title: Pressure effects on lipidic direct phases: the dodecyl trimethyl ammonium chloride-water system.
abstract: The direct lyotropic polymorphism of dodecyltrimethylammonium chloride (DTAC) was investigated by synchrotron X-ray diffraction at different water concentrations under compression up to 2 kbar, i.e., in the pressure intermediate range where interesting biophysical transformations occur and the functional characteristics of cell membranes are altered. The results show that pressure induces the transition from the hexagonal phase to the micellar Pm3n cubic phase in hydrated samples (c between 0.5 and 0.6, c being the weight concentration of lipid in the mixture) and the transition from the bicontinuous Ia3d cubic phase to the hexagonal phase in drier samples (c = 0.8). By increasing the pressure on very dry samples, a lamellar L(alpha) phase was observed to form transitorily at the Ia3d cubic-hexagonal phase transition. Phase compressibility and then the lipid and water partial molecular compressibilities were derived as a function of pressure and concentration. As a result, we assessed the very low compressibility of the hydration water within the lipid phases, and we demonstrated that the compressibility of DTAC is very dependent on pressure. Moreover, the molecular parameters of DTAC calculated in the different phases during compression confirmed that pressure induces small but continuous conformational changes, definitely different from the large changes observed in lipid molecules forming type II structures.
authors.affiliation: Dipartimento di Scienze Applicate ai Sistemi Complessi, Università Politecnica delle Marche, Via Ranieri 65, I-60131 Ancona, Italy.
authors.forename: Lydia
authors.initials: L
authors.lastname: Paccamiccio
authors.forename: Michela
authors.initials: M
authors.lastname: Pisani
authors.forename: Francesco
authors.initials: F
authors.lastname: Spinozzi
authors.forename: Claudio
authors.initials: C
authors.lastname: Ferrero
authors.forename: Stephanie
authors.initials: S
authors.lastname: Finet
authors.forename: Paolo
authors.initials: P
authors.lastname: Mariani
mesh_terms: D008055:Lipids / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D014867:Water / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D008055:Lipids; D000644:Quaternary Ammonium Compounds; D014867:Water; C022806:dodecyltrimethylammonium