CA2-MEDIATED INTERACTION BETWEEN DEXTRAN SULFATE AND DIMYRISTOYL-SN-GLYCERO-3-PHOSPHOCHOLINE SURFACES STUDIED BY H-2 NUCLEAR-MAGNETIC-RESONANCE()

The binding of dextran sulfates (DSs) with varying chain lengths to ph osphatidylcholine multilamellar vesicles was investigated as a functio n of polyelectrolyte, NaCl, and Ca2+ concentration. Attractive forces between negatively charged polyelectrolytes and zwitterionic phospholi pids arise from the assembly of calcium bridges. The formation of calc ium bridges between the sulfate groups on the dextran sulfate and the phosphate group of the lipid results in increased calcium binding in m ixtures of DS and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). At high NaCl concentration, the plateau adsorption of DS 500 is increa sed. The strength of dextran sulfate binding to DMPC is reflected in t he changes of the H-2 NMR quadrupolar splittings of the headgroup meth ylenes. Association forces increase with the number of calcium bridges formed. Low-molecular-weight DS does not bind to DMPC surfaces wherea s longer-chain DSs strongly influence headgroup structure as a result of strong association. DS binding increases with increasing concentrat ion; however, further association of the polyelectrolyte can be promot ed only if negative charges are sufficiently screened. DS binding to l ipid bilayers is a complicated balance of calcium bridging and charge screening. From our data we postulate that the structure of the adsorb ed layer resembles a lattice of DS strands sandwiched between the bila yer lamellae.