The stacking order of a 14.30-Å Mg-vermiculite from Santa-Olalla, Spain, has been determined from Weissenberg photographs. The results prove that the Mg-vermiculite structure is not a 2-layer polytype structure, but a semi-ordered one. Because the structure is semi-ordered, its resolution needed a dual approach: (1) a direct approach using an electron density projection along the y axis in conjunction with a one-dimensional electron-density projection onto the z axis; and (2) an indirect approach in which the observed intensities along the (0,k) and (1,k) reciprocal rods were compared to the calculated intensities given by model defect structures. The semi-ordered structure of the Mg-vermiculite results from ±b/3 shifts in the stacking of the silicate layers. The shifts are randomly either along +b or -b.

The stacking order of a bi-ionic K/Mg vermiculite from Malawi (Nyasaland), has been determined from Weissenberg data. The sample corresponds to a K/Mg interstratified vermiculite containing 50% K layers (dK = 1.01 nm) and 50% Mg layers with two layers of water (dMg = 1.44 nm). The observed intensities along (0, 0), (h, 0), (0, k) and (1, k) reciprocal rods were compared to the calculated intensities given by model defect structures. It was found that: 1) The (h, 0), (0, k) and (1, k) rods reveal the interstratification phenomenon which was previously observed on the (0, 0) rod; 2) The exchange of Mg by K does not occur randomly in a single interlayer, but occurs in interlayer domains sufficiently extensive to allow the reorganisation of the layer stacking and development of the ordered K-vermiculite structure from the semi-ordered structure characteristic of magnesium vermiculites; 3) For this sample, the interlayer water of the Mg phase occurs in two coexisting configurations; one configuration has water molecules forming octahedral coordination around the Mg2+, the other has water which is not linked to the Mg2+ cation forming two planes.

The organoclay quasicrystals formed by exchange reactions of fluoromagnesian smectite (FMS) and two kinds of cationic surfactants were investigated by X-ray diffraction (XRD) of both aqueous suspensions and dried powders. ζ potential measurements were also carried out. The two cationic surfactants employed in this study, dodecyltriphenylphosphonium bromide (C12PBr) and dodecyltrimethylammonium bromide (C12NBr), have the same alkyl chain length but different head group structures. In aqueous suspensions of C12N/FMS, regular stacking with a basal spacing of 2.28 nm was observed within the range 0.4 to 1.0 of the cation exchange capacity (CEC) of the C12N/FMS ratios. The basal spacing was constant in this range. The most intense diffraction for the C12N/FMS suspension was observed at 0.6 CEC, and a further increase in the C12N/FMS ratio resulted in a decline of the diffraction intensity and a slight increase in the peak width. On the other hand, in C12P/FMS suspensions with the surfactant/FMS ratios <0.6 CEC, only weak, broad XRD peaks were observed. With higher C12P/FMS ratios (0.8 and 1.0 CEC), strong diffraction peaks appeared with a basal spacing of 3.20 nm. The difference in the observed basal spacings of C12N/FMS and C12P/FMS tactoids was ∼0.9 nm, which did not agree with the difference in the estimated height of the two surfactants (∼0.1 nm). These differences of the tactoid structures between C12N/FMS and C12P/FMS suspensions could be explained in terms of the difference in the adsorption manner of the surfactants on the silicate surface and in the size of the surfactant head groups.

Radio and mm observations play an important role in determining the star formation properties of high redshift galaxies. However, most galaxies at high redshift are too faint to be detected individually at these wavelengths. A way to study this population of galaxies is to use stacking. By averaging the emission of a large number of galaxies detected in optical or near infrared surveys, we can achieve statistical detection.

We investigate methods for stacking data from interferometric surveys. Interferometry poses unique challenges in stacking due to the nature of imaging of this data. We have compared directly stacking the uv data with stacking of the imaged data, the latter being the typically used approach. Using simulated data, we find that uv-stacking may provide around 50% less noise and that image based stacking systematically loses around 10% of the flux.

The spatial and temporal pattern of cone packing during marmoset foveal development was explored to understand the variables involved in creating a high acuity area. Retinal ages were between fetal day (Fd) 125 and 6 years. Cone density was determined in wholemounts using a new hexagonal quantification method. Wholemounts were labeled immunocytochemically with rod markers to identify reliably the foveal center. Cones were counted in small windows and density was expressed as cones × 103/mm2 (K). Two weeks before birth (Fd 125–130), cone density had a flat distribution of 20–30 K across the central retina encompassing the fovea. Density began to rise at postnatal day 1 (Pd 1) around, but not in, the foveal center and reached a parafoveal peak of 45–55 K by Pd 10. Between Pd 10 and 33, there was an inversion such that cone density at the foveal center rose rapidly, reaching 283 K by 3 months and 600 K by 5.4 months. Peak foveal density then diminished to 440 K at 6 months and older. Counts done in sections showed the same pattern of low foveal density up to Pd 1, a rapid rise from Pd 30 to 90, followed by a small decrease into adulthood. Increasing foveal cone density was accompanied by 1) a reduction in the amount of Müller cell cytoplasm surrounding each cone, 2) increased stacking of foveal cone nuclei into a mound 6–10 deep, and 3) a progressive narrowing of the rod-free zone surrounding the fovea. Retaining foveal cones in a monolayer precludes final foveal cone densities above 60 K. However, high foveal adult cone density (300 K) can be achieved by having cone nuclei stack into columns and without reducing their nuclear diameter. Marmosets reach adult peak cone density by 3–6 months postnatal, while macaques and humans take much longer. Early weaning and an arboreal environment may require rapid postnatal maturation of the marmoset fovea.

We study a microfluidic flow model where the movement of several charged species is coupled with electric field and the motion of ambient fluid. The main numerical difficulty in this model is the net charge neutrality assumption which makes the system essentially overdetermined. Hence we propose to use the involutive and the associated augmented form of the system in numerical computations. Numerical experiments on electrophoresis and stacking show that the completed system significantly improves electroneutrality constraint conservation and recovers analytical results while a direct implementation of the initial model fails.

Chemically modified nucleotide analogs have gained widespread popularity for probing structure–function relationships. Among the modifications that were incorporated into RNAs for assessing the role of individual functional groups, the phenyl nucleotide has displayed surprising effects both in the contexts of the hammerhead ribozyme and pre-mRNA splicing. To examine the conformational properties of this hydrophobic base analog, we determined the crystal structure of an RNA double helix with incorporated phenyl ribonucleotides at 1.97 Å resolution. In the structure, phenyl residues are engaged in self-pairing and their arrangements suggest energetically favorable stacking interactions with 3′-adjacent guanines. The presence of the phenyl rings in the center of the duplex results in only moderate changes of the helical geometry. This finding is in line with those of earlier experiments that showed the phenyl analog to be a remarkably good mimetic of natural base function. Because the stacking interactions displayed by phenyl residues appear to be similar to those for natural bases, reduced conformational restriction due to the lack of hydrogen bonds with phenyl as well as alterations in its solvent structure may be the main causes of the activity changes with phenyl-modified RNAs.

The wobble GoU pairs have been implicated in several biological processes where RNA molecules play a key role. We review the geometrical and conformational properties of wobble GoU pairs on the basis of available crystal structures of RNAs at high resolution. The similarities with the wobble A+oC pairs and UoU pairs are illustrated, while the differences with the recently discovered bifurcated G[bull ]U pairs are contrasted.