Fig 3.
Pressure-area isotherms for DPPC alone and with isovaleric acid (Panel A), methylmalonic acid (Panel B), or propionic acid (Panel C) added to the subphase. IVA = isovaleric acid. MMA = methylmalonic acid. PPA = Propionic acid. Surface pressure, Π (total lateral pressure) is expressed in units of milliNewtons/meter (mN/m) on the y-axis. Mean molecular area is in units of Å2 on the x-axis. The organic acids shift the isotherms to higher pressures, decrease the plateaus (two-phase coexistence region), and change the slopes (compressibility) of the monolayers. Panel D shows the surface pressure at a mean molecular area of 60 Å2, which is approximately that obtained in bilayers (25). The acids increase the pressures in a concentration dependent manner, with methylmalonic acid having the greatest effect and propionic acid the least effect. All experiments were performed at 23.1°C.