Model membrane/substrate interactions: ethanol and procaine interactions

The ability of ethanol to lower the surface tension of water plays a major role in its ability to affect membrane lipids. Typical lipids will show little expansion on exposure to ethanol substrates and may even show condensations. At the same time, the overall stability of the lipid phase is significantly reduced. Previously, we reported experimental and theoretical studies of the stearic acid (SA)/procaine (PR) system. PR substrate concentrations were examined in the 10(-4)-10(-2) M range as Gibbs monolayers, in order to establish the surface activity of both charged and uncharged species, and to estimate the orientation of the PR species at the air/water interface. SA interactions with PR substrates were studied by compressing films of the former and recording the surface pressure/area per molecule isotherms at both pH 2 and 8, so that the SA was in an uncharged and charged state, respectively. More recently, we have carried out similar studies with L-alpha-dipalmitoyl phosphatidylcholine (DPPC), maintaining the substrate pH at between 5 and 6, at PR concentrations of 10(-6)-10(-2)M. We also carried out studies of the DPPC/PR system using fluorescence microscopy in order to examine the effects of PR on the biphasic liquid expanded/liquid condensed (LE/LC) transition region. In the absence of any lipid film, PR species appear to be horizontally oriented at the air/water interface, while the surface activity of PR species increases in the order PRH(2+)