Effects of counterions on molecular transport across liposome bilayer: Probed by second harmonic generation

The transport rate of an organic cation, malachite green (MG), across a unilamellar bilayer (∼105 nm) of the dioleoylphosphatidylglycerol (DOPG) liposome has been studied by the SHG technique. This is the first time to our knowledge that the effects of anions on molecular cation transport have been observed. Our studies show four results. First, in the presence of sodium chloride (NaCl) or sodium bromide (NaBr), the time constant for transport of MG across the DOPG bilayer increases with the increase in the concentration of the counterion (i.e., Cl^- or Br^-). Second, with the organic electrolytes, sodium citrate (NaCitrate) and sodium ethanesulfonate (NaEtSO₃), the transport rate is independent of the concentration of the counterion (i.e., Citrate^- and EtSO₃^-). Third, at the same counterion concentration, the transport rate depends on the species of the counterion used. The rate of MG transport is the slowest with Cl^-, taster with Br^-, and the fastest with the two organic counterions, Citrate^- and EtSO₃^-. Last, at the low counterion concentration of 1mM, the transport rates of MG were found to approach the same value for the four anions. A brief discussion of a possible mechanism is presented.