15N,(1)H Heteronuclear correlation NMR of gramicidin A in DMPC-d(67)

We have studied gramicidin A, an environmentally sensitive polymorphic pentadecapeptide, fully 15N-labelled and dispersed in a highly deuterated phospholipid bilayer system. By submitting the sample to fast magic angle spinning, we were able to reduce the polypeptide amide hydrogen linewidths to 160 Hz, and hence to partially resolve them. By correlating these resonances with the 40 Hz wide dipolar coupled 15N in a 2D-CROPSY (cross-polarization spectroscopy) experiment, it was possible to observe the 20 partially overlapping 1H-15N signal pairs from the polypeptide backbone and sidechains. Both chemical shift distributions closely match those of the same peptide in SDS micelles, but only poorly match those of conformationally different gramicidin A in trifluoroethanol, dimethylsulfoxide, or methanol/chloroform mixture. Our results are indicative of the N-to-N right-handed beta6.3-helix conformation of gramicidin A and offer sufficient resolution to encourage development of experiments to measure orientational or distance restraints using through-space dipolar couplings.     

Solution conformation by NMR and molecular modeling of three sulfide-free somatostatin octapeptide analogs compared to angiopeptin

Summary The conformation in dimethylsulfoxide of the somatostatin derivative angiopeptin and of three disulfide-free analogs was estimated by two-dimensional nuclear magnetic resonance spectroscopy at room temperature. The resulting 3D molecular graphics were compared and shown to reflect the observed differences in the inhibition of restenosis after rat aorta balloon injury by these octapeptide inhibitors. Angiopeptin and its active analog 2 displayed a relatively rigid conformation of the cyclic hexapeptide backbone due to the presence of two well-defined hydrogen bonds, further stabilized by a third hydrogen bond outside the ring. No such constraints were detected for the two biologically inactive analogs, which, compared to 2, had a two-atom longer or shorter hexapeptide ring. The well-defined structure of compound 2 may serve as an improved pharmacophore for this new class of drugs.     

Shape-tunable polymer nodules grown from liposomes via ring-opening metathesis polymerization

A new inisurf (acting as surfactant and initiator) molecule for ring-opening metathesis polymerization (ROMP) was synthesized and used in aqueous solution in order to control the size and shape of polymer nodules grown from liposomes. Nodules were observed to grow in size with conversion of monomer, and depending on the monomer used, they adopted either a spherical or comet-like shape. Here, we investigate polymer production from a liposome surface. We use a hydrophobic derivative of the Grubbs catalyst positioned at the liposome surface to allow for ROMP of monomers dissolved in the aqueous outer phase. We obtain nodules of polymer that can grow up to tens of micrometers, unveiling new efficient possibilities of polymerization from a membrane in an aqueous solution.     

Noncovalent functionalization of multi-walled carbon nanotubes by pyrene containing polymers

Multi-walled carbon nanotubes (MWNTs) have been solubilized in water and in various organic solvents by noncovalent side-wall functionalization by pyrene containing polymers.     

Highly enantiomerically enriched chlorophosphine boranes: synthesis and applications as P-chirogenic electrophilic blocks

The stereoselective synthesis of P-chirogenic chlorophosphine boranes 4 was investigated by HCl acidolysis of the corresponding aminophosphine boranes 10. The reaction afforded the P-N bond cleavage with inversion of the configuration at the phosphorus center, leading to the chlorophosphine boranes 4 with high to excellent enantiomeric purities (80-99% ee), except in the case of the chloro-1-naphthylphenylphosphine borane 4d. Reaction conditions and workup significantly influence the enantiomeric purity of the product, with the exception of the o-anisyl- and o-tolylchlorophenylphosphine boranes, 4b and 4c, which were found to be particularly stable even after purification by chromatography on silica gel. Reaction of the chlorophosphine boranes 4 with various nucleophiles, such as carbanions, phenolates, thiophenolates, or amides, afforded the corresponding organophosphorus borane complexes via P-C, P-O, P-S, and P-N bond formation, respectively, in 34-93% yield and with up to 99% ee. This work demonstrates the importance of chlorophosphine boranes 4 as new and powerful electrophilic building blocks for the highly stereoselective synthesis of P-chirogenic organophosphorus compounds.     

Matrix-assisted laser desorption/ionisation mass spectrometry for the direct analysis of enzymatically digested kappa- iota- and hybrid iota/nu-carrageenans

Enzymatically digested oligosaccharides of kappa-, iota- and hybrid iota/nu-carrageenans were analysed using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry in the negative-ion mode. nor-Harmane was used as matrix. Depending on the stock concentration and the laser intensity applied, the oligosaccharides exhibited losses of sulphate units (neutralised by the Na+ ion, and thus non-stable), leaving the primary backbone structure in most cases with only the deprotonated sulphate groups (carrying the negative charge, stable). This meant that kappa- and iota-oligosaccharides could not be easily distinguished from one another since they share the same primary backbone structure. However, for the hybrid iota/nu-oligosaccharides the primary backbone structure could be identified since the nu-carrageenan repeating unit differs from that of the kappa/iota-carrageenan unit. For all types of oligosaccharides, the results indicated cleavage of an anhydrogalactose unit from the non-reducing end. Specifically, for the hybrid oligosaccharides of iota/nu-carrageenans, this type of fragmentation means that the nu-carrageenan unit is not positioned on the non-reducing end of the hybrid oligosaccharides. Dehydration reactions, and exchange reactions of Na+ with K+ and Ca2+, were also observed.     

Fundamental role of the fostriecin unsaturated lactone and implications for selective protein phosphatase inhibition

Key derivatives and analogues of fostriecin were prepared and examined that revealed a fundamental role for the unsaturated lactone and confirmed the essential nature of the phosphate monoester. Thus, an identical 200-fold reduction in protein phosphatase 2A (PP2A) inhibition is observed with either the saturated lactone (7) or with an analogue that lacks the entire lactone (15). This 200-fold increase in PP2A inhibition attributable to the unsaturated lactone potentially may be due to reversible C269 alkylation within the PP beta12-beta13 active site loop accounting for PP2A/4 potency and selectivity.     

Evidence of a phase transition of RbHCO3 from high-resolution solid-state 13C and 87Rb NMR by comparison with KHCO3

A variable-temperature high-resolution 13C and 87Rb solid-state NMR study of powder rubidium hydrogencarbonate, RbHCO3, is presented for the first time. At ambient temperature, RbHCO3 is formed by centrosymmetric dimers linked by hydrogen bonds, but almost no information is available on this compound concerning proton disorder and the low-temperature phase. However, potassium hydrogencarbonate, KHCO3, which has an isomorphic structure for the high temperature phase, was well studied: it undergoes a non-ferroic, non-ferroelectric phase transition at Tc = 318 K between two monoclinic structures. The protons are disordered in an asymmetric double-well potential in the low-temperature phase, and the double-well potential becomes symmetric in the high-temperature phase. By comparison with recent solid-state NMR experimental results on KHCO3, we show that RbHCO3 undergoes a phase transition at Tc approximately 245 K, and give evidence that the proton dynamic disorder in both compounds is very similar.     

Exploring the free-energy landscape of a short peptide using an average force

The reversible folding of deca-alanine is chosen as a test case for characterizing a method that uses an adaptive biasing force (ABF) to escape from the minima and overcome the barriers of the free-energy landscape. This approach relies on the continuous estimation of a biasing force that yields a Hamiltonian in which no average force is exerted along the ordering parameter xi. Optimizing the parameters that control how the ABF is applied, the method is shown to be extremely effective when a nonequivocal ordering parameter can be defined to explore the folding pathway of the peptide. Starting from a beta-turn motif and restraining xi to a region of the conformational space that extends from the alpha-helical state to an ensemble of extended structures, the ABF scheme is successful in folding the peptide chain into a compact alpha helix. Sampling of this conformation is, however, marginal when the range of xi values embraces arrangements of greater compactness, hence demonstrating the inherent limitations of free-energy methods when ambiguous ordering parameters are utilized.