Article pubs.acs.org/Langmuir
Cite This: Langmuir XXXX, XXX, XXX−XXX
Hopanoid-like Sterols Form Compact but Fluid Films Agustín Mangiarotti,†,‡ Vanesa V. Galassi,§ Elida N. Puentes,†,‡ Rafael G. Oliveira,†,‡ Mario G. Del Pópolo,§ and Natalia Wilke*,†,‡ Facultad de Ciencias Químicas, Departamento de Química Biológica Ranwel Caputto, and ‡CONICET, Centro de Investigaciones en Química Biológica de Córdoba (CIQUIBIC), Universidad Nacional de Córdoba, Haya de la Torre y Medina Allende s/n, Ciudad Universitaria, X5000HUA Córdoba, Argentina § CONICET y Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Cuyo, Padre Jorge Contreras 1300, Parque General San Martín, M5502JMA Mendoza, Argentina Downloaded via UNIV AUTONOMA DE COAHUILA on July 19, 2019 at 02:11:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: Hopanoids are pentacyclic molecules present in membranes from some bacteria, recently proposed as sterol surrogates in these organisms. Diplopterol is an abundant hopanoid that, similar to sterols, does not self-aggregate in lamellar structures when pure, but forms monolayers at the air−water interface. Here, we analyze the interfacial behavior of pure diplopterol and compare it with sterols from different organisms: cholesterol from mammals, ergosterol from fungi, and stigmasterol from plants. We prepared Langmuir monolayers of the compounds and studied their surface properties using different experimental approaches and molecular dynamics simulations. Our results indicate that the films formed by diplopterol, despite being compact with low mean molecular areas, high surface potentials, and high refractive index, depict shear viscosity values similar to that for fluid films. Altogether, our results reveal that hopanoids have similar interfacial behavior than that of sterols, and thus they may have the capacity of modulating bacterial membrane properties in a similar way sterols do in eukaryotes.
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INTRODUCTION Sterols are a subgroup of the steroids with a hydroxyl group at the C3-position and a branched chain of eight or more carbon atoms at C17. The most studied sterol is cholesterol (cholest5-en-3β-ol, see the structure in Figure 1), which is the sterol present in mammalian cells. It was first isolated from human gallstones in 1815 by Michel E. Chevreul, and it does not get distributed uniformly among cell membranes; while plasma membranes contain a high concentration of cholesterol, mitochondria, endoplasmic reticulum, and other organelles have much less cholesterol.1−3 The function of the high amounts of this sterol and the possible interactions with other lipids and proteins has been an active research field since 1970s.3 It is now accepted that cholesterol is an important regulator of membrane rheology and permeability, and it imposes ordering in fluid membranes and fluidity in solid membranes, inducing the so-called “liquid ordered phase”.4 Pure cholesterol does not form lamellar phases but does form stable monolayers, and their behavior upon compression has been studied.5−9 These films are very compact, with values for the compressional moduli comparable to those of liquidcondensed or solid monolayers. However, they are fluid, with a viscosity comparable to those of liquid-expanded phases.10 This dual behavior makes pure monolayers of this compound unique. © XXXX American Chemical Society
In yeast, it was found that ergosterol is the most abundant sterol;11 see the structure in Figure 1. In contrast to animal and fungal cells, which contain only one major sterol, plant cells synthesize a complex array of sterol mixtures in which sitosterol, stigmasterol (see the structure in Figure 1), and 24-methylcholesterol often predominate.12 All of the studied sterols form very compact monolayers,13−19 but the fluidity of the pure films has not been tested as far as we know. Several sterols have been proved to induce the liquidordered phase, and they have been referred to as “membraneactive” sterols.20 The minimal structural requirements for a sterol to be “membrane-active” have been discussed. Different authors proposed that sterol’s structure requirements include a flat fused ring system, a β-hydroxyl (or other very small polar group) at position 3, a cholesterol-like tail, and a relatively small minimal area (
