Fig. 5. Titration of a low potential heme b in DC membranes. (A) Effect of sodium dithionite on the optical spectrum of DC membranes in the presence of phenosafranin as redox indicator. Conditions of titration are described in Materials and Methods. Selected traces (1 to 8) illustrate the effect of the dithionite-dependent reduction on the increase of the peaks of heme b at 426 nm and heme a₃ at 444 nm, and on the decrease of the peak of phenosafranin at 525 nm. (B) Relationship between the spectral modifications of phenosafranin (525 nm) , heme b (426 nm) and heme a₃ (444 nm) upon reduction by sodium dithionite. (C) Nernst plot of the redox titration of the low potential heme b in DC membranes. The ox/red ratio of the low potential heme b was calculated from the second wave of absorbance increase (at high concentration of sodium dithionite) at 426 nm. Forward titration () and backward titration ().