Interaction of phenols with old yellow enzyme. Physical evidence for charge-transfer complexes

Photoreduction of flavoproteins and other biological compounds catalyzed by deazaflavins

Deazaflavins have been found to act as potent catalysts in the photoreduction of flavoproteins in the presence of EDTA and other "photosubstrates". In distinction to the catalysis brought about by normal flavins which involves dark reaction of the photoreduced flavin catalyst, the mechanism of the catalysis by deazaflavins has been shown to involve unstable, strongly reducing radicals which are generated by photolysis of a preformed covalent dimer. By this new method it is possible to reduce not only flavoproteins but a variety of other redox proteins, including heme proteins and iron-sulfur proteins. By virtue of its great catalytic efficiency, it is possible to employ concentrations of deazaflavin sufficiently low as not to interfere with the spectral evaluation of the reduced proteins obtained.

Determination of the redox potential of deazariboflavin by equilibration with flavins

The redox potential of deazariboflavin has been determined for pH values from 5.5 to 9.2 by equilibration with riboflavin and lumiflavin 3-acetate. The position of the equilibrium with riboflavin was measured spectrophotometrically and fluorimetrically; the equilibrium potential with lumiflavin 3-acetate was measured spectrophotometrically and potentiometrically. The Em7 for deazariboflavin was found to be--0.273 +/- 0.003 V against the standard hydrogen electrode. Equilibrium with flavodoxin at pH 9.5 and 10.0 was also used to determine the redox potential of deazariboflavin at high pH values. The pK of dihydrodeazariboflavin was found from the break in the potential vs. pH diagram and from spectrophotometric pH titration. The pK value obtained by both methods is 7.00 +/- 0.05. We found that borate, a product of the reducing agent borohydride, complexed with the ribityl sidechain of deazariboflavin, causing a shift in the pK for the reduced form to values of about 8.