Towards a phenomenological definition of the term ‘gel’

The term ‘gel’ is used so indiscriminately that it has become ambiguous. Existing definitions are reviewed, examples of unfortunate uses of the term are discussed, and important phenomenological characteristics of gels are identified. We propose that the term ‘gel’ should be limited to systems which fulfil the following phenomenological characteristics: (a) they consist of two or more components one of which is a liquid, present in substantial quantity and (b) they are soft, solid, or solid-like materials. We further propose a definition of the solid-like characteristics of gels in terms of the dynamic mechanical properties, viz. a storage modulus, G′(ω), which exhibits a pronounced plateau extending to times at least of the order of seconds and a loss modulus, G″(ω), which is considerably smaller thatn the storage modulus in the plateau region.     

Towards a Rational Synthesis of Large-Pore Zeolite-Type Materials?

A range of novel structures , such as the metalloaluminosilicate UCSB-8 (depicted schematically), are accessible by synthetic strategies that could provide the basis for the rational design of other large-pore zeolites and zeolite-type materials. The combination of experimental approaches with computational methods, for example host–guest shape analysis, may provide further breakthroughs in this field.     

Towards Substrate–Reagent Interaction of Lochmann–Schlosser Bases in THF: Bridging THF Hides Potential Reaction Site of a Chiral Superbase

The metalation of N,N-dimethylaminomethylferrocene in THF by the superbasic mixture of ⁿBuLi/KO^tBu proceeds readily at low temperatures to afford a bimetallic Li₂K₂ aggregate containing ferrocenyl anions and tert-butoxide. Starting from an enantiomerically enriched ortho-lithiated aminomethylferrocene, an enantiomerically pure superbase can be prepared. The molecular compound exhibits superbasic behavior deprotonating N,N-dimethylbenzylamine in the α-position and is also capable of deprotonating toluene. Quantum chemical calculations provide insight into the role of the bridging THF molecule to the possible substrate–reagent interaction. In addition, a benzylpotassium alkoxide adduct gives a closer look into the corresponding reaction site of the Lochmann–Schlosser base that is reported herein.     

Towards the total synthesis of etnangien: synthesis of C32–C42 fragment by using a desymmetrization strategy

The construction of the C32–C42 fragment of etnangien is described using a desymmetrization strategy generating five stereogenic centers from a bicyclic lactone. Another notable feature includes the use of the Sharpless asymmetric epoxidation to generate a stereogenic center at C40.     

Towards a consistent mechanism of emulsion polymerization—new experimental details

The application of atypical experimental methods such as conductivity measurements, optical microscopy, and nonstirred polymerizations to investigations of the ‘classical’ batch ab initio emulsion polymerization of styrene revealed astonishing facts. The most important result is the discovery of spontaneous emulsification leading to monomer droplets even in the quiescent styrene in water system. These monomer droplets with a size between a few and some hundreds of nanometers, which are formed by spontaneous emulsification as soon as styrene and water are brought into contact, have a strong influence on the particle nucleation, the particle morphology, and the swelling of the particles. Experimental results confirm that micelles of low-molecular-weight surfactants are not a major locus of particle nucleation. Brownian dynamics simulations show that the capture of matter by the particles strongly depends on the polymer volume fraction and the size of the captured species (primary free radicals, oligomers, single monomer molecules, or clusters).     

Towards a comprehensive exploitation of agrofood residues: Olive tree – olive oil as example

A comprehensive use of residues from the agrofood industry requires development of a number of steps – discussed in this article taking as a model the olive tree – olive oil binomial – consisting of: (i) extraction, identification and quantitation of valuable products (with isolation of individual compounds, if required); (ii) assessment of beneficial effects (which could be made mainly through metabolomics); (iii) improvement of cultivation varieties (through cross-breeding or other agricultural resources); (iv) commercialization as nutraceuticals, food supplements, pharmaceuticals or even as antibacterial and antifoam agents in water treatment plants; (v) use of the final residues to produce compost, foodstuffs, biogas or heat (after checking the characteristics of these residues which can be very different from those of the original residue). This working scheme may be applied to any other system to break the traditional one cultivation–one commercial product scheme.     

Unicycle graphs with extremal Merrifield–Simmons Index

The Merrifield–Simmons index f(G) of a (molecular) graph G is defined as the number of subsets of the vertex set, in which any two vertices are non-adjacent, i.e., the number of independent-vertex sets of G. By we denote the set of unicycle graphs in which the length of its unique cycle is k. In this paper, we investigate the Merrifield–Simmons index f(G) for an unicycle graph G in . Unicycle graphs with the largest or smallest Merrifield–Simmons index are uniquely determined.     

Synthetic Studies Towards MPC1001: Preparation of a β-hydroxyl-Tyrosine Derivative

A procedure for the asymmetric synthesis of a novel β-hydroxy-α-amino acid deriverative to be used in the preparation of the natural product MPC1001 and analogs has been developed. The amino acid was efficiently prepared in six steps via a Mukaiyama aldol reaction via a diphenyloxazinone and 3-bromo-4-methoxybenzaldehyde.     

pH measurements in non-aqueous and aqueous–organic solvents – definition of standard procedures

pH standardisation procedures in non-aqueous and aqueous-organic solvents are discussed in the light of the newly prepared IUPAC recommendation on measurement of pH in dilute aqueous solutions. Both scientific and metrological aspects are considered, as required by the definitions of primary and secondary methods of measurements recently endorsed by BIPM (Bureau International de Poids et de Mesures, France).     

What is the best definition of a liquid cluster at the molecular scale?

We investigate the ability of different cluster definitions to serve as a good reaction coordinate in molecular simulations of nucleation. In particular, the most commonly used Stillinger criterion [J. Chem. Phys. 38, 1486 (1963)] is compared with the cluster definition introduced by ten Wolde and Frenkel [J. Chem. Phys. 109, 9901 (1998)]. The accuracy of these two different cluster definitions is tested by using molecular dynamics to study the vapor-liquid nucleation of Lennard-Jones argon as a model system. We are able to compare the size of the critical cluster identified by each cluster definition with a completely model-independent value provided by the nucleation theorem, aided by a recently introduced method that accurately extracts the location of the transition state directly from the kinetics. It is found that the Stillinger definition strongly overestimates the size of small molecular clusters by up to a factor of 2. A simple change of the Stillinger radius is unable to rectify this deficiency. On the contrary, the ten Wolde-Frenkel definition, while being only slightly more elaborate than a simple Stillinger criterion, is remarkably successful in identifying the correct molecular excess of the small clusters if the parameters are chosen adequately. The method described here can also be generalized to identify a proper reaction coordinate in other activated processes.     

Towards an accurate and precise determination of the solid–solid transition temperature of enantiotropic systems

This paper presents two distinct methods for the determination of the solid–solid transition temperature (T_tr) separating the temperature ranges of stability of two crystallographic forms, hereafter called morphs, of a same substance. The first method, based on thermodynamic calculations, consists in determining T_tr as the temperature at which the Gibbs free energies of the two morphs are equal to each other. For this purpose, some thermodynamic characteristics of both morphs are required, such as the specific heat capacities, the melting temperatures and the melting enthalpies. These are obtained using the Differential Scanning Calorimetry (DSC). In the second method, T_tr is determined directly by an experimental study of the temperature ranges of stability of each morph. The three main originalities of the method developed are (i) to prepare samples composed by an isomassic mixture of crystals of both morphs, (ii) to set them in a thermostated and agitated suspension, and (iii) to use an in situ Raman spectroscopic probe for the determination of the crystallographic form of the crystals in suspension at equilibrium. Both methods are applied to determine the solid–solid transition temperature of the enantiotropic system of Etiracetam, and both of its two crystallographic forms so far identified, named morph I and morph II. The first method is shown to be very sensitive to the experimental data obtained by DSC while the second experimental method is a more accurate, precise, time- and effort-friendly method for the determination of T_tr. The solid–solid transition temperature of the Etiracetam system, determined with the second method, using three different solvents, is found to be equal to 303.65 K ± 0.5 K.     

Individually double minimum-distance definition of protein–RNA binding residues and application to structure-based prediction

Identifying protein–RNA binding residues is essential for understanding the mechanism of protein–RNA interactions. So far, rigid distance thresholds are commonly used to define protein–RNA binding residues. However, after investigating 182 non-redundant protein–RNA complexes, we find that it would be unsuitable for a certain amount of complexes since the distances between proteins and RNAs vary widely. In this work, a novel definition method was proposed based on a flexible distance cutoff. This method can fully consider the individual differences among complexes by setting a variable tolerance limit of protein–RNA interactions, i.e. the double minimum-distance by which different distance thresholds are achieved for different complexes. In order to validate our method, a comprehensive comparison between our flexible method and traditional rigid methods was implemented in terms of interface structure, amino acid composition, interface area and interaction force, etc. The results indicate that this method is more reasonable because it incorporates the specificity of different complexes by extracting the important residues lost by rigid distance methods and discarding some redundant residues. Finally, to further test our double minimum-distance definition strategy, we developed a classifier to predict those binding sites derived from our new method by using structural features and a random forest machine learning algorithm. The model achieved a satisfactory prediction performance and the accuracy on independent data sets reaches to 85.0%. To the best of our knowledge, it is the first prediction model to define positive and negative samples using a flexible cutoff. So the comparison analysis and modeling results have demonstrated that our method would be a very promising strategy for more precisely defining protein–RNA binding sites.     

Towards a correlation of absolute configuration and chiroptical properties of alkyl aryl sulfoxides: a coupled-oscillator foundation of the empirical Mislow rule?

The absorption and circular dichroism (CD) data for a series of alkyl aryl sulfoxides 1-16 of known S configuration have been analyzed. The strong bathochromic effect exerted by the nitro group in the para position of the phenyl sulfoxides indicates that the sulfur atom acts as an electron donor moiety towards the phenyl ring. Such behavior requires a significant 2p(C)-3sp3(S) overlap, and therefore the phenyl (and p-substituted phenyl) sulfoxides 1-12, as well as the 2-naphthyl sulfoxides 15 and 16, must assume a conformation which permits such orbital overlap. The steric effect of the peri hydrogen in 1-naphthyl-substituted compounds 13 and 14 does not allow a conformation of this type, and in these compounds the above-mentioned 2p(C) and 3sp3(S) orbitals are positioned in almost orthogonal planes. This conformational difference is clearly shown by the absorption spectra: compounds 1-12, 15, and 16 show the lowest energy sigma --> sigma* transition of the sulfoxide chromophore at approximately 250 nm, indicating the existence of a conjugated S=O chromophore. In contrast, the corresponding absorption in 13 and 14 occurs at about 200 nm, indicating the presence of an isolated S=O chromophore. The CD spectra of 13 and 14 show a negative, couplet-like feature between 250 and 200 nm. This spectral feature can be interpreted in terms of exciton coupling between the allowed sigma --> sigma* transition of the isolated S=O chromophore at 200 nm and the 1B transition of the naphthalene chromophore. In fact, the Harada-Nakanishi rule predicts a negative CD couplet for an S-configured sulfoxide in the conformation found by UV analysis, as found experimentally. The CD spectrum of 13 is quantitatively reproduced by DeVoe coupled-oscillator calculations, strongly implying that a coupled-oscillator mechanism is operative in determining the optical activity of 13 and 14. This approach has also tentatively been extended to the conjugated sulfoxides 1-12, taking into account the coupling of the benzene chromophore 1La transition with the sigma --> sigma* transition of the S=O chromophore. In this case the Harada-Nakanishi rule also predicts a negative CD couplet for the S-configured sulfoxides, as found experimentally.     

Studies Towards the Total Synthesis of Methyl Isosartortuoate： The Synthesis of a Cyclic Precusor of the Diene Unit

The preparation of the key intermediate 4 of the methyl isosartortuoate is described. The macrocyclization was completed through an intramolecular [ 2,3]-Wittig ring contraction. The four necessary stereogenic carbons were established by Sharpless asymmetric AE and AD reaction.