Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 67

groups consist of various acidic and/or basic moieties, the majority of lipids
are charged in aqueous environments. The resulting electrostatic inter-
actions within the lipid membranes, as well as between lipid head groups
and ions from solution, are crucial for phase transition temperatures. 50 Here
we present results that demonstrate how streaming current measurements
were used in combination with measurements of the fluorescence recovery
after photobleaching (FRAP) to analyze correlations between the charging
and the fluidity of supported bilayer lipid membranes (sBLMs) prepared
from 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC). 51,52
The head group of DOPC contains both an acidic and a basic moiety that
should charge up equally and in opposite directions at neutral pH. There-
fore, on the basis of the molecular structure, one would expect an electrically
neutral lipid membrane (no net charge) in the intermediate pH range. 53-55
However, streaming current measurements performed at different concen-
trations of the neutral background electrolyte KCl revealed a significant
negative charge at neutral pH (Figure 3.4a). The isoelectric point was found
to be about 4 at all salt concentrations in solution. Therefore, the authors of
this study 51 concluded that the charging of the zwitterionic DOPC mem-
branes is superposed by unsymmetrical water ion adsorption, i.e., hydroxide
ions (OH ) are preferentially adsorbed as compared to hydronium ions

d n 3 r 4 n g | 5

.

Figure 3.4 Ratio of streaming current over applied pressure, I str /DP, and zeta
potential, z, of sBLMs prepared from DOPC on SiO 2 substrates as a
function of pH for concentrations of 0.1 mM, 1 mM and 10 mM of the
background electrolyte KCl (a) and diffusion coecient of the DOPC
membranes (doped with 1 mol% NBD-PE for FRAP) as a function of pH
in 1 mM KCl solution at 22 1C and 45 1C (b). The streaming current
measurements were performed at a channel height of 30 mm and a
temperature of 22 1C.
Reprinted with permission from R. Zimmermann, D. K¨ttner, L. Renner,
M. Kaufmann, C. Werner. Fluidity modulation of phospholipid bilayers
by electrolyte ions: insights from fluorescence microscopy and microslit
electrokinetic experiments, J. Phys. Chem. A, 2012, 116, 6519-6525.
Copyright 2012 American Chemical Society.