How the chain configuration governs the packing of inverted micelles in the cubic Fd3m-phase

Abstract

The inverted micellar cubic Fd3m phase has attracted significant interest in drug delivery and is also of special biological relevance in the early steps of fat digestion. Applying small angle X-ray diffraction (SAXD) the stability of the Fd3m phase with respect to the chain configuration has been examined. In particular, the fully hydrated monoelaidin system containing the trans elaidic or its counterpart cis oleic fatty acid was investigated. Re-analyzed data of a fully hydrated monoolein–oleic acid (MO–OA) mixture complete this study (data taken from Luzzati et al.). On the basis of determined electron density maps, minute structural details are presented. The decomposition of the cubic Fd3m nanostructure into its apolar and polar moieties allowed the estimation of chain length and water radius depending on the fatty acid content. An increase in the elaidic chain concentration increases the membrane monolayer thickness, and in particular reduces the sphericity of the polar envelope of the small micelles. Furthermore, the electron density maps revealed the geometry of the space filling 5¹² and 5¹²6⁴ cages of the Fd3m phase as well as the loci of the greatest packing stress, which are situated at vertices of the 5¹²6⁴ cages that are placed opposite the hexagonal faces.