Influences of membrane curvature in lipid hexagonal phases studied by deuterium NMR spectroscopy

The presence of reversed hexagonal phase, H_II, favoring lipids in membranes has been proposed to be significant in various biological processes. Therefore an understanding of the H_II phase and the transition from the lamellar to hexagonal phase is of importance. We have applied deuterium NMR spectroscopy to study the bilayer and reversed hexagonal phases of 1-perdeuteriopalmitoyl-2-linoleoyl-sn-glycero-3-phosphoethanolamine. The difference in packing between the H_II and L_α phases leads to smaller segmental order parameters in the former case. Since the order profiles are sensitive to the geometry of the aggregates, they can be used to extract structural information about the phases. We present a new means of calculating the radius of curvature, R₁, for the H_II phase from ²H NMR data. This method gives a value of R₁ = 18.1 Å, which is in agreement with current understanding of the structure of the H_II phase and with x-ray diffraction data.