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 di oleoylphosphatidylcholine (DOPC), dioleoylphosphatidylserine (DOPS) an d dioleoylphosphatidic acid (DOPA) deposited on a hanging mercury drop electrode was measured over the concentration range from 1 X 10(-7) t o 2 X 10(-5)M, and its electroreduction was investigated over the pH r ange from 2 to 6 by a computerized chronocoulometric technique. The me chanism of SFR electroreduction on a DOPC-coated mercury electrode was elucidated on the basis of the dependence of the applied potential up on the electrolysis time t at constant pH and upon pH at constant t, w hile keeping the faradaic charge constant: the SFR cation is electrore duced in a two-electron process which takes place via the reversible u ptake of the first transferring electron with formation of the neutral radical SFR, followed by the rate-determining protonation of this rad ical with SFRH+ formation. On DOPA and DOPS monolayers the electroredu ction rate of SFR is higher than on DOPC monolayers, possibly due to a n acceleration of the protonation step by the protonated phosphate gro ups of the polar heads of these phospholipids.