The Adsorption of Phloretin to Lipid Monolayers and Bilayers Cannot Be Explained by Langmuir Adsorption Isotherms Alone

Abstract

Phloretin and its analogs adsorb to the surfaces of lipid monolayers and bilayers and decrease the dipole potential. This reduces the conductance for anions and increases that for cations on artificial and biological membranes. The relationship between the change in the dipole potential and the aqueous concentration of phloretin has been explained previously by a Langmuir adsorption isotherm and a weak and therefore negligible contribution of the dipole-dipole interactions in the lipid surface. We demonstrate here that the Langmuir adsorption isotherm alone is not able to properly describe the effects of dipole molecule binding to lipid surfaces--we found significant deviations between experimental data and the fit with the Langmuir adsorption isotherm. We present here an alternative theoretical treatment that takes into account the strong interaction between membrane (monolayer) dipole field and the dipole moment of the adsorbed molecule. This treatment provides a much better fit of the experimental results derived from the measurements of surface potentials of lipid monolayers in the presence of phloretin. Similarly, the theory provides a much better fit of the phloretin-induced changes in the dipole potential of lipid bilayers, as assessed by the transport kinetics of the lipophilic ion dipicrylamine.

Electrochemical Properties of Phospholipid Monolayers at Liquid–Liquid Interfaces

“…Likewise, the magnitude of the dipole potential depends on the lateral packing of the monolayer. Values between 270 mV and 411 mV were measured for air-water monolayers of 30-40 mN m À1 , [13] similar to dipole potentials observed for lipid bilayers. [53] A sketch of the hydrocarbon region (hc), the aqueous (w) and organic (o) bulk phases with relative permittivities, e i (where i = hc, w, o), and the salt ions represented by their concentrations, c b i;k (k labels the ionic species in solution) are shown in Figure 4.…”

“…[7,8,[12][13][14][15] As a result of their unique properties, biomembrane models with variable complexities and can be formed at liquid-liquid interfaces and are well-suited to elucidate structural details of the bilayers membrane and to mimic its functions. [2][3][4] In this context, lipid monolayers at liquid-liquid interfaces have been the basis for numerous technical applications in fields such as biochemistry, electrochemistry, chemistry and biology, and have attracted particular interest for studies of the charge transfer, electroanalysis, drug delivery and membrane activity.…”

Interactions of Myristoylated Alanine-Rich C Kinase Substrate (MARCKS)-Related Protein with a Novel Solid-Supported Lipid Membrane System (TRANSIL)

“…The resulting plasmid, pET3dMRPHis, was used to transform the E. coli strain JM109(DE3) by electropo- 6 Abbreviations used: DEPC, 1,2-dielaidoyl-sn-glycero-3-phosphatidylcholine; K d,eff , effective dissociation constant; K p , molar partition coefficient; MARCKS, myristoylated alanine-rich C kinase substrate; MRP, MARCKS-related protein; myr, myristoylated; PC, phosphatidylcholine; PG, phosphatidylglycerol; PIC, protease inhibitor cocktail; POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine; POPS, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine; PS, phosphatidylserine; unmyr, unmyristoylated; TRANSIL PC, silica gel coated with 100% DEPC; TRANSIL PC/PS 98/2 ; silica gel coated with 98% DEPC and 2% POPS; TRANSIL PC/PS 80/20 , silica gel coated with 80% DEPC and 20% POPS. ration.…”

“…This signal can usually be followed by monitoring the spectroscopic properties of the membrane active substance, including fluorescence, circular dichroism, and infrared spectroscopy (2). Other methods focus on other intrinsic properties of the system, including heat changes, i.e., microcalorimetry (3) and potential changes induced at the surface of the membrane, e.g., inner field compensation (4,5) or the vibrating plate technique (5,6). Finally, spectroscopic techniques such as surface plasmon resonance (7,8) or two-mode waveguide spectroscopy (9,10) have been recently adapted to the field of membrane biology to allow an observation of the interaction of membraneactive molecules with bilayers in real time.…”

“…We used MARCKS-related protein (MRP), 6 a peripheral membrane protein, for our studies. MRP is essential for brain development and among the bestcharacterized examples for cross-talk between the signal transduction pathways mediated by protein kinase C and calmodulin (13).…”