Effect of chirality on monoacylglycerol ester monolayer characteristics: 3-Monopalmitoyl-sn-glycerol

Abstract

The effect of chirality on the thermodynamic behavior, the morphological features, and the 2D lattice structures of 3-monopalmitoyl-sn-glycerol monolayers is studied. The results are based on measurements of the surface pressure-area (π-A) isotherms, Brewster angle microscopy (BAM), and Synchrotron X-ray diffraction at grazing incidence (GIXD). The π-A isotherms of the enantiomeric 3-monopalmitoyl-sn-glycerol monolayers measured between 10 and 34°C are similar to those of the racemic 1-monopalmitoyl-rac-glycerol monolayers. Evaluating the temperature dependence of the phase transition pressure (πt) gives access to the transition entropy. The absolute ΔS values increase as the temperature decreases demonstrating that the ordering of the condensed phase increases at lower temperatures. The comparison with the corresponding racemate shows that the condensed phase of the racemate has a higher stability indicating preferred heterochiral interactions. Similar to the racemic monoglycerol esters round and cardioid-like domains are observable but the tendency to irregular deviations in the inner texture and shape from the perfect circular domain in the mesoscopic domain topography is larger for the chiral 3-monopalmitoyl-sn-glycerol than for the racemic 1-monopalmitoyl-rac-glycerol. Systematic GIXD measurements of 3-monopalmitoyl-sn-glycerol monolayers are carried out over large pressure intervals at 5, 10, and 15°C. The comparison of the contour plots for the three temperatures shows a shift of the contour plots displaying three separated diffraction signals, which indicate the dominance of the chiral nature, to higher lateral pressures with increasing temperature. The similarity of the lattice data obtained by fitting 3 and 2 peaks for conditions where the contour plots resemble those of NN tilted or NNN tilted orthorhombic structures demonstrates the small effect of chirality on the lattice structures in this region.The cross-sectional area, A0, is almost unaffected by surface pressure and temperature and amounts to values between 19.7Å2 and 19.8Å2, as expected for a rotator phase at the lower limit. The tilt angle with respect to the surface normal t decreases with increasing pressure. The tilting transition pressure was obtained by plotting 1/cos(t) vs. the lateral pressure and extrapolating to the zero tilt angle. The values for the untilted state are quite high and have not been reached experimentally, but they are slightly smaller than those of the corresponding racemic mixture. The lattice distortion d plotted versus sin2(t) and extrapolated to zero tilt provides the d0 values which are slightly smaller than 0 showing that the influence of chirality on the lattice distortion is weak and decreases with increasing temperature.