The effect of twin‐tailed sidearms on sodium cation transport in synthetic hydraphile cation channels

The preparation of novel tris(macrocycle)s that transport Na⁺ through phospholipid bilayers is reported. All of the reported structures have the following structural elements: sidechain‐macrocycle‐spacer‐macro‐cycle‐spacer‐macrocycle‐sidechain. The overall extension of the channel system is defined by the lengths of the spacer chains. The flexible sidearms are thought to be aligned with the lipid axis and opposite to the spacer chains. To the extent the spacer chains contribute to organization of the structure and isolation of the cation‐containing pore from the surrounding lipid chains, more hydrophobic chains are expected to enhance cation transport. A comparison of C₁₂H₂₅<N18N>(CH₂)₁₂<N18N>(CH₂)₁₂<N18N> C₁₂H₂₅, 1, with (C₁₂H₂₅)₂N(CH₂)₂<N18N>(CH₂)₁₂<N18N>(CH₂)₁₂<N18N>(CH₂)2N(C₁₂H₂₅)₂, 11, shows that the Na⁺ transport is, indeed, enhanced.     

Numerical approximation of non-homogeneous, non-convex vector variational problems

Summary. In this paper we study a numerical scheme for non-convex vector variational problems allowing for microstructure, based on the approximation of gradient Young measures. We present a convergence result and some numerical experiments. Received March 26, 2000 / Revised version received November 13, 2000 / Published online March 20, 2001     

On discrete frames associated with semidirect products

For groups which are the semidirect product of some vector group with a unimodular group we prove that the existence of a discrete frame obtained from an at-most countable set of vectors through the action of a given unitary representation implies that the representation in use has to be square-integrable.     

P-wave velocity-porosity relations and homogeneity lengths in a realistic deposition model of sedimentary rock

A realistic model of a porous rock is generated by simulating critical processes in sedimentary rock formation. The simulated processes include deposition of grains in different energy environments, compression with grain rearrangement effects due to the stratigraphic column pressure and cementation. It has been shown that such processes are fundamental in determining the appropriate correlations for the system to exhibit realistic electrical and hydraulic conductivities of sandstones. Here we analyse the theoretical model from the geometrical and acoustic standpoint, determining the relations of the homogenization volumes and sound speed to the resulting porosity of the model. For the quartz cemented contact model we find the correct velocity magnitudes and concavities in the velocity-porosity curves. We also show that using anisotropic rescaling as a compaction stage can lead to inadequate acoustic properties for the rock model, due to the importance of grain rearrangement.     

Dysideasterols A–E,Five New Uncommon Polyhydroxylated Steroids from the South China Sea Sponge Dysidea sp.

Five new uncommon polyhydroxylated steroids, dysideasterols A–E ( 1 – 5 ), were isolated from the South China Sea sponge Dysidea sp. Their structures were established by detailed analysis of spectral data and verified via single‐crystal X‐ray‐diffraction analysis of compound 1 . The absolute configuration of 1 was determined by the modified Mosher method.     

Magnetic properties of Fe and Tb in TbxFe1−x amorphous films studied with soft X-ray circular and linear dichroism

We have used linearly and circularly polarized X-rays to determine the magnetic properties of several Tb_xFe_1−x amorphous films. Absorption measurements on the M_4.5 edges of Tb and the L_2.3 edges of Fe allowed us to obtain information about the size and direction of local magnetic moments. Our results confirm that linear dichorism in rare earth M_4.5 edges can give useful information about both crystal field and magnetic effects.     

A model for hydrogen chemisorption including image charge effects

The Green function method is applied to an Anderson hamiltonian including the interaction of the chemisorbed hydrogen atom with the surface plasmon of the metal substrate. The resulting adatom density of states presents a periodic structure with a period equal to the surface plasmon energy. Numerical results are obtained for the case of hydrogen adsorbed on tungsten explaining the peaks observed in photo-emission experiments.