Adsorption and reduction kinetics of safranine T in self-assembled phospholipid monolayers deposited on mercury

The adsorption of safranine-T (SFR) in self-assembled monolayers of dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylserine (DOPS) and dioleoylphosphatidic acid (DOPA) deposited on a hanging mercury drop electrode was measured over the concentration range from 1 X 10(-7) to 2 X 10(-5)M, and its electroreduction was investigated over the pH range from 2 to 6 by a computerized chronocoulometric technique. The mechanism of SFR electroreduction on a DOPC-coated mercury electrode was elucidated on the basis of the dependence of the applied potential upon the electrolysis time t at constant pH and upon pH at constant t, while keeping the faradaic charge constant: the SFR cation is electroreduced in a two-electron process which takes place via the reversible uptake of the first transferring electron with formation of the neutral radical SFR, followed by the rate-determining protonation of this radical with SFRH+ formation. On DOPA and DOPS monolayers the electroreduction rate of SFR is higher than on DOPC monolayers, possibly due to an acceleration of the protonation step by the protonated phosphate groups of the polar heads of these phospholipids.