Ionspecific Effects

(supported by the Austrian Science Fund, Project No. I1304-B20 and the Slovenian Research Agency)

start of project: January 1, 2014
end of project: December 31, 2016

Living matter interacts with each other frequently through an aqueous medium. Prime examples for such systems are cell membranes, which are fundamental elements of life. All these aqueous media contain electrolytes, i.e. ions, which, depending on type, valency and concentration are able to modulate intermembrane interactions. Ion-specific effects are general, but not well understood phenomena, that occur in diverse matter and under diverse conditions and are known since the studies of Franz Hofmeister on the stability of proteins in different salt solutions about 130 years ago.

With respect to cell membranes, we hypothesize that ions do not only affect membrane interactions, but also intramembrane structure and in particular the formation, size, connectivity, elasticity, and surface charge of lipid domains. Hence, ions may have a more significant influence of membrane function than conceived previously. Within the proposed work we seek answers to three major questions. (i) How does ion mediated attraction between membranes affect the fluctuation spectrum of membranes as compared to membranes that are condensed by an external (osmotic) pressure? (ii) How do salts change lateral phase separation occurring in membranes, including the physical properties of the coexisting domains? (iii) Do specific interactions of ions with membrane domains create specific charge distributions on the membrane surface? To achieve our goals we will combine the experimental expertise of the Pabst group (University of Graz, Austria) in biomembrane physics with the theoretical expertise of the Podgornik group (University of Ljubljana, Slovenia) in (bio)macromolecular interactions. We will study a set of charged and charge-neutral lipid membranes, including complex lipid mixtures, in different ionic solutions, following a Hofmeister series that comprises also polyvalent ions. From these studies, we will derive an analytical description of ion-mediated interactions based on an extension of Poisson-Boltzmann theory, and elucidate its coupling to lateral membrane structure. We expect to delineate unprecedented insight on the physics pertaining to ion-mediated inter- and intramembrane coupling, knowledge which is important for understanding and treating a multitude of diseases relating to lipid disorder.