The binding of a neutral aromatic molecule to a negatively-charged lipid membrane. I. Thermodynamics and mode of binding.
This paper presents a detailed study of the binding of the fluorescent dye N-phenyl naphthylamine (NPN) to bilayers composed of the negatively-charged phospholipid methylphosphatidic acid. Binding to the liquid-crystalline membrane is enthalpy-driven. It is shown by determination of the binding constant and confirmed by n.m.r. that most of the dye ("guest") molecules reside between the lipid hydrocarbon chains at a fixed distance from the head-group, and are not distributed uniformly throughout the hydrocarbon phase. Each guest molecule is surrounded by about four lipid molecules. Transition of the membrane from the liquid-crystalline to the crystalline state results in almost total expulsion of the bound NPN into the water phase. Electrostatic theory is developed to find the effect of electrostatics upon the binding of a neutral molecule to charged membranes. Although the charge product is zero, electrostatic interactions play a part in determining the strength of binding, if each guest molecule incorporated increases the area of the membrane. For NPN this increase was found to be ca. 41 A(2).