Biophys J, August 1999, p. 879-887, Vol. 77, No. 2

Investigation of Phospholipid Area Compression Induced by Calcium-Mediated Dextran Sulfate Interaction

 ^*Institute of Medical Physics and Biophysics, University of Leipzig, 04103 Leipzig, Germany;  ^#Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Rockville, Maryland 20852 USA; and  ^§Institute for Experimental Physics I, Department of Physics of Biomembranes, University of Leipzig, 04103 Leipzig, Germany

The association of anionic polyelectrolytes such as dextran sulfate (DS) to zwitterionic phospholipid surfaces via Ca²⁺ bridges results in a perturbation of lipid packing at physiologically relevant Ca²⁺ concentrations. Lipid area compression was investigated in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar bilayer dispersions by ²H-NMR and in monolayer studies. Binding of DS to DMPC surfaces via Ca²⁺ results in denser lipid packing, as indicated by higher lipid chain order. DMPC order parameters are homogeneously increased throughout the lipid bilayer. Higher order translates into more extended hydrocarbon chains and decreased average lipid area per molecule. Area compression is reported as a function of DS concentration and molecular weight. Altering the NaCl and Ca²⁺ concentrations modified electrostatic interactions between DS and phospholipid. A maximal area reduction of A = 2.7 Å² per DMPC molecule is observed. The lipid main-phase transition temperature increases upon formation of DMPC/Ca²⁺/DS-complexes. Lipid area compression after addition of DS and Ca²⁺ to the subphase was also observed in monolayer experiments. A decrease in surface tension of up to 3.5 mN/m at constant molecular area was observed. DS binds to the lipid headgroups by formation of Ca²⁺ bridges without penetrating the hydrophobic region. We suggest that area compression is the result of an attractive electrostatic interaction between neighboring lipid molecules induced by high local Ca²⁺ concentration due to the presence of DS. X-ray diffraction experiments demonstrate that DS binding to apposing bilayers reduces bilayer separation. We speculate that DS binding alters the phase state of low-density lipoproteins that associate with polyelectrolytes of the arterial connective tissue in the early stages of arteriosclerosis.

Biophys J, August 1999, p. 879-887, Vol. 77, No. 2
© 1999 by the Biophysical Society   0006-3495/99/08/879/09  $2.00

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