In the current work, the microviscosity, encapsulation, and permeability of 2-ketooctanoic acid (KOCOOH) vesicle membranes were investigated by steady-state and time-resolved fluorescence techniques, using 1,6-diphenyl-1,3,5-hexatriene (DPH), riboflavin, and calcein as fluorescence probes. Our results show that the microviscosity of KOCOOH membranes is similar to that of common bilayer aggregates, the KOCOOH vesicles have the ability to encapsulate hydrophilic guests, and the KOCOOH membranes are permeable to ions. The permeation of OH− across KOCOOH membranes can be well described using a first-order kinetic model. The KOCOOH vesicles may be a good alternative protocell model that possesses some functional properties necessary for early cell membranes. To the best of our knowledge, this is the first report on the characteristics of vesicle membranes of single-tailed keto-acid amphiphiles.
Single-tailed short-chain alkyl keto-acids/salts, a class of fatty acid/salt derivatives, such as sodium 2-ketooctanate (KOCOONa), are a kind of weakly acid/salt type amphiphiles and plausible prebiotic molecules, and the current understanding of their aggregation behavior in aqueous solutions is still limited. Herein, the aggregation behavior of KOCOONa in aqueous solution was studied by changing its concentration ( C ), using equilibrium surface tension, conductivity, and fluorescence measurements. The aggregates formed were characterized using freeze-fracture and cryogenic transmission electron microscopy, dynamic light scattering, atomic force microscopy, and confocal laser scanning microscopy. A concentration-driven stepwise aggregation was identified in the KOCOONa solution. Vesicles can spontaneously form from the single-component aqueous solution, with a critical vesicle concentration (CVC) of ∼15 mM, which is obviously lower than that of octanoic acid/salt (120–200 mM). With increasing C , a vesicle-to-micelle transition can occur, showing a critical micelle concentration (CMC) of ∼80 mM. In addition, the membrane permeability of the KOCOONa vesicles was examined using small-size Calcein and large-size FITC-BSA as fluorescence probes, showing a size-selective permeability, similar to short-chain (C8-C11) fatty acid vesicles. For the first time, the aggregation behavior of single-tailed keto-acid salt surfactant is reported.
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