The bending elasticity of 1-monoolein upon relief of packing stress

The bending elasticity of the monoglyceride 1-monoolein (IMO) in water has been measured in the inverse hexagonal(HII) phase at 37 degrees C. At this temperature, fully hydrated MO is normally in an inverse bicontinuous cubic phase based on Schwarz's D surface, Q(II)(D);. The addition of either of t he C-23 chainlength hydrocarbons, tricosane or 9-cis-tricosene, at mole fra ctions with respect to MO in excess of 0.05, induces a phase transition int o the H-II phase. This transition is understood to occur because packing st resses in the hydrophobic regions of the! H-II phase are reduced to levels where this phase is at a lower free energy than the Q(II)(D) phase. We have used X-ray diffraction on gravimetrically prepared samples and samples sub jected to an osmotic stress to determine the bending energy of MO in this p hase. The evidence suggests that 9-cis-tricosene can relieve almost all of the packing stress in the Hn phase. In this case, we fmd that the spontaneo us radius of curvature at the pivotal surface, R-0, is -20.0 +/-. 0.3 Angst rom, and the monolayer bending rigidity, kappa, is (1.2 +/- 0.1) x 10(-20) J. Comparing these energetics with those of the Q(II)(D) phase indicates th at the packing stress for an excess water, H-II phase at 37 degrees C in th e absence of 9-cis-tricosene would constitute at least 50% of the total equ ilibrium free energy. With tricosane, the packing stress cannot be complete ly relieved because tricosane melts at 47.6 degrees C in the bulk. The resu lts from geometric measurements on this system suggest that when the packin g stress is not fully relieved in the H-II phase, the polar/apolar interfac e is deformed away from being cylindrical. Treating the interface as if it were cylindrical leads to a nonphysical location of the pivotal surface tha t is in disagreement with all previous measurements, a 35% increase in the magnitude of R-0, and a 4-fold increase in the calculated bending rigidity.