Effects of Counterions on Molecular Transport Across Liposome Bilayer:  Probed by Second Harmonic Generation

Abstract

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 (NaEtSO 3 ), the transport rate is independent of the concentration of the counterion (i.e., Citrateand EtSO 3 -). 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 -, faster with Br -, and the fastest with the two organic counterions, Citrate -and EtSO 3 -. 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.

Adsorption and transport of charged vs. neutral hydrophobic molecules at the membrane of murine erythroleukemia (MEL) cells

“…In addition to probing adsorption and transport across living cell membranes [17,18,20], SHG has been applied to other biologically relevant systems including liposomes [21][22][23][24][25], neurons [26], and collagen networks [27,28].…”

Molecule‐Membrane Interactions in Biological Cells Studied with Second Harmonic Light Scattering

“…It was expected that the rate of transport across the CM should be similar to those obtained from prior liposome‐based studies . In both cases, the molecular probe has to directly diffuse across the lipid bilayer.…”

“…It was expected that the rate of transport across the CM shouldb es imilar to those obtained from prior liposome-based studies. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] In both cases, the molecular probe has to directly diffusea cross the lipid bilayer.S till, the deduced average moleculart ransport rate for the bacterial CM is found to be an order of magnitude slower compared to unilamellar liposomes (ca. 10 À3 s À1 )w hose lipid composition was homogeneous.…”

“…Following this demonstration,anumber of studies have since examined the effects of lipid composition and av ariety of environmental factorso nt he propensity and characteristics of molecular transport across membranes in model liposome systems. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The studies on liposomes suggested the feasibility that SHS could be used for monitoring molecular transport across membranesi nl iving biological cells. Indeed, in 2010, our laboratory reportedt he first demonstration that SHS could be applied to study molecular transport across membranes in living cells.…”

Nonlinear Optical Spectroscopy of Soft Matter Interfaces

“…SHS has also been applied to study changing membrane fluidity of liposomes by monitoring the scattered SH intensity of malachite green as a function of cholesterol, [67] and ionic strength. [68] Since water has a high absorption coefficient for infrared radiation, vibrational SFS experiments in water are more involved than similar SHS experiments, but they can be performed. Thanks to the advances made in the development of pulsed infrared laser sources combined with the notion that it is possible to perform pump-probe experiments with femtosecond infrared pules in bulk water [69] and on water surfaces [70,71] it is likely that in the near future SFS experiments can be performed routinely in aqueous environment.…”