Isotherms of dipalmitoylphosphatidylcholine (DPPC) monolayers: Features revealed and features obscured

Abstract

Pressure-area isotherms of dipalmitoylphosphatidylcholine (DPPC) exhibit a two-phase region wherein domains of a liquid-condensed (LC) phase are dispersed in the less ordered liquid-expanded (LE) phase. Fluorescence microscopy has been used over the past years to visualize the shapes displayed by DPPC domains throughout the coexistence region; characteristic domain shapes include those resembling dimpled beans and S-like figures. In this paper we show that the types and distributions of domain shapes formed throughout the coexistence region depend sensitively on the rate of monolayer compression. Additionally, by comparing the relative proportion of LE and LC phases observed to that which is predicted by the isotherm, we find apparent violations of the lever rule. Most importantly, we find that if the monolayer is allowed to age in a state of two-phase coexistence for long times (approx. 12 h), the DPPC domains all become nearly circular. Finally, isotherms generated by compressing very slowly, or those generated at quick speeds but only after the film was left stagnant for a few hours, are markedly different from isotherms produced if the film was compressed within an hour after it was spread. One likely explanation for this behavior is the accumulation of air-borne impurities.