Self-assembly of the peptide A10K – Intermediate state in aggregate formation

We have studied the synthetic surfactant-like peptide A10K in solution. Upon mixing the freeze dried, essentially amorphous peptide powder in heavy water, the peptides self-assemble into long ribbon-like aggregates with a fixed cross section of circa 3x8 nm. It is still unclear whether this self-assembly is equilibrium like a surfactant micelle formation or whether it corresponds to a precipitation of a solid phase. Through light scattering measurements on dilution series, the solubility of the ribbons has been determined to 4.7 μM. However, quantitative NMR spectroscopy shows a monomer concentration of 3 mM, corresponding to a roughly 60 times supersaturation, and independent of the total concentration. Samples prepared directly at, or below this specific concentration shows no, or only minor signs of aggregation. In combination with a broad peak in high resolution 1H NMR spectroscopy we conclude that the formation of the A10K aggregates occurs through an intermediate state in equilibrium with the peptide monomers. Please click Additional Files below to see the full abstract

A desymmetrization route to fused Troger's base analogues: Synthesis, isolation, and characterization of the first anti-anti diastereomer of a fused tris-Troger's base analogue

A desymmetrization route to fused Troger's base analogues was developed. In this way, the synthesis of the first example of an anti-anti diastereomer of a fused tris-Troger's base analogue was accomplished. The resulting compound 5b is a nonlinear symmetric regioisomer obtained from p-bromoaniline in 7% yield. The corresponding syn-anti diastereomer 5a was obtained in 4% yield

Determination of the degree of branching in normal and amylopectin type potato starch with 1H-NMR spectroscopy : Improved resolution and two-dimensional spectroscopy

Starch from genetically modified potatoes was found to be highly branched compared with normal potato varieties through the use of 1H-NMR spectroscopy. The average chain length, blue-value,and the wavelength at maximum absorptivity clearly show that the new potato varieties produce amylopectin starch. Correlation between the degree of branching as determined by 1H-NMR and starch-iodine complexation, expressed as blue-value, was good and the NMR-method gives low standard deviation. For the first time, the anomeric proton, H-I, of a (1→4)-α-linked D-glucose residue and the H-I of the glucose residue of a non-reducing end have been assigned separate chemical shifts in starch. Assignments were made as determined from two-dimensional homonuclear and 1H-13C heteronuclear spectroscopy (COSY, HMQC, and HMBC). The molecular weight in degraded starch and pullulan were determined by means of NMR-spectroscopy. These results were in accordance with determinations by size exclusion chromatography and with the known molecular weights of pullulan standards

Expanding the scope of Lewis acid catalysis in water: Remarkable ligand acceleration of aqueous ytterbium triflate catalyzed Michael addition reactions

Significant rate acceleration of metal-catalyzed Michael addition reactions in water was observed upon addition of small, dibasic ligands. Ytterbium triflate and TMEDA was the most effective combination leading to a nearly 20-fold faster reaction than in the absence of ligand

Differentiation and quantification of synthetic phosphatidylethanol (PEth) homologues by (1)H- and (13)C-NMR in polar organic solvents.

Various phosphatidylethanol (PEth) derivatives, the corresponding reversed positional isomers (RPI-PEths), lyso-PEth-16:0, and penta-deuterium-labeled PEth analogs (d5-PEths), were synthesized by enzyme-independent synthetic routes. A general solvent system consisting of a mixture of acetone-d6 and methanol-d4 (97:3; v/v) was found to provide a good solubilizing capacity and excellent hydrogen-1 NMR ((1)H-NMR) peak resolution of various PEth homologues. Analytical differentiation of PEth from the corresponding RPI-PEth by carbon-13 NMR ((13)C-NMR) was demonstrated by comparison of the (13)C-NMR signals of the carbonyl groups, the allylic positions, and of the β-carbons. An exemplary stable long-term room temperature, DMSO-d6-based, and proton-sensitive quantitative nuclear magnetic resonance ((1)H-qNMR) independently quantified calibrator comprising PEth-16:0/18:1 for liquid chromatography (tandem) mass spectrometry (LC-MS/MS) analytical applications were prepared by employment of sodium dodecyl sulfate (SDS) as a solubilizing additive. In summary, novel hypothetically occurring PEth derivatives, e.g., RPI-PEths, have been independently synthesized with regio- and stereochemical control. Use of polar organic solvents, e.g., mixtures of acetone-d6 and methanol-d4 or DMSO-d6, improves spectral line shapes as compared to traditional hydrophobic solvents and allow for analytical differentiation between closely related PEth derivatives, as well as LC-MS/MS-independent concentration determination of dissolved single species by employment of (1)H-qNMR

Expanding the Scope of Lewis Acid Catalysis in Water: Remarkable Ligand Acceleration of Aqueous Ytterbium Triflate Catalyzed Michael Addition Reactions.

Significant rate acceleration of metal-catalyzed Michael addition reactions in water was observed upon addition of small, dibasic ligands. Ytterbium triflate and TMEDA was the most effective combination leading to a nearly 20-fold faster reaction than in the absence of ligand

Synthesis and self-aggregation of enantiopure and racemic molecular tweezers based on the bicyclo[3.3.1]nonane framework.

A pair of molecular tweezers (syn-4) that consists of quinoline and pyrazine units fused to a bicyclic framework is presented. The tweezers were synthesised both as a racemic mixture (rac-4) and an enantiomerically pure form ((R,R,R,R)-4) starting from either racemic or enantiomerically pure bicyclo[3.3.1]nonane-2,6-dione (3). Homochiral dimers were observed in the solid state for rac-4. The self-association of both rac-4 and (R,R,R,R)-4 was studied in solution. A weak self-association constant in CDCl(3) was estimated by (1)H NMR spectroscopic dilution titration experiments in both cases, following several proton resonances. For this purpose, a general normalisation model for the accurate determination of association constants from multiple datasets was developed. In contrast to the solid state, no diastereomeric discrimination was observed for rac-4 in solution

Absence of reverse anomeric effect in furanosides

A series of conformationally restricted N-"furanosides" has been synthesized, where the carbons of the tetrahydrofuran ring are kept in one plane by a rigid norbornane skeleton, permitting only the ring oxygen to move above or below the tetrahydrofuran ring plane. This causes the substituents of the anomeric carbon to occupy a pseudoaxial or a pseudoequatorial position. On protonation of these "norbornane-furanosides" with trifluoromethanesulfonic acid, all three compounds exhibited decreasing coupling constants for the anomeric proton, indicating a shift toward the pseudoaxial conformation. The coupling constant measurements were supported by volume integration of NOESY cross-peaks, which also showed a change toward the pseudoaxial conformation upon protonation of the nitrogen. These results provide no evidence for the so-called reverse anomeric effect; on the contrary they are in full agreement with a small normal anomeric effect

Potential Determining Salts in Microemulsions: Interfacial Distribution and Effect on the Phase Behavior

In this work we consider potential determining salts, also referred to as phase transfer agents for a future objective of electrochemistry at the oil-water interface in microemulsions. We have studied these salts, composed of a hydrophilic and a hydrophobic ion, in microemulsion stabilized by nonionic surfactants with an oligo ethylene oxide headgroup. NMR measurements show that the salts preferentially dissociate across the surfactant interface between the oil and water domains, and hence create a potential drop across the surfactant film, and back to back diffuse double layers in the oil and water phases. These observations are also supported by Poisson-Boltzmann calculations. This adsorption like event stabilizes the microemulsion. Repulsive long-range interactions between thin lamellae of surfactant and water lamellae in oil were observed using SAXS, thus confirming the presence of electrostatic forces mediated through the oil domain. We also observed that reversing the charges on the potential determining salts had opposite effects on the phase inversion temperature