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    Crucial role of the hydroxyl group orientation in Langmuir monolayers organization-The case of 7-hydroxycholesterol epimers

    Year: 2019

    Journal: Colloid Surf. A-Physicochem. Eng. Asp., Volume 563, 1-Feb, page 330–339

    Authors: Wnetrzak, Anita; Chachaj-Brekiesz, Anna; Janikowska-Sagan, Maria; Fidalgo Rodriguez, Jose Luis; Minones Conde, Jose; Dynarowicz-Latka, Patrycja

    Keywords: Langmuir monolayers; Phase transitions; Molecular orientation; Film thickness; Brewster angle microscopy

    Oxysterols, which are products of cholesterol oxidation, are important biological molecules. For our investigations we have selected two epimers: 7 alpha-hydroxycholesterol (7 alpha-OH) and 7 beta-hydroxycholesterol (7 beta-OH). Both molecules have been found to be capable of stable Langmuir monolayer formation, however, the characteristic of their monolayers were quite different. Contrary to 7 beta -OH, which isotherm devoid any transition, the pi/A dependence for 7 alpha-OH displayed a broad plateau. Temperature strongly affected this plateau transition - the lower the temperature, the higher the surface pressure of the transition. Such an inverse temperature dependence is uncommon for classical film-forming molecules. Film thickness results excluded multilayers as the probable cause for this plateau. Based on theoretical calculations it was possible to infer that the plateau can be attributed to the change of orientation upon film compression, from being anchored mainly with both polar groups in the pre-plateau region to that with one polar group being detached from the water surface in the postplateau region. Differences in surface behavior of both diastereoisomers have been explained as being due to orientation of OH groups in their molecules; i.e. for 7 beta-OH both hydroxyl groups (at C3 and C7) are situated on the same side of the rings A and B, while for 7 alpha-OH-on different sides, which induces the lifting off of one OH group from the water surface.
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