Polymerization of styrene to isotactic polymer with MAO-Ni(acac)2. Examination of the factors that influence activity and stereospecificity

The soluble catalyst system methylaluminoxane (MAO)-Ni(acac), (acac: acetylacetonate) gives polystyrene consisting of an amorphous (aPS) and a crystalline isotactic (ips) fraction. Al(CH₃)₃, which is always present in commercial samples of MAO, decreases both polymer yield and stereospecificity. The polymer yield increases with increasing the MAO/Ni ratio but, at the same time, the iPS/aPS ratio decreases. Addition of N(C₂H₅)₃ (mole ratio N/Ni = 1) increases the proportion of the isotactic fraction, while it decreases the polymer yield. A tentative interpretation of the stereospecificity is reported.     

Multicategory classification via discrete support vector machines

Discrete support vector machines (DSVM), originally proposed for binary classification problems, have been shown to outperform other competing approaches on well-known benchmark datasets. Here we address their extension to multicategory classification, by developing three different methods. Two of them are based respectively on one-against-all and round-robin classification schemes, in which a number of binary discrimination problems are solved by means of a variant of DSVM. The third method directly addresses the multicategory classification task, by building a decision tree in which an optimal split to separate classes is derived at each node by a new extended formulation of DSVM. Computational tests on publicly available datasets are then conducted to compare the three multicategory classifiers based on DSVM with other methods, indicating that the proposed techniques achieve significantly higher accuracies. This research was partially supported by PRIN grant 2004132117.     

Gemini pyridinium surfactants: synthesis and conductometric study of a novel class of amphiphiles1

A new series of pyridinium cationic gemini surfactants was prepared by quaternization of the 2,2'-(alpha,omega-alkanediyl)bispyridines with N-alkylating agents, whose reactivity is briefly discussed. Particularly useful was the use of long-chain alkyl triflates (trifluoromethanesulfonates) for both overcoming the sterical hindrance in the pyridines and obtaining higher synthetic yields. Well-known 4,4'-(alpha,omega-alkanediyl)bis(1-alkylpyridinium) structures showed narrow temperature ranges for practical applications, due to their high Krafft points, while the new 2,2'-(alpha,omega-alkanediyl)bis(1-alkylpyridinium) series, accounted for good surface active properties. Due to the Krafft points below 0 degrees C, they could be exploited as solutions in water at any temperature. The characterization of the behavior of the series was performed by conductivity measurements. Some of the proposed structures exhibited unusual surface active behavior, which was interpreted in terms of particular conformational arrangements.     

4‐Naphthylmethyl Proline Forms a Channel Structure

Proline and proline derivatives are useful tools to control the structural properties of peptides and proteins, and thereby modulate numerous processes. Here, we show that proline derivatives can have unique structural properties in the solid state by presenting the crystal structure of zwitterionic (2S,4S)/(2S,4R)‐4‐[(naphthalen‐2‐yl)methyl]proline (H‐Nap‐OH). This amphiphilic proline derivative forms a columnar structure around large hydrophilic and small hydrophobic channels with diameters of 9 Å and 4 Å, respectively. We show that this architecture, which is unprecedented for amino acids, results from the combination of a hydrogen bond network between the ammonium and carboxylate moieties and π–π as well as CH–π interactions between the aromatic moieties.     

Charge renormalization in planar and spherical charged lipidic aqueous interfaces

The charge renormalization in planar and spherical charged lipidic aqueous interfaces has been investigated by means of thermodynamic and electrokinetic measurements. We analyzed the behavior of mixed DOTAP/DOPE monolayers at the air-electrolyte solution interface and DOTAP/DOPE liposomes 100 nm in size dispersed in an aqueous phase of varying ionic strength. For the two systems, we have compared the "effective" surface charge derived from the measurements of surface potential and zeta-potential to the "bare" charge based on the stoichiometry of the lipid mixture investigated. The results confirm that a strong charge renormalization occurs, whose strength depends on the geometry of the mesoscopic system. The dependence of the "effective" charge on the "bare" charge is discussed in light of an analytical approximation based on the Poisson-Boltzmann equation recently proposed.     

Thermodynamic properties of the aqueous solution of potassium salts of some 4-((alkylcarbonyl)amino)-2-hydroxybenzoic acids at 298 and 313 K

To understand the aggregation behavior of surface-active ligands with a salycilic polar head, we undertook a systematic study of some classes of anionic surfactants where the presence and the position of the -OH and the carboxylic group differ. This paper reports the dilution heats at 298 and 313 K of aqueous solutions of potassium 4-((alkylcarbonyl)amino)-2-hydroxybenzoate (KPAS-C(n) where n stands for the number of carbon atoms in the chain) in KOH at 0.1 m, measured as a function of concentration. From the experimental data, apparent and partial molar enthalpies vs concentration were obtained. By using a pseudo-phase-transition approach, the enthalpy changes upon micelle formation (DeltaH(m)) and assuming that in the restricted range of temperature examined heat capacities are constant, the heat capacity changes have been obtained. Micelle formation enthalpies are seen to be additive with a group contribution for the methylene group of -1.5+/-0.1 kJ mol(-1) per group at 298 K and -2.3+/-0.1 kJ mol(-1) per group at 313 K, comparable with that obtained for similar anionic compounds in the same experimental conditions and for N-alkylnicotinamide chlorides (cationic surfactants). The -CH(2)- group contribution to the micelle formation heat capacities is -53+/-1 J K(-1) mol(-1).     

Accurately learning from few examples with a polyhedral classifier

In the context of learning theory many efforts have been devoted to developing classification algorithms able to scale up with massive data problems. In this paper the complementary issue is addressed, aimed at deriving powerful classification rules by accurately learning from few data. This task is accomplished by solving a new mixed integer programming model that extends the notion of discrete support vector machines, in order to derive an optimal set of separating hyperplanes for binary classification problems. According to the cardinality of the set of hyperplanes, the classification region may take the form of a convex polyhedron or a polytope in the original space where the examples are defined. Computational tests on benchmark datasets highlight the effectiveness of the proposed model, that yields the greatest accuracy when compared to other classification approaches. This research was partially supported by PRIN grant 2004132117.     

Flavonoid distribution in tissues of Phillyrea latifolia L. leaves as estimated by microspectrofluorometry and multispectral fluorescence microimaging

A new method for detecting the tissue-specific distribution of flavonoids has been developed by coupling microspectrofluorometry and multispectral fluorescence microimaging techniques. Fluorescence responses of cross sections taken from 1 year old Phillyrea latifolia leaves exposed to full (sun leaves) or 15% (shade leaves) solar radiation in a coastal area of Southern Tuscany were analyzed. Fluorescence spectra of different tissue layers, each normalized at its fluorescence maximum, that were stained or not stained with Naturstoff reagent A (in ethanol), under excitation with UV light (lambdaexc = 365 nm) or blue light (lambdaexc = 436 nm) were recorded. The shape of the fluorescence spectra of tissue layers from shade and sun leaves differed only under UV excitation. The fluorescence of stained cross sections from sun and shade leaves as well as from different layers of sun leaves received a markedly different contribution from the blue (470 nm) and the yellow-red (580 nm) wavebands. Such changes in tissue fluorescence signatures were related to light-induced changes of extractable caffeic acid derivatives and flavonoid glycosides, namely quercetin 3-O-rutinoside and luteolin 7-O-glucoside. Wall-bound phenolics, i.e. hydroxycinnamic acids (p-coumaric, ferulic and caffeic acid) and flavonoids (apigenin and luteolin derivatives), did not substantially differ between sun and shade leaves. A Gaussian deconvolution analysis of fluorescence spectra was subsequently performed to estimate the contribution of flavonoids (emitting at 600 nm, F600 [red fluorescence contribution = signal integrated over a Gaussian band centered at about 600 nm]) relative to the tissue fluorescence (Ftot [total fluorescence = signal integrated over the whole fluorescence spectrum]). The F600/ Ftot ratios sharply differed between analogous tissues of sun and shade leaves, as well as among tissue layers within each leaf type. A highly resolved picture of the tissue flavonoid distribution was finally provided through a fluorescence microimaging technique by acquiring fluorescence images at the blue (fluorescence at about 470 nm [F470]) and yellow-red (fluorescence at about 580 nm [F580]) wavelengths and correcting the F580 image for the contribution of nonflavonoids to the fluorescence at 580 nm. Monochrome images were elaborated by adequate computing functions to visualize the exclusive accumulation of flavonoids in different layers of P. latifolia leaves. Our data show that in shade leaves flavonoids almost exclusively occurred in the adaxial epidermal layer. In sun leaves flavonoids largely accumulated in the adaxial epidermal and subepidermal cells and followed a steep gradient passing from the adaxial epidermis to the inner spongy layers. Flavonoids also largely occurred in the abaxial epidermal cells and constituted the exclusive class of phenylpropanoids synthesized by the cells of glandular trichomes. The proposed method also allowed for the discrimination of the relative abundance of hydroxycinnamic derivatives and flavonoids in different layers of the P. latifolia leaves.