Surface properties of 1,2-dipalmitoyl-3-acyl-sn-glycerols

Abstract

Stereospecific 1,2-dipalmitoyl-sn-glycerol and a series of 1,2-dipalmitoyl-3-acyl-sn-glycerols (TGs) with 3-acyl chains of two through six and eight carbons in length were synthesized. Pressure-area isotherms at 27 .degree.C, surface melting temperatures (Ts), and equilibrium spreading pressures (esp) measured at the bulk melting temperature (Tf) were obtained for each TG and for dipalmitin. Whereas dipalmitin and the 3-acetyl-TG condense directly to an expanded mesomorphous state (30-33 .ANG.2/palmitoyl chain at the vapor pressure, .pi.v), the 3-propionyl- through 3-octanoyl-TGs show an area per molecule (in the liquid at .pi.v) that increases linearly from 105 to 130 .ANG.2/molecule (slope = 5 .ANG.2/CH2 group). This slope suggests that the 3-acyl chains are lying flat on the water at the end of the gas-liquid transition. Before solidification at 42-47 .ANG.2/molecule, the 3-propionyl- through 3-hexanoyl-TGs show a transition corresponding to the immersion of the 3-acyl chain. The pressure at this transition, .pi.tr, vs. 3-acyl carbon number is linear and indicates a chain immersion energy of 497 cal mol-1 per CH2. In contrast, the 3-octanoyl chain is not forced into the water but rather is pushed into the monolayer to lie parallel to the palmitoyl chains. As the sn-3 chain is lengthened, Ts decreases from 68 to 25.degree.C, but the 3-octanoyl monolayer does not solidify even at 5.degree.C because the short upright octanoyl chains fluidize the palmitoyl chains. The esp (at Tf) drops from 31.7 mN m-1 for dipalmitin to 20.6 mN m-1 for the 3-acetyl-TG. The esp for the 3-octanoyl-TG is 14.8 mN m-1. In summary, increasing the length of the shorter 3-acyl chain of these diacid TGs decreases the Ts and the esp. In monolayers, the shorter 3-acyl chains, lying flat on the surface at .pi.v, either submerge into the aqueous phase if .pi.tr is less than esp or stand up and fluidize the monolayer if .pi.tr is greater than esp.