MEASURED EFFECTS OF DIACYLGLYCEROL ON STRUCTURAL AND ELASTIC PROPERTIES OF PHOSPHOLIPID-MEMBRANES

Diacylglycerol, a biological membrane second messenger, is a strong pe rturber of phospholipid planar bilayers. It converts multibilayers to the reverse hexagonal phase (H-II), composed of highly curved monolaye rs. We have used x-ray diffraction and osmotic stress of the H-II phas e to measure structural dimensions, spontaneous curvature, and bending moduli of dioleoylphosphatidylethanolamine (DOPE) monolayers doped wi th increasing amounts of dioleoylglycerol (DOG). The diameter of the H -II phase cylinders equilibrated in excess water decreases significant ly with increasing DOG content, Remarkably, however, all structural di mensions at any specific water/lipid ratio that is less than full hydr ation are insensitive to DOG, By plotting structural parameters of the H-II phase with changing water content in a newly defined coordinate system, we show that the elastic deformation of the lipid monolayers c an be described as bending around a pivotal plane of constant area, Th is dividing surface includes 30% of the lipid volume independent of th e DOG content (polar heads and a small fraction of hydrocarbon chains) , As the mole fraction of DOG increases to 0.3, the radius of spontane ous curvature defined for the pivotal surface decreases from 29 Angstr om to 19 Angstrom, and the bending modulus increases from similar to 1 1 to 14 (+/- 0.5)kT. We derive the conversion factors and estimate the spontaneous curvatures and bending moduli for the neutral surface whi ch, unlike the pivotal plane parameters, are intrinsic properties that apply to other deformations and geometries. The spontaneous curvature of the neutral surface differs from that of the pivotal plane by less than 10%, but the difference in the bending moduli is up to 40%. Our estimate shows that the neutral surface bending modulus is similar to 9kT and practically does not depend on the DOG content.