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Copyright © 1997 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 72, Issue 3, 1404-1413, 1 March 1997
doi:10.1016/S0006-3495(97)78787-3
Research Article
Atomic force microscope measurements of long-range forces near lipid-coated surfaces in electrolytes
W. Xu, B.L. Blackford, J.G. Cordes, M.H. Jericho, D.A. Pink, V.G. Levadny and T. Beveridge
Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada.
Abstract
The interaction of DMPC (L-alpha-dimyristoyl-1,2-diterradecanoyl-sn-glycero-3-phosphoch oli ne, C36H72NO8P) lipid-coated Si3N4 surfaces immersed in an electrolyte was investigated with an atomic force microscope. A long-range interaction was observed, even when the Si3N4 surfaces were covered with nominally neutral lipid layers. The interaction was attributed to Coulomb interactions of charges located at the lipid surface. The experimental force curves were compared with solutions for the linearized as well as with exact solutions of the Poisson-Boltzmann equation. The comparison suggested that in 0.5 mM KCl electrolyte the DMPC lipids carried about one unit of charge per 100 lipid molecules. The presence of this surface charge made it impossible to observe an effective charge density recently predicted for dipole layers near a dielectric when immersed in an electrolyte. A discrepancy between the theoretical results and the data at short separations was interpreted in terms of a decrease in the surface charge with separation distance.
