Chromone-Containing Allylmorpholines Influence Ion Channels in Lipid Membranes via Dipole Potential and Packing Stress

Abstract

Herein, we report that chromone-containing allylmorpholines can affect ion channels formed by pore-forming antibiotics in model lipid membranes, which correlates with their ability to influence membrane boundary potential and lipid-packing stress. At 100 µg/mL, allylmorpholines 1, 6, 7, and 8 decrease the boundary potential of the bilayers composed of palmitoyloleoylphosphocholine (POPC) by about 100 mV. At the same time, the compounds do not affect the zeta-potential of POPC liposomes, but reduce the membrane dipole potential by 80-120 mV. The allylmorpholine-induced drop in the dipole potential produce 10-30% enhancement in the conductance of gramicidin A channels. Chromone-containing allylmorpholines also affect the thermotropic behavior of dipalmytoylphosphocholine (DPPC), abolishing the pretransition, lowering melting cooperativity, and turning the main phase transition peak into a multicomponent profile. Compounds 4, 6, 7, and 8 are able to decrease DPPC's melting temperature by about 0.5-1.9 °C. Moreover, derivative 7 is shown to increase the temperature of transition of palmitoyloleoylphosphoethanolamine from lamellar to inverted hexagonal phase. The effects on lipid-phase transitions are attributed to the changes in the spontaneous curvature stress. Alterations in lipid packing induced by allylmorpholines are believed to potentiate the pore-forming ability of amphotericin B and gramicidin A by several times.

Keywords: allylmorpholine; chromone; ion channel; membrane.

Figure 1

The relationships between the structures of allylmorpholines and their potential-modifying abilities: the dependence of dipole modifying efficiency of allylmorpholines on (A) the length of hydrophobic chain in R₅ position; (B) lengths of hydrocarbon radicals in R₄ and R₅ positions; (C) the type of halogen in the R₁ position; (D) the existence of halogen in R₁ and R₃ positions.

Figure 2

(A) Current fluctuations corresponding to opening and closing single gramicidin A channels in the absence (control) and presence of 5 and 7 at 100 μg/mL. V = 150 mV. C—closed state of the channel, O—open state of the channel. (B) g_sc(V) curves of single gramicidin A channels in the absence and presence of chromone-containing allylmorpholines at 100 μg/mL. The relationship between the color of the symbol and the compound is given in the figure legend. (C) The effects of 5 and 7 on pore-forming activity of gramicidin A. A peptide was added into the bathing solution at both sides of the bilayers up to 1 nM. The moments when up to 100 μg/mL of 5 (upper panel) and 7 (lower panel) was added in the bilayer bathing solution are indicated by arrows. The membranes were composed of POPC and bathed in 2.0 M KCl (pH 7.4). V = 150 mV.

Figure 3

The schematic representations of the mechanisms of the action of allylmorpholine 7 on the conductance of GrA channels via alterations in membrane dipole potential.

Figure 4

Heating thermograms of DPPC liposomes in the absence and presence of various chromone-containing allylmorpholine derivatives at 100 μg/mL. (A) control, 1, 2, 3, 4, 5, and 6; (B) 7, 8, 9, 10, 11, and 12. The relationship between the profile and the compound is given in the figure.

Figure 5

The effects of chromone-containing allylmorpholines (4, 6, 7, 8) on the steady-state transmembrane current flowing through membranes modified by one-sided addition of AmB. The moments when 100 μg/mL of 4 (A), 6 (B), 7 (C), and 8 (D) was added to the bilayer bathing solution are indicated by arrows. The lipid bilayers were composed of POPC/CHOL (80/20 mol%) and were bathed in 2.0 M 4KCl, pH 7.4. V = 50 mV.

Figure 6

The schematic representations of the mechanisms of action of allylmorpholine 7 on the pore-forming activity of AmB via alterations in lipid-packing stress.