Rhamnolipids are amphiphilic glycolipids biosynthesized by bacteria that, due to their low toxicity and biodegradability, are potential replacements for synthetic surfactants. The previously limited access to pure materials at the gram scale has hindered extensive characterization of rhamnolipid structure-performance behavior. Here, we present an efficient and versatile synthetic methodology from which four diastereomers of the most common monorhamnolipid, α-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate, are prepared and subsequently characterized. Exploration of their behavior at the air-water interface is reported and analyzed in terms of the absolute configuration of the lipid tail carbinols at pH 4.0 and 8.0. All diastereomers exhibit a minimum surface tension of about 28 mN/m without a significant difference between the protonated (nonionic) or deprotonated (anionic) states. At pH 4.0 (nonionic), all diastereomers have a critical micelle concentration (CMC) in the micromolar range. At pH 8.0 (anionic), CMC values for the (R,R), (S,S), and (S,R) diastereomers are approximately an order of magnitude higher than in their nonionic states, whereas the (R,S) diastereomer exhibits a CMC about five times larger.
The evolution of solution aggregates of the anionic form of the native monorhamnolipid (mRL) mixture produced by Pseudomonas aeruginosa ATCC 9027 is explored at pH 8.0 using both experimental and computational approaches. Experiments utilizing surface tension measurements, dynamic light scattering, and both steady-state and time-resolved fluorescence spectroscopy reveal solution aggregation properties. All-atom molecular dynamics simulations on self-assemblies of the most abundant monorhamnolipid molecule, L-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-C10-C10), in its anionic state explore the formation of aggregates and the role of hydrogen bonding, substantiating the experimental results. At pH 8.0, at concentrations above the critical aggregation concentration of 201 μM but below ~7.5 mM, small premicelles exist in solution; above ~7.5 mM, micelles with hydrodynamic radii of ~2.5 nm dominate, although two discrete populations of larger lamellar aggregates (hydrodynamic radii of ~10 and 90 nm) are also present in solution in much smaller number densities. The critical aggregation number for the micelles is determined to be ~26 monomers/micelle using fluorescence quenching measurements, with micelles gradually increasing in size with monorhamnolipid concentration. Molecular dynamics simulations on systems with between 10 and 100 molecules of Rha-C10-C10 indicate the presence of stable premicelles of seven monomers with the most prevalent micelle being ~25 monomers and relatively spherical. A range of slightly larger micelles of comparable stability can also exist that become increasing elliptical with increasing monomer number. Intermolecular hydrogen bonding is shown to play a significant role in stabilization of these aggregates. In total, the computational results are in excellent agreement with the experimental results.
2-Aminothiazolobenzazepine (2-ATBA), 7-[(1-methyl-1H-pyrazol-4-yl)methyl]-6,7,8,9-tetrahydro-5H-[1,3]thiazolo[4,5-h][3]benzazepin-2-amine, is a D2 partial agonist that has demonstrated antipsychotic effects in a rodent in vivo efficacy model. The metabolite profile showed that 2-ATBA is mainly metabolized by oxidation. However, identification of the oxidation site(s) in the 2-aminothiazole group presents a challenge for the traditional metabolite identification methods such as liquid chromatography/mass spectrometry and NMR due to the lack of unique tandem mass spectrometry fragmentation patterns for ions with the 2-aminothiazole group oxidized at different sites and the lack of stability for purification or reference standard synthesis. We describe the characterization of the oxidized heteroatoms of the 2-aminothiazole group via gas-phase ion-molecule reactions (GPIMR) in a modified linear quadrupole ion trap mass spectrometer. The GPIMR reagents used were dimethyl disulfide, tert-butyl peroxide, and tri(dimethylamino)borane. Each reagent was introduced into the ion trap through the helium line and was allowed to react with the protonated metabolites. The ionic ion-molecule reaction products and their fragmentation profiles were compared with the profiles of the ionic ion-molecule reaction products of protonated reference compounds that had specific heteroatom functionalities. The oxidized 2-aminothiazole metabolite of 2-ATBA showed a similar GPIMR profile to that of the reference compounds with a tertiary N-oxide functionality and distinct from the profiles of the reference compounds with N-aryl hydroxylamine, nitroso, or pyridine N-oxide functionalities. This study demonstrates the feasibility of fingerprinting the chemical nature of oxidized nitrogen functional groups via GPIMR profiling for metabolite structure elucidation.
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