The mixture of myelin lipids segregates into two phases at surface pressures lower than 1 mN m-1. This was observed by the addition of 0.5 mole % of a fluorescent lipid probe that partitions unequally among the phases. The phase segregation is stabilized by the addition of myelin basic protein (MBP), which partitions to the probe-enriched phase.
Oliveira et al. [1] showed that in whole myelin monolayers, some of the negatively charged lipid components segregate from the positively charged MBP. The aim of this work is to analyze the effect of electrostatics on the domain stability and the partition of charged components among the phases. To this purpose, different concentrations of positively and negatively charged fluorescent molecules were added to the lipid-protein mixture. The phase-fusion pressures, the migration of domains under the influence of an external electric field and the probe partition in films on different subphases were studied using epifluorescence microscopy coupled to a Langmuir balance. The results indicate that low concentration of charged fluorescent probes markedly modifies the phase coexistence stability. The influence of the electrostatic properties of the molecule on the partition coefficient was analyzed from the dependence of the latter with the ionic strength of the subphase. The observed migration of domains under an electric field corresponds to dipoles oriented with the resultant positive end pointing toward the subphase [2], apparently in disagreement with the measured surface potential density in monolayers of the pure component and the net charge of the MBP. An insight of the influence of electrostatics on the phase stability and component distribution in a complex mixed monolayer was obtained by varying the probe concentration of different charged probes on subphases of different ionic strength.
