin radicals have a blue color (the wavelength of the absorption maximum, A.max, is -560 nm) but either protonation at N-l or dissociation of a proton from N-5 leads to red cation or anion radicals with imax at -477 nm. Both blue and red radicals are... 

Flavin adenine dinucleotide. See FAD Flavin adenine diphosphate. See FAD Flavin coenzymes 766,780 - 795 modified 788, 789 reduced 794 Flavin radicals 792 color of 794 formation constant 794 Flavocytochrome b2 782, 794, 847 Flavodoxins 793, 799, 800 Flavoprotein(s) 513, 788... 

The neutral flavin radical has an absorption maximum at 580 nm and hence a blue color it is sometimes referred to as the blue radical. It can undergo either protonation atN-1 to yield a cation radical or deprotonation atN-5 to yield an anion radical, if the enzyme has appropriate proton donating or withdrawing amino acid residues at the catalytic site. Both protonation and deprotonation result in the same spectral shift to give an absorption maximum at 470 nm and hence a red color. Both the blue and red radicals are seen as intermediates in enzyme reactions, suggesting that some enzymes form the neutral radical, whereas others form one of the charged radicals. 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

The nitroblue tetrazolium assay (111) is another indirect method that is used especially for detecting SOD activity on gel electrophoresis. Superoxide radicals are generated by xanthine/xanthine oxidase or by the photoreduction of flavins (typically riboflavin), which oxidize H2O to O2. The gel on which SOD samples have been loaded is then stained with nitroblue tetrazolium chloride. This reagent is reduced by superoxide to the blue-colored formazan. SOD competes with nitroblue tetrazolium and produces colorless zones on the blue gels. This method, which is highly speciflc toward superoxide dismutase, is limited by its low reliability with respect to quantitative determinations. 

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