V.Noethig-Lasloa, P. Cevcb, D. Arconb and M. Šentjurcb

(a) "R. Boskovic" Institute, 10002 Zagreb, CROATIA, (b) "J. Stefan" Institue, 10001 Ljubljana, SLOVENIA

A fundamental property of biological membranes is to act as a barrier to permeation of polar molecules. This barrier effect is largely due to a hydrophobicity of a phospholipid bilayer, determined by the extent of water penetration into the membrane.
In order to study the depth of water peneration into liposomes, we used the electron spin echo modulation, a pulsed electron spin resonance method [1], that takes advantage of the solvent effect on the hyperfine interaction of the nitroxide spin labels. A fatty acid with the paramagnetic nitroxide group of the spin label covalently attached at various positions along the fatty acid chain was introduced into the multilamellar liposomes. The hydrophobic barrier for water molecules, as well as, for small molecules such as glycine, was studied as a function of the liposome composition and size.

[1] A. Schweiger (1991) Angew. Chem. Int. Ed. Engl. 30, 265-292.