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Contents : 12816 J. Phys. Chem. B 2001 105 12816-12822 Effects of Counterions on Molecular Transport Across Liposome Bilayer: Probed by Second Harmonic Generation Xiaoming Shang Yan Liu Elsa Yan and Kenneth B. Eisenthal* Department of Chemistry Columbia UniVersity New York New York 10027 ReceiVed: June 1 2001 In Final Form: October 1 2001 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. Citrateand 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- faster 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. 1. Introduction SCHEME 1 Liposomes have been widely used to investigate membrane functions because of the similarity between a liposome bilayer and biological membranes. Studies of liposomes in various fields of science and technology 1-3 extend from basic studies of the shapes and size distribution of cells reconstitution of protein membranes to applications such as drug delivery systems photoconversion of solar energy chemical catalysis and cosmetics. In all these applications molecular transport kinetics across the liposome bilayer plays an important and basic role which has been studied extensively for several decades.4-12 This includes the mechanism of transport 10 13 14 the effects of bilayer chemical composition phase structure 15 16 chemical and electrical gradients across a phospholipid bilayer 17 18 as well as a variety of practical problems including time-controlled drug release using liposomes as delivery vehicles.19 However studies of the effects of the electrolyte on molecular transport across the liposome bilayer have not been reported to our knowledge. The effect of the counterion (anion) on the molecular transport rate of an organic cation across the liposome membrane is reported in the present work. We used an organic cation malachite green (MG) which has a charge of +1 as the transport probe and dioleoylphosphatidylglycerol (DOPG) ( 105 nm) as a negatively charged liposome. The molecular structures of MG and the DOPG lipid are shown in Scheme 1. DOPG consists of two unsaturated 18-carbon chains with one double bond located between the 9th and the 10th carbons in each carbon chain. We used second harmonic generation (SHG) to measure the transport rates of MG across the liposome bilayer as a function of electrolyte concentration in four solutions with the same cation (Na+) and different anions (Cl- Br- Citrate- and EtSO3-). Our results show that the different counterions (i.e. Cl- Br- Citrate- and EtSO3-) affect the MG transport rate in different ways. For example the transport rate is the slowest with Cl- faster with Br- and the fastest with Citrate- and EtSO3-. In addition we observed a concentration dependence of the transport rate with the two inorganic anions Cl- and * To whom correspondence should be addressed. Br-. More specifically the transport rate of MG across the DOPG liposome bilayer becomes slower when increasing the concentration of the counterion (Cl- or Br-). However for the two organic anions (Citrate- and NaEtSO3-) the transport is independent of their concentration and the time constants of MG transport for these two organic electrolytes are the same within our experimental precision. 2. SHG as a Probe of Ion Transport Kinetics across a Liposome Bilayer Second harmonic generation (SHG) is a second-order nonlinear optical effect in which the incident light field at frequency interacts with the medium to generate a light wave at 2 . As a second-order process it is forbidden in centrosymmetric media in the electric dipole approximation but is allowed at surfaces or interfaces where the centrosymmetry is broken. This leads to a unique property of SHG namely its surface specificity. Some excellent reviews summarize recent progress in this field.20-25 10.1021/jp0120918 CCC: $20.00 2001 American Chemical Society Published on Web 12/01/2001 Molecular Transport Across Liposome Bilayer J. Phys. Chem. B Vo

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