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.