Amphiphilic Compounds Assemble into Membranous Vesicles in Hydrothermal Hot Spring Water but Not in Seawater

Abstract

There is a general assumption that amphiphilic compounds, such as fatty acids, readily form membranous vesicles when dispersed in aqueous phases. However, from earlier studies, it is known that vesicle stability depends strongly on pH, temperature, chain length, ionic concentration and the presence or absence of divalent cations. To test how robust simple amphiphilic compounds are in terms of their ability to assemble into stable vesicles, we chose to study 10- and 12-carbon monocarboxylic acids and a mixture of the latter with its monoglyceride. These were dispersed in hydrothermal water samples drawn directly from hot springs in Yellowstone National Park at two pH ranges, and the results were compared with sea water under the same conditions. We found that the pure acids could form membranous vesicles in hydrothermal pool water, but that a mixture of dodecanoic acid and glycerol monododecanoate was less temperature-sensitive and assembled into relatively stable membranes at both acidic and alkaline pH ranges. Furthermore, the vesicles were able to encapsulate nucleic acids and pyranine, a fluorescent anionic dye. None of the amphiphiles that were tested formed stable vesicles in sea water because the high ionic concentrations disrupted membrane stability.

Keywords: hydrothermal environments; lipid vesicles; self-assembly.

Figure 1

Acidic clay-lined acid pool (A), silica-rich alkaline Bison Pool (B) and sample vial (C) with Bison Pool water added and shaken to disperse the amphiphilic compounds.

Figure 2

Decanoic acid (10 carbons) formed droplets at an acidic pH range (Geyser Basin sample) but partially crystallized at the alkaline pH range (Bison Pool sample) even above the transition temperature of 31 °C. All bars show 20 micrometer scales.

Figure 3

Dodecanoic acid at pH 3.3 (Geyser Basin sample) and pH 7.9 (Bison Pool sample). At both pH ranges the vesicles emerged from melted droplets of the acids.

Figure 4

Dodecanoic acid (12 carbons), mixed in a 1:1 mole ratio with its monoglyceride, formed stable membranes at both acidic (Geyser Basin) and alkaline (Bison Pool) pH ranges.

Figure 5

Dodecanoic acid (12 carbons) were mixed in a 1:1 mole ratio with its monoglyceride in pH 7.9 Bison Pool water, then were put through a wet-dry-wet cycle in the presence of 0.1 mM pyranine dye. Large vesicles are barely visible in the phase image (arrows, A) because the very thin bilayer membranes are not clearly resolved. However, the encapsulated pyranine was revealed within vesicles by fluorescence microscopy (B). Some vesicles have excluded the dye for reasons explained in the text.

Figure 6

Lauric acid–glycerol monolaurate (LA–GML, 10 mM) was mixed with yeast ribosomal RNA in a 2:1 ratio by weight in pH 3.3 Geyser Basin water, then dried on a microscope and rehydrated with the same volume of water. Phase (A) and fluorescence (B) images. (C) A control in which the same lipid mixture was prepared without a wet–dry cycle and put through the gel permeation column in the absence of RNA.

Figure 7

LA–GML (10 mM) was mixed with lambda DNA in a 2:1 ratio by weight in pH 3.3 Geyser Basin water, then dried on a microscope slide and rehydrated with the same volume of water. Phase (top) and fluorescence (bottom) images.

Figure 8

Dodecanoic acid (12-carbon lauric acid) in seawater formed crystals protruding from a central mass (A). A mixture of dodecanoic acid and its monoglyceride also formed particulate aggregates surrounded by crystals (B).

Figure 9

LA–GML put through a wet-dry-wet cycle in seawater formed intractable aggregates (A) that did not encapsulate pyranine dye (B).

Figure 10

Ionic composition of seawater compared with hydrothermal water from Mammoth Hot Springs in Yellowstone National Park. Data from [16].