Appl. Magn. Reson. (2008) 33, 311322 DOI 10.1007/s00723-008-0068-x Printed in The Netherlands Applied Magnetic Resonance Effect of Polyacrylic Acid on Phase State of Lipids and Diffusion in Lipid-Water System A. Filippov 1 , A. Suleymanova 1 , and A. Berkovich 2 1 Kazan State University, Kazan, Russian Federation 2 Moscow State University, Moscow, Russian Federation Received 18 July 2006; revised 10 April 2007 © Springer-Verlag 2008 Abstract. Lipid vesicles interacting with polyanions are promising for controlled drug delivery. However, different aspects of the interaction of these polymers with lipids are far from complete understanding. In this work we studied the influence of polyacrylic acid (PAA) with small concen- trations (14 mol%) on the change of the phase state, lateral diffusion of these lipids in lamellar phase and transmembrane water diffusion in macroscopically oriented bilayers of lipid-water systems formed by dimiristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine. Measurements were performed by 31 P nuclear magnetic resonance (NMR) spectroscopy and the 1 H NMR technique with a pulsed field gradient. It was found that the presence of PAA does not change the lamellar struc- ture of the system. However, a part of bilayers changes their originally flat geometry and forms vesicles with a higher surface curvature. Macroscopic orientation of bilayers disappeared. For DMPC the presence of PAA leads to a shift of the gel-to-liquid crystalline phase-transition temperature to higher temperatures. An increase of PAA concentration leads to a monotonous decrease in the lat- eral diffusion coefficient of lipids that is caused, probably, by the ordering of lipids in bilayers. The transbilayer diffusion coefficient of water increases in the presence of PAA, but it depends slightly on the PAA concentration. An increase of pH leads to a change of the lipid lateral and transbilayer diffusion coefficients to the values typical for a pure bilayer. 1 Introduction Interest in the application of water-soluble polymers in biotechnology and medi- cine is growing rapidly [1]. It was shown for various model systems that differ- ent classes of polymers interact with lipid membranes in a different way and, as a rule, essentially change the structure and properties of membranes [26], in- cluding the membrane fusion, selective membrane permeabilization, and forma- tion of lateral domains. So, a vesicle with covalently attached, adherent or en- trapped polymer can be used as a novel responsive delivery system, particularly to deliver deoxyribonucleic acid and drugs. For the practical application of the polymers for drug delivery it is important to know, firstly, the details of changes produced by the molecular interaction of the polymer with lipids, and the ef-