Palladium/BINAP(S)-catalyzed asymmetric allylic amination

The enantioselective allylic amination of acyclic allylic carbonates catalyzed by a palladium/(S)-BINAP(S) system was investigated. Amination of several substrates proceeded with high ee. Crotyl carbonates show an unusually high regioselectivity for the branched isomer. The use of (S)-TolBINAP(S) and (S)-3,5-xylyl-BINAP(S) as ligands was found to increase the enantioselectivity of the aminations. A P,S binding mode of the BINAP(S) ligand was found in an X-ray crystallographic study. [reaction: see text]     

Scattering study of vitrifying Cr-Ti solutions

The configurations involved in the spontaneous vitrification of quenched Cr-Ti -phase alloys were investigated by X-ray diffraction and elastic neutron scattering. The two methods yield complementary information: X-rays are sensitive to the topology whereas neutrons emphasize the chemical fluctuations. The results show the presence of two simultaneous but opposing processes, the formation of CsCl-type chemical short range order on one hand and progressive structural disordering on the other. The latter ultimately leads to amorphization. The probable reason for this behavior is that at the temperature in question only the chromium atoms are mobile.     

Chirality changes in reactions of asymmetric molybdenum complexes

Optical rotatory dispersion and circular dichroism studies have allowed the determination of the changes in configuration at the molybdenum center upon displacement of carbonyl and iodide in neomenthylcyclopentadienyl—Mo(allyl)(NO)X systems. Displacement of carbonyl by iodide occurs with retention of configuration. Replacement of iodide with benzenesulfonate followed by replacement of the sulfonate with iodide occurs stereospecifically with net retention of configuration. In the case of cyclopentadienylMo(cyclooctenyl)(NO)I, the enantiomers were separated via a spontaneous resolution through crystallization of the complex in the space group P2₁2₁2₁. These studies have allowed the correlation not only of the absolute configuration at the metal center with CD studies, but also have established that a long wavelength optically active transition at approximately 400 nm can be correlated with endo-exo isomerism. Comparison of the rates of interconversion suggest that endo to exo isomerization occurs via a clockwise rotation of the allyl in the (R)-isomer. Crystallographic details: (—)-(S)-(NMCp)Mo(allyl)(NO)I crystallizes in the space group P2₁2₁2₁ with a 7.221(1), b 12.686(7), c 21.603(7) Å, Z = 4, V = 1979(2) ų; R₁ = 0.039, R₂ = 0.046; (—)-(S)-(Cp)Mo(cyclooctenyl)(NO)I crystallizes in the space group P2₁2₁2₁ with a 8.466(1), b 10.449(2), c 16.372(2) Å, Z = 4, V = 1448.3(6), R₁ = 0.038, and R₂ = 0.046.     

Neutron and X-ray scattering studies of cholera toxin interactions with lipid monolayers at the air-liquid interface

Using neutron/X-ray reflectivity and X-ray grazing incidence diffraction (GID), we have characterized the structure of mixed DPPE:GM₁ lipid monolayers before and during the binding of cholera toxin (CTAB₅) or its B subunit (CTB₅). Structural parameters such as the density and thickness of the lipid layer, extension of the GM₁ oligosaccharide headgroup, and orientation and position of the protein upon binding are reported. Both CTAB₅ and CTB₅ were measured to have 50% coverage when bound to the lipid monolayer. X-ray GID experiments show that both the lipid monolayer and the cholera toxin layer are crystalline. The effects of X-ray beam damage have been assessed and the monolayer/toxin structure does not change with time after protein binding has saturated.     

Finite element error bounds for a curve shrinking with prescribed normal contact to a fixed boundary

We approximate the evolution of a curve subject to motion bycurvature by linear finite elements. The curve evolves insidea given domain and meets orthogonally. We derive optimal boundsfor the error with respect to the L²- and H¹-norms and presentsome computed examples.     

Chasing the Elusive “In-Between” State of the Copper-Amyloid β Complex by X-ray Absorption through Partial Thermal Relaxation after Photoreduction

The redox activity of Cu ions bound to the amyloid-β (Aβ) peptide is implicated as a source of oxidative stress in the context of Alzheimer's disease. In order to explain the efficient redox cycling between Cu^II-Aβ (distorted square-pyramidal) and Cu^I-Aβ (digonal) resting states, the existence of a low-populated “in-between” state, prone to bind Cu in both oxidation states, has been postulated. Here, we exploited the partial X-ray induced photoreduction at 10 K, followed by a thermal relaxation at 200 K, to trap and characterize by X-ray Absorption Spectroscopy (XAS) a partially reduced Cu-Aβ_1–16 species different from the resting states. Remarkably, the XAS spectrum is well-fitted by a previously proposed model of the “in-between” state, hence providing the first direct spectroscopic characterization of an intermediate state. The present approach could be used to explore and identify the catalytic intermediates of other relevant metal complexes.     

Rearrangements in Allylpalladium Complexes with Hemilabile Chelating Ligands

In addition to η³‐ to η¹‐allyl rearrangement, η²‐ to η¹‐chelating‐ligand rearrangements can affect the dynamic properties in allylpalladium complexes containing hemilabile ligands. These rearrangements have the potential of altering stereochemistry, which can ultimately affect the stereochemistry and regiochemistry of reactions upon the allyl group, e.g., allylic alkylation. Apparent rotation of an η³‐allyl in four‐coordinate Pd^II‐complexes can be the result of ligand exchange or intramolecular processes. NMR Studies indicate that the effects of η²‐ to η¹‐chelating‐ligand stereochemistry and rearrangements may be hidden or subtle, and both intra‐ and intermolecular processes may be important.     

Neuropeptide delivery to the brain: a von Willebrand factor signal peptide to direct neuropeptide secretion

Background  

Multiple neuropeptides, sometimes with opposing functions, can be produced from one precursor gene. To study the roles of the different neuropeptides encoded by one large precursor we developed a method to overexpress minigenes and establish local secretion.