We intend to manipulate inter and intramembrane interactions of lipid multibilayers (stacks) by very short DNA fragments at the size limit between point-like multivalent ions and polyions. Primarily, we will use single-stranded DNA fragments (3, 6, 10 nucleotides) and polystyrene sulphonate (PSS; 4 monomers) as polyvalent anions to tune the separation of bilayers. In order to probe for the role of structural flexibility of the polyelectrolytes we will also use rigid biopolyelectrolytes, such as mono- and polydisperse double-stranded DNA, hyaluronic acid and longer PSS chains (100-1000 monomers). Single and multi-component lipid multi-lamellar vesicles (MLV) will be prepared in Graz either from zwitterionic lipids or mixed with some cationic lipids in the presence of ions of various size and valency. Using small-angle x-ray scattering (SAXS), in Graz or at a synchrotron beamline, in combination with osmotic stress, we will determine the interactions between membranes as a function of (poly)anion size, concentration and, in the case of larger polyanions, conformational dynamics (mesh size). The latter information will be obtained from fluorescence correlation spectroscopy (FCS) experiments and/or dielectric spectroscopy (DS) by the Zagreb group. The Croatian partner will also determine the appearance and extent of Manning condensation in the polyelectrolytes by the electrical transport techniques combined with FCS. Additionally, we will study the restructuring of coexisting fluid lipid domains in the presence of the diverse ions using SAXS, FCS and fluorescence microscopy.