Structure and phase behavior of a charged glycolipid (1,2-O-dialkyl-3-O-beta-D-glucuronosyl-sn-glycerol)

In order to investigate the effects of a net surface charge on the properties of glycolipid membranes, we have synthesized a glyceroglycolipid, 1,2-O-dialkyl-3-O-beta-D-glucuronosyl- sn-glycerol (GlcUA lipid), with saturated alkyl chains of varying length (14, 16, and 18 carbon atoms; 14-, 16-, and 18-GlcUA, respectively) and glucuronic acid with an ionizable 6-carboxyl group as polar residue. Aqueous dispersions of GlcUA lipids have been characterized by differential scanning calorimetry, densitometry, and X-ray diffraction methods as a function of pH. The carboxyl group deprotonation of apparent pK about 5.5 leads to a decrease of the melting temperatures by about 7 degrees C for all three compounds and to a chain-length-dependent reduction of the transition enthalpies by 0, 7, and 14% for 14-, 16-, and 18-GlcUA, respectively. The decrease of the transition temperature is consistent with current electrostatic concepts and models of charged membrane interfaces, but the chain-length-specific dependence of the enthalpy decrease with an increase of pH shows that the pH effects in GlcUA lipids are not of purely electrostatic origin. However, these effects appear to be simpler in some instances than corresponding effects in phospholipids with multiply ionizable head groups. For this reason, the lipids with the glucuronic acid head group appear to represent an appropriate model system for studies of net electric charge effects on the membrane properties.(ABSTRACT TRUNCATED AT 250 WORDS)