We found that interactions of dipalmitoyphosphatidylcholine (DPPC) lipid monolayers with sugars are influenced by addition of NaCl. This work is of general importance in understanding how sugar-lipid-salt interactions impact biological systems. Using Langmuir isothermal compressions, fluorescence microscopy, atomic force microscopy, and neutron reflectometry we examined DPPC monolayers upon addition of sugars/polyols and/or monovalent salts. Sugar-lipid interactions in the presence of NaCl increased with increasing complexity of the sugar/polyol in the order glycerol < glucose < trehalose. When the anion was altered in the series NaF, NaCl, NaBr, only minor differences were observed. When comparing LiCl, NaCl, and KCl, sodium chloride had the greatest influence on glucose and trehalose interactions with DPPC. We propose that heterogeneity created by cation binding allowed for sugars to bind the lipid headgroups. While cation binding increases in the order K+ < Na+ < Li+, lithium ions may also compete with glucose for binding sites. Thus, both cooperative and competitive factors contribute to the overall influence of salts on sugar-lipid interactions.
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