Effects of salt on the lamellar and bicontinuous cubic phases of fully hydrated monoacylglycerol (monoelaidin)

The effects of various sodium salts on the phase behavior of monoacylglycerol were investigated for fully hydrated monoelaidin systems by means of X-ray diffraction and differential scanning calorimetry. The hydrated monoelaidin systems form lamellar and bicontinuous cubic phases, depending on the temperature. Calorimetric data showed that the sign and magnitude of the phase transition temperature shifts on addition of salts are consistent with the Hofmeister series which is determined according to the efficiency of salting-out of proteins. Kosmotropic salts, having a great ability for the salting-out of proteins, increase the lamellar gel to lamellar liquid crystalline phase transition temperatures and decrease the primitive cubic to body-centered cubic (Im3m to Pn3m) phase transition temperatures. On the other hand, chaotropic salts, having a weak ability for the salting-out of proteins, exhibit the opposite effect. X-ray diffraction data demonstrated that chaotropic salts expand the lattice constants of fully hydrated monoelaidin in the Pn3m cubic phase whereas kosmotropic salts reduce the lattice constants. The above results are discussed in comparison with previous results reported for phospholipids and glycolipids. In addition, the structures of the cubic phases are discussed, based upon the above results.