We show that low-field proton nuclear magnetic resonance (NMR) relaxation and diffusion experiments can be used to study asphaltene aggregation directly in crude oils. Relaxation was found to be multiexponential, reflecting the composition of a complex fluid. Remarkably, the relaxation data for samples with different asphaltene concentrations can be collapsed onto each other by a simple rescaling of the time dimension with a concentration-dependent factor xi, whereas the observed diffusion behavior is unaffected by asphaltene concentration. We interpret this finding in terms of a theoretical model that explains the enhanced relaxation by the transitory entanglement of solvent hydrocarbons within asphaltene clusters and their subsequent slowed motion and diffusion within the cluster. We relate the measured scaling parameters xi to cluster sizes, which we find to be on the order of 2.2-4.4 nm for an effective sphere diameter. These sizes are in agreement with the typical values reported in the literature as well as with the small-angle X-ray scattering (SAXS) experiments performed on our samples.
Calix[4]pyrroles and their congeners are conformationally flexible, nonplanar tetrapyrrolic macrocycles that can recognize anionic substrates. Although the mother calix[4]pyrrole was reported more than 100 years ago, anion recognition accompanied with conformational changes was first discovered in 1996. These compounds have shown preferential binding of halide anions such as fluoride and chloride as well as that of phosphate and carboxylate anions. Extensive efforts have been made to improve the affinity and selectivity. Continuous expansion of related research makes this easy‐to‐make macrocycle more promising. In this review, recent synthetic developments and the anion binding properties of newly developed calix[4]pyrroles are discussed and accomplishments to date are summarized. We hope that this review provides a seed for future developments in this area of chemistry.
In this article, we categorize presently available experimental and theoretical knowledge of various physicochemical and biochemical features of amino acids, as collected in the AAindex database of known 544 amino acid (AA) indices. Previously reported 402 indices were categorized into six groups using hierarchical clustering technique and 142 were left unclustered. However, due to the increasing diversity of the database these indices are overlapping, therefore crisp clustering method may not provide optimal results. Moreover, in various large-scale bioinformatics analyses of whole proteomes, the proper selection of amino acid indices representing their biological significance is crucial for efficient and error-prone encoding of the short functional sequence motifs. In most cases, researchers perform exhaustive manual selection of the most informative indices. These two facts motivated us to analyse the widely used AA indices. The main goal of this article is twofold. First, we present a novel method of partitioning the bioinformatics data using consensus fuzzy clustering, where the recently proposed fuzzy clustering techniques are exploited. Second, we prepare three high quality subsets of all available indices. Superiority of the consensus fuzzy clustering method is demonstrated quantitatively, visually and statistically by comparing it with the previously proposed hierarchical clustered results. The processed AAindex1 database, supplementary material and the software are available at http://sysbio.icm.edu.pl/aaindex/.
Maghemite nanoparticles protected with a shell of Simanek (melamine) dendrons of three different generations were achieved via multiple-step syntheses. The resulting nanoclusters exhibited switchable solubility in various solvents and were examined as soluble matrices for supporting magnetically recoverable homogeneous Pd catalysts in organic solvents and as potential MRI contrast agents in aqueous media. Successful promotion of Suzuki reactions and the observation of generation-dependent dendritic effects with strong MR relaxivities suggest broad application possibilities for dendron-maghemite nanoparticles across many fields.
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