Different hydration degrees of the topological arrangements of phospholipids in aqueous solutions

BOUCHET, ANA; FRIAS, M DE LOS ANGELES; DIAZ, SONIA; LAIRION, FABIANA; DISALVO

Buenos Aires

Workshop; Worshop on Infrared spectroscopy Applied to Biological and Biomietic systems.; 2007

Grupo Fisicoquimica membranas Lipidicas

Molecules with different molecular shapes stabilize in water forming different supramolecular structures due to the unbalance between the area of the polar head group and the hydrocarbon chains. Cylindrically-shaped molecules, such as phosphatidylcholines, stabilize forming lamellar structures. Conical shaped molecules such as lysoPCs forms spherical micelles and inverted cone molecules such as phosphatidylethanolamines stabilize in the inverted hexagonal HII form. Although many works have done with X ray diffraction and NMR, little is known about the surface properties of different polymorphic forms. In particular, it is of importance to determine how the different hydration degrees inherent to each conformation are related to the exposure of the groups that may fix water, that is carbonyl and phosphates. In this work, we have determined by FTIR spectrometry the antisymmetric stretching of phosphate groups of DMPC, DMPE, DOPE and LysoPC at different temperatures and correlated the position of the center band with the topological phase stable at each condition. The rate of hydration of the phosphates groups was measured by the shift of the frequency corresponding to the antisymmetric stretching band of the phosphate group when anhydrous lipid was hydrated in an excess of water. The shift to lower frequencies upon hydration of phosphatidylcholines was congruent with the increase in area measured in lipid monolayers. However, the trend was opposite when the lipid head groups were phosphatidylethanolamines. It is concluded that hydration is not able to disrupt the P-- N+ interaction in the crystal of phosphatidylethanolamine, in contrast to the solid lattice of PCs. In DOPE, the displacement is much lower than in fluid PE denoting that the P--N+ interactions prevails in the inverted hexagonal HII phases. In addition, the presence of carbonyl affects the exposure of the phosphates in PCs but not in PE´s.