Properties of mixed monolayers and bilayers of long-chain 5- n-alkylresorcinols and dipalmitoylphosphatidylcholine

Abstract

5- n-Alkylresorcinols with different chain length are known to occur in different plants and procaryonts, for example in the cysts of the Azotobacter genus. To study the monolayer and bilayer behaviour of these amphiphilic compounds, we synthesized three analogues with different chain length, namely 5- n-octadecylresorcinol (18AR), 5- n-nonadecylresorcinol (19AR) and 5- n-heneicosylresorcinol (21AR). The pure alkylresorcinols form stable monolayers, 18AR and 19AR, show a phase transition from a liquid-condensed to a liquid-expanded state at temperatures between 20 and 40°C and at lateral pressures below 15 dyn/cm. In mixtures with dipalmitoylphosphatidylcholine (DPPC), monolayer isobars at higher lateral pressures (20–30 dyn/cm) show an increase in the transition temperature to values greater than 50°C for the liquid-condensed to liquid-expanded state of DPPC when the alkylresorcinol content is increased. Particularly in the liquid-expanded state, alkylresorcinols show a strong condensing effect on DPPC monolayers. 18AR/DPPC monolayers at a pressure of 5 dyn/cm show indications of an azeotropic behaviour. A similar increase in transition temperature is observed by differential scanning calorimetry (DSC) and differential scanning densitometry (DSD) in DPPC/alkylresorcinol bilayers. The transition enthalpy decreases with increasing alkylresorcinol content. All alkylresorcinols seem to be completely miscible with DPPC up to at least 50 mol%. The DSD data indicate a slight condensing effect of the alkylresorcinols on the liquid-crystalline phase of DPPC.