New thionaphthoquinones and their hydroxyl derivatives, bearing alkyl side chains that match the phospholipids POPC and POPE, were synthesized in order to investigate their interactions with lipids. It was observed that, in general, these additives destabilize the lipid bilayer and induce less organized structures with higher curvature, in particular the induction of an hexagonal phase on aqueous POPC mixtures. Moreover, cubic phases, not normally observed in the pure lipids when fully hydrated, were detected. Coexistence of lamellar phases was interpreted as a consequence of microsegregation of the components in the mixtures. These results are in line with previous observations on the effect of structurally similar (hydro)quinones in phase behavior of these lipids. 1. Introduction Quinones are structures present in many naturally occurring compounds [1]. 1,4-Naphthoquinones like vitamin K [2], doxorubicin, mitomycin [3], lapachol [4], plumbagin [5, 6], and others [7–10] are among the examples of this vast class of chemicals used in the treatment of bleeding, lymphoma, carcinoma, and so forth. Only one sulfurated naphthoquinone was found in nature [11], but many were synthesized and proved to be potent inhibitors of Staphylococcus aureus [12], better in vitro antibacterial agents than gentamycin against Staphylococcus aureus and markedly in vitro antifungal against Cryptococcus neoformans, Sporothrix schenckii, and Trichophyton mentagrophytes, when compared with fluconazole [13]. They are moderate-to-good antitubercular agents against Mycobacterium tuberculosis [14]. Furthermore, several thionaphthoquinones have been recently synthesized because of their interesting spectroscopic properties [15] and also as attractive organic dyes due to their high solubility in organic solvents. Their red color in the solid state [16] also leads to applications as organic nonlinear optical materials [17]. Here, we describe some newly prepared thionaphthoquinones, to be used as additives for interacting with lipids normally present in cell membranes, and their influence on the structural behavior of the lipid matrix. Attached to the quinonoid ring of the newly prepared thionaphthoquinones, there is one (ANQ/ANHQ; Scheme 1) or two (BANQ/BANHQ; Scheme 1) SC16H33 chains, similar to those present in the lipids used. The lipids were selected for having different polarities (POPC, low and POPE, high) in water solution. Scheme 1: Synthetic route for thionaphthoquinone additives. 2. Methods Generally, all chemicals, commercially available, were of pure grade and used without
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