Total synthesis of d-(+)-biotin

A total synthesis of d-(+)-biotin is described starting from d-(+)-glucosamine using acyliminium chemistry.     

In situ measurement of biaxial modulus of Si anode for Li-ion batteries

We report in situ measurement of biaxial moduli of a Si thin-film electrode as a function of its lithium concentration. During lithiation, biaxial compressive stress is induced in the Si film and it undergoes plastic flow. At any state-of-charge (SOC), a relatively small delithiation–relithiation sequence unloads and reloads the film elastically. From the stress and strain changes during a delithiation–relithiation cycle, the biaxial modulus of the film is calculated. Stress change is obtained by measuring the change in substrate curvature using a Multi-beam Optical Sensor; the elastic strain change is obtained from the change in SOC. By repeating these measurements at several different values of SOC, the biaxial modulus was seen to decrease from ca. 70 GPa for Li_0.32Si to ca. 35 GPa for Li_3.0Si. Such a significant reduction in elastic modulus has important implications for modeling stress evolution and mechanical degradation in Si-based anodes.     

Real-time measurement of stress and damage evolution during initial lithiation of crystalline silicon

Crystalline to amorphous phase transformation during initial lithiation in (100) Si wafers is studied in an electrochemical cell with Li metal as the counter and reference electrode. During initial lithiation, a moving phase boundary advances into the wafer starting from the surface facing the lithium electrode, transforming crystalline Si into amorphous Li(x)Si. The resulting biaxial compressive stress in the amorphous layer is measured in situ, and it was observed to be ca. 0.5 GPa. High-resolution TEM images reveal a very sharp crystalline-amorphous phase boundary, with a thickness of ～1 nm. Upon delithiation, the stress rapidly reverses and becomes tensile, and the amorphous layer begins to deform plastically at around 0.5 GPa. With continued delithiation, the yield stress increases in magnitude, culminating in a sudden fracture of the amorphous layer into microfragments, and the cracks extend into the underlying crystalline Si.     

Total synthesis of the originally proposed and revised structures of palmerolide A and isomers thereof

Palmerolide A is a recently disclosed marine natural product possessing striking biological properties, including potent and selective activity against the melanoma cancer cell line UACC-62. The total syntheses of five palmerolide A stereoisomers, including the originally proposed (1) and the revised [ent-(19-epi-20-epi-1)] structures, have been accomplished. The highly convergent and flexible strategy developed for these syntheses involved the construction of key building blocks 2, 19-epi-2, 20-epi-2, ent-2, 3, ent-3, 4, and ent-4, and their assembly and elaboration to the target compounds. For the union of the building blocks, the Stille coupling reaction, Yamaguchi esterification, Horner-Wadsworth-Emmons olefination, and ring-closing metathesis reaction were employed, the latter being crucial for the stereoselective formation of the macrocycle of the palmerolide structure. The Horner-Wadsworth-Emmons olefination and the Yamaguchi lactonization were also investigated and found successful as a means to construct the palmerolide macrocycle. The syntheses were completed by attachment of the enamide moiety through a copper-catalyzed coupling process.     

Shell Buckling of Imperfect Multiwalled Carbon Nanotubes—Experiments and Analysis

Experiments were conducted in which multiwalled carbon nanotubes were subjected to uniaxial compression and shell-buckling loads were measured. A comparison with existing theoretical models shows that the predictions are about 40–50% smaller than the experimentally measured buckling loads. This is in contrast to the classical elastic shell studies in which the experimental values were always substantially lower than the predicted values due to imperfection sensitivity. It is proposed that the discrepancy between the predicted and measured value might be due to imperfections in the multiwalled nanotubes in the form of sp ³ bonds between the tube walls, which introduce shear coupling between them. An analytical model is presented to estimate the effect of the shear coupling on the critical buckling strain, which shows that the contribution from shear coupling increases linearly with the effective shear modulus between the walls. Further, this contribution increases with the number of walls; the increment from each additional wall progressively decreases.     

Chemical synthesis and biological evaluation of palmerolide A analogues

Molecular design and chemical synthesis of several palmerolide A analogues allowed the first structure activity relationships (SARs) of this newly discovered marine antitumor agent. From several analogues synthesized and tested (ent- 1, 5- 14, 21- 26, 50, 51), compounds 25 (with a phenyl substituent on the side chain) and 51 (lacking the C-7 hydroxyl group) were the most interesting, exhibiting approximately a 10-fold increase in potency and equipotency, respectively, to the natural product. These findings point the way to more focused structure activity relationship studies.     

Blood compatibility and permeability of heparin-modified polysulfone as potential membrane for simultaneous hemodialysis and LDL removal

Heparin was covalently immobilized on PSf membranes to obtain a dialysis membrane with high affinity for LDL. WCA and streaming potential measurements were performed to investigate wettability and surface charge of the membranes. The morphology of the membranes was investigated by SEM. An ELISA was used to measure the adsorption and desorption of LDL on plain and modified PSf. Blood compatibility was studied by measurement of thrombin time, partial thromboplastin time, kallikrein activity and platelet adhesion. It was found that the blood compatibility of the membrane was improved by covalent immobilization of heparin at its surface. However, PSf-Hep membrane showed higher flux recovery after BSA solution filtration, which revealed antifouling property of PSf-Hep membranes.     

Orientation-dependent adhesion strength of a rigid cylinder in non-slipping contact with a transversely isotropic half-space

Recently, Chen and Gao [Chen, S., Gao, H., 2007. Bio-inspired mechanics of reversible adhesion: orientation-dependent adhesion strength for non-slipping adhesive contact with transversely isotropic elastic materials. J. Mech. Phys. solids 55, 1001–1015] studied the problem of a rigid cylinder in non-slipping adhesive contact with a transversely isotropic solid subjected to an inclined pulling force. An implicit assumption made in their study was that the contact region remains symmetric with respect to the center of the cylinder. This assumption is, however, not self-consistent because the resulting energy release rates at two contact edges, which are supposed to be identical, actually differ from each other. Here we revisit the original problem of Chen and Gao and derive the correct solution by removing this problematic assumption. The corrected solution provides a proper insight into the concept of orientation-dependent adhesion strength in anisotropic elastic solids.     

Derivatized fullerenes bearing multiple electron donors: synthesis and charge transfer properties

A series of methano-C60 adducts bearing up to six electron donating N,N-dimethylaniline units (denoted as D compounds), along with their analogues without the dimethylamino groups as references (R compounds), were synthesized. The redox properties of the D compounds in solutions were evaluated spectroscopically in reference to the R compounds. According to UV/vis absorption results, there are obviously ground-state intramolecular charge-transfer complexes in the D series, and the charge-transfer effects apparently become saturated with only two donor units in the molecule. The photoinduced intramolecular electron-transfer properties of the D compounds were investigated via fluorescence measurements. The emission from intramolecular exciplexes can be found only in the D molecule with two electron donor units. Throughout the D series, the fluorescence properties are highly sensitive to the solvent polarity, with the emission completely quenched for all of the molecules in a polar solvent like methylene chloride. Mechanistic implications of the results are discussed.     

The Influence of Elastic Strain on Catalytic Activity in the Hydrogen Evolution Reaction

Understanding the role of elastic strain in modifying catalytic reaction rates is crucial for catalyst design, but experimentally, this effect is often coupled with a ligand effect. To isolate the strain effect, we have investigated the influence of externally applied elastic strain on the catalytic activity of metal films in the hydrogen evolution reaction (HER). We show that elastic strain tunes the catalytic activity in a controlled and predictable way. Both theory and experiment show strain controls reactivity in a controlled manner consistent with the qualitative predictions of the HER volcano plot and the d‐band theory: Ni and Pt's activities were accelerated by compression, while Cu's activity was accelerated by tension. By isolating the elastic strain effect from the ligand effect, this study provides a greater insight into the role of elastic strain in controlling electrocatalytic activity.