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+)<PRH(+)<PR, with the neutral species being the most surface active. "Penetration" of a lipid film appears to be predominately due to the insertion of the lipophilic portion of the PR species, though some deeper penetration may occur. Penetration also appears to be greater when the film is charged, thus, SA films show greater expansion at pH 8 than they do at pH 2. For both SA and DPPC the penetration of PR species may be reversed by increasing surface pressure and the penetration goes through a maximum value which is dependent on both the substrate PR concentration and the nature of the lipid film. This phenomenon may be related to that of pressure anesthesia reversal. For DPPC, the maximum penetration occurs where the film is undergoing a LE/LC transition and it appears that PRH(+) penetrates DPPC more easily than SA. Fluorescence microscopy indicates that the presence of PR species creates much smaller LC domains, while maintaining the total amount of that phase. This is interpreted in terms of a reduction of the line tension between the LE and LC phases due to a line activity of the PR species. Thus, PR will preferentially locate at interfaces within the membrane and this observation should also apply to the lipid-protein interface.
Cadenhead
D A.
DA
Department of Chemistry, University at Buffalo, 410 NSC, 14260, Buffalo, NY, USA