Measuring the bending rigidity of microbial glucolipid (biosurfactant) bioamphiphile self-assembled structures by neutron spin-echo (NSE): Interdigitated vesicles, lamellae and fibers

Bending rigidity, k, is classically measured for lipid membranes to characterize their nanoscale mechanical properties as a function of composition. Widely employed as a comparative tool, it helps understanding the relationship between the lipid's molecular structure and the elastic properties of its corresponding bilayer. Widely measured for phospholipid membranes in the shape of giant unilamellar vesicles (GUVs), bending rigidity is determined here for three self-assembled structures formed by a new biobased glucolipid bioamphiphile, rather associated to the family of glycolipid biosurfactants than phospholipids. In its oleyl form, glucolipid G-C18:1 can assemble into vesicles or crystalline fibers, while in its stearyl form, glucolipid G-C18:0 can assemble into lamellar gels. Neutron spin-echo (NSE) is employed in the q-range between 0.3 nm-1 (21 nm) and 1.5 nm-1 (4.1 nm) with a spin-echo time in the range of up to 500 ns to characterize the bending rigidity of three different structures (Vesicle suspension, Lamellar gel, Fiber gel) solely composed of a single glucolipid. The low (k = 0.30 ± 0.04 kbT) values found for the Vesicle suspension and high values found for the Lamellar (k = 130 ± 40 kbT) and Fiber gels (k = 900 ± 500 kbT) are unusual when compared to most phospholipid membranes. By attempting to quantify for the first time the bending rigidity of self-assembled bioamphiphiles, this work not only contributes to the fundamental understanding of these new molecular systems, but it also opens new perspectives in their integration in the field of soft materials.

Membrane adaptation in the hyperthermophilic archaeon Pyrococcus furiosus relies upon a novel strategy involving glycerol monoalkyl glycerol tetraether lipids

Microbes preserve membrane functionality under fluctuating environmental conditions by modulating their membrane lipid composition. Although several studies have documented membrane adaptations in Archaea, the influence of most biotic and abiotic factors on archaeal lipid compositions remains underexplored. Here, we studied the influence of temperature, pH, salinity, the presence/absence of elemental sulfur, the carbon source, and the genetic background on the lipid core composition of the hyperthermophilic neutrophilic marine archaeon Pyrococcus furiosus. Every growth parameter tested affected the lipid core composition to some extent, the carbon source and the genetic background having the greatest influence. Surprisingly, P. furiosus appeared to only marginally rely on the two major responses implemented by Archaea, i.e., the regulation of the ratio of diether to tetraether lipids and that of the number of cyclopentane rings in tetraethers. Instead, this species increased the ratio of glycerol monoalkyl glycerol tetraethers (GMGT, aka. H-shaped tetraethers) to glycerol dialkyl glycerol tetrathers (GDGT) in response to decreasing temperature and pH and increasing salinity, thus providing for the first time evidence of adaptive functions for GMGT. Besides P. furiosus, numerous other species synthesize significant proportions of GMGT, which suggests that this unprecedented adaptive strategy might be common in Archaea. This article is protected by copyright. All rights reserved.

Diblock, Triblock and Cyclic Amphiphilic Copolymers with CO2 Switchability: Effects of Topology

The combination of topology and CO2 switchability could provide new options for amphiphilic copolymers. Cyclic molecules supply novel topologies, and CO2 switching provides stimulus responsiveness. Cyclic poly(2-(diethylamino)ethyl methacrylate)-b-poly(ethylene oxide) and their corresponding block copolymers were prepared from poly(ethylene oxide) and 2-(diethylamino)ethyl methacrylate via atom transfer radical polymerization and Keck allylation with a Hoveyda-Grubbs catalyst. Changes in conductivity, surface activity, and hydrodynamic size were examined to illustrate the switchability of the produced amphiphilic copolymers upon contact with CO2 in the presence of water. The reversible emulsification and switchable viscosity behaviors of the copolymers were also demonstrated.

Entropic effects enable life at extreme temperatures

Maintaining membrane integrity is a challenge at extreme temperatures. Biochemical synthesis of membrane-spanning lipids is one adaptation that organisms such as thermophilic archaea have evolved to meet this challenge and preserve vital cellular function at high temperatures. The molecular-level details of how these tethered lipids affect membrane dynamics and function, however, remain unclear. Using synthetic monolayer-forming lipids with transmembrane tethers, here, we reveal that lipid tethering makes membrane permeation an entropically controlled process that helps to limit membrane leakage at elevated temperatures relative to bilayer-forming lipid membranes. All-atom molecular dynamics simulations support a view that permeation through membranes made of tethered lipids reduces the torsional entropy of the lipids and leads to tighter lipid packing, providing a molecular interpretation for the increased transition-state entropy of leakage.

Characterization of drug encapsulation and retention in archaea-inspired tetraether liposomes

Archaea-inspired lipids exhibit reduced membrane permeability and increased retention of hydrophilic drugs in liposomes.

Formation and Properties of Vesicles from Cyclic Amphiphilic PS-PEO Block Copolymers

Linear polystyrene-poly(ethylene oxide)-polystyrene (PS-PEO-PS) block copolymers and corresponding cyclized PS-PEO counterparts with three different PS molecular weights were synthesized and self-assembled to investigate the effects arising from the topology. Linear PS₅-PEO₄₅-PS₅ (L1) and cyclic PS₁₀-PEO₄₅ (C1) formed micelles. As previously reported for poly(n-butyl acrylate) and PEO block copolymers, the micelles from C1 showed more than 30 °C higher phase transition temperature (cloud point, T_c) than those from L1. Linear PS₁₀-PEO₄₅-PS₁₀ (L2) and cyclic PS₂₀-PEO₄₅ (C2) resulted in the formation of a structure called large compound micelles. Self-assembly of linear PS₄₀-PEO₄₈-PS₄₀ (L3) and cyclic PS₈₆-PEO₄₈ (C3) lead to the formation of vesicles. The vesicles were characterized by TEM, DLS, and SLS. Remarkably, the vesicles from L3 (T_c = 69, 59, and 48 °C in the presence of 1, 5, and 10 wt % of NaCl, respectively) were found to be somewhat more thermally stable than those from C3 (T_c = 62, 52, and 43 °C in the presence of 1, 5, and 10 wt % of NaCl, respectively). This trend of the thermal stability was counterintuitively opposed to the case of the micelles. Moreover, T_c of the vesicles was controlled by the ratio of L3 and C3.

Contrasting Topological Effect of PEG-Containing Amphiphiles to Natural Lipids on Stability of Vesicles

Topology of amphiphiles is important to control physicochemical properties of supramolecular assemblies. Nature demonstrates higher stability of membrane composed of lipids with a macrocyclic aliphatic tail than those with linear tails, which likely results from the restricted molecular structures of the macrocyclic lipids, allowing for closer molecular packing. In contrast, here we report that a PEG-containing macrocyclic amphiphile shows lower stability of vesicles than the corresponding acyclic one. The macrocyclic amphiphile consists of an aromatic hydrophobic part with chirality in which both ends are strapped by octaethylene glycol via phosphoric ester groups, while the acyclic amphiphile bears tetraethylene glycol chains attached to both ends of the hydrophobic part. Because of the thermoresponsive property of PEG to change its conformation, the hydrophobic part of the macrocyclic amphiphile undergoes a larger thermal conformational change than that of the acyclic one. In addition, the cyclic amphiphile has a larger molecular area, which likely reduces the vesicular stability compared with the acyclic one. Such a contrasting topological effect caused by macrocyclization at the aliphatic part seen in the natural system and at the hydrophilic part demonstrated in this study leads to expand the molecular design of amphiphiles for both increasing and decreasing the stability of vesicles by molecular topology.

Tuneable enhancement of the salt and thermal stability of polymeric micelles by cyclized amphiphiles

Cyclic molecules provide better stability for their aggregates. Typically in nature, the unique cyclic cell membrane lipids allow thermophilic archaea to inhabit extreme conditions. By mimicking the biological design, the robustness of self-assembled synthetic nanostructures is expected to be improved. Here we report topology effects by cyclized polymeric amphiphiles against their linear counterparts, demonstrating a drastic enhancement in the thermal, as well as salt stability of self-assembled micelles. Furthermore, through coassembly of the linear and cyclic amphiphiles, the stability was successfully tuned for a wide range of temperatures and salt concentrations. The enhanced thermal/salt stability was exploited in a halogen exchange reaction to stimulate the catalytic activity. The mechanism for the enhancement was also investigated. These topology effects by the cyclic amphiphiles offer unprecedented opportunities in polymer materials design unattainable by traditional means.

Construction of a 3-Miktoarm Star from Cyclic Polymers

Cyclic polymers have intriguing physical properties, including those found in biological membranes for greater temperature, salt and acid stability. Although, many unique and complex synthetic cyclic structures have been prepared, there are no reports of ABC miktoarm stars constructed of three cyclic polymers with very different chemical compositions. We report such a structure in one pot at 25 °C by modulating the copper catalyst activity using combinations of solvents and ligands.

Preservation of microbial lipids in geothermal sinters

Lipid biomarkers are widely used to study the earliest life on Earth and have been invoked as potential astrobiological markers, but few studies have assessed their survival and persistence in geothermal settings. Here, we investigate lipid preservation in active and inactive geothermal silica sinters, with ages of up to 900 years, from Champagne Pool, Waiotapu, New Zealand. Analyses revealed a wide range of bacterial biomarkers, including free and bound fatty acids, 1,2-di-O-alkylglycerols (diethers), and various hopanoids. Dominant archaeal lipids include archaeol and glycerol dialkyl glycerol tetraethers (GDGTs). The predominance of generally similar biomarker groups in all sinters suggests a stable microbial community throughout Champagne Pool's history and indicates that incorporated lipids can be well preserved. Moreover, subtle differences in lipid distributions suggest that past changes in environmental conditions can be elucidated. In this case, higher archaeol abundances relative to the bacterial diethers, a greater proportion of cyclic GDGTs, the high average chain length of the bacterial diethers, and greater concentrations of hopanoic acids in the older sinters all suggest hotter conditions at Champagne Pool in the past.

Effect of growth temperature and growth phase on the lipid composition of the archaeal membrane from Thermococcus kodakaraensis

Archaea have unique membrane lipids typified by ether linkages of the glycerol-to-isoprenoid chains with sn-2,3 stereochemistry that runs against the naturally occurring sn-1,2 stereochemistry of the glycerophospholipids of Bacteria and Eukarya. Membrane lipids were extracted and analyzed from the hyperthermophilic archaeon, Thermococcus kodakaraensis, cultivated at various temperatures. At all growth temperatures examined, both the diphytanylglycerol diether (archaeol, C(20)) and diphytanyldiglycerol tetraether (caldarchaeol, C(40)) were identified as saturated forms, and no other lipids could be identified. The ratio of caldarchaeol to archaeol increased with increasing growth temperature, particularly at 93 degrees C. A larger amount of archaeol was detected from cells in the logarithmic phase than from those in the stationary phase at all temperatures examined. These results indicate that T. kodakaraensis modulated the membrane lipid composition depending on both the growth phase and the growth temperature, and suggest that the membrane fluidity to environmental change was maintained by altering the length of the hydrocarbon chains, and not by side-chain saturation such as double-bond hydrogenation nor by such a modification as cyclopentane ring formation.

Temperature- and pH-Induced Structural Changes in the Membrane of the Hyperthermophilic Archaeon Aeropyrum pernix K1

The influence of pH and temperature on the structural organization, fluidity and permeability of the hyperthermophilic archaeon membrane was investigated in situ by a combination of electron paramagnetic resonance (EPR) and fluorescence emission spectroscopy. For EPR measurements, Aeropyrum pernix cells, after growing at different pHs, were spin-labeled with the doxyl derivative of palmitic acid methylester (MeFASL(10,3)). From the EPR spectra maximal hyperfine splitting (2A (max)) and empirical correlation time (tau (emp)), which are related to mean membrane fluidity, were determined. The mean membrane fluidity increases with temperature and depends on the pH of the growth medium. Computer simulation of the EPR spectra shows that membrane of A. pernix is heterogeneous and consists of the regions characterized with three different types of motional characteristics, which define three types of membrane domains. Order parameter and proportion of the spin probes in the three types of domains define mean membrane fluidity. The fluidity changes of the membrane with pH and temperature correlate well with the ratio between the fluorescence emission intensity of the first and third bands in the vibronic spectra of pyrene, I(1)/I(3). At pH 7.0 a decrease of I(1)/I(3) from 2.0 to 1.2, due to the penetration of pyrene into the nonpolar membrane region, is achieved at temperatures above 65 degrees C, the lower temperature limit of A. pernix growth.

In vitro biosynthesis of ether-type glycolipids in the methanoarchaeon Methanothermobacter thermautotrophicus

The biosynthesis of archaeal ether-type glycolipids was investigated in vitro using Methanothermobacter thermautotrophicus cell-free homogenates. The sole sugar moiety of glycolipids and phosphoglycolipids of the organism is the beta-D-glucosyl-(1-->6)-D-glucosyl (gentiobiosyl) unit. The enzyme activities of archaeol:UDP-glucose beta-glucosyltransferase (monoglucosylarchaeol [MGA] synthase) and MGA:UDP-glucose beta-1,6-glucosyltransferase (diglucosylarchaeol [DGA] synthase) were found in the methanoarchaeon. The synthesis of DGA is probably a two-step glucosylation: (i) archaeol + UDP-glucose --> MGA + UDP, and (ii) MGA + UDP-glucose --> DGA + UDP. Both enzymes required the addition of K(+) ions and archaetidylinositol for their activities. DGA synthase was stimulated by 10 mM MgCl(2), in contrast to MGA synthase, which did not require Mg(2+). It was likely that the activities of MGA synthesis and DGA synthesis were carried out by different proteins because of the Mg(2+) requirement and their cellular localization. MGA synthase and DGA synthase can be distinguished in cell extracts greatly enriched for each activity by demonstrating the differing Mg(2+) requirements of each enzyme. MGA synthase preferred a lipid substrate with the sn-2,3 stereostructure of the glycerol backbone on which two saturated isoprenoid chains are bound at the sn-2 and sn-3 positions. A lipid substrate with unsaturated isoprenoid chains or sn-1,2-dialkylglycerol configuration exhibited low activity. Tetraether-type caldarchaetidylinositol was also actively glucosylated by the homogenates to form monoglucosyl caldarchaetidylinositol and a small amount of diglucosyl caldarchaetidylinositol. The addition of Mg(2+) increased the formation of diglucosyl caldarchaetidylinositol. This suggested that the same enzyme set synthesized the sole sugar moiety of diether-type glycolipids and tetraether-type phosphoglycolipids.

Astonishing diversity of natural surfactants: 3. Carotenoid glycosides and isoprenoid glycolipids

Carotenoid glycosides and isoprenoid glycolipids are of great interest, especially for the medicinal, pharmaceutical, food, cosmetic, flavor, and fragrance industries. These biologically active natural surfactants have good prospects for the future chemical preparation of compounds useful as antimicrobial, antibacterial, and antitumor agents, or in industry. More than 300 unusual natural surfactants are described in this review article, including their chemical structures and biological activities.