W. Xu et al., ATOMIC-FORCE MICROSCOPE MEASUREMENTS OF LONG-RANGE FORCES NEAR LIPID-COATED SURFACES IN ELECTROLYTES, Biophysical journal, 72(3), 1997, pp. 1404-1413
The interaction of DMPC yl-1,2-diterradecanoyl-sn-glycero-3-phosphocho
line C36H72NO8P) lipid-coated Si3N4 surfaces immersed in an electrolyt
e was investigated with an atomic force microscope. A long-range inter
action was observed, even when the Si3N4 surfaces were covered with no
minally neutral lipid layers. The interaction was attributed to Coulom
b interactions of charges located at the lipid surface. The experiment
al force curves were compared with solutions for the linearized as wel
l as with exact solutions of the Poisson-Boltzmann equation. The compa
rison suggested that in 0.5 mM KCl electrolyte the DMPC lipids carried
about one unit of charge per 100 lipid molecules. The presence of thi
s surface charge made it impossible to observe an effective charge den
sity recently predicted for dipole layers near a dielectric when immer
sed in an electrolyte. A discrepancy between the theoretical results a
nd the data at short separations was interpreted in terms of a decreas
e in the surface charge with separation distance.