Effect of temperature and pH on the aggregation and the surface hydrophobicity of bovine κ-casein

κ-Casein (κ-CN) aggregation by heating has been studied at pH 7.2 and 5.2 using UV-visible spectrophotometry, sodium dodecyl sulfate polyacrylamide gel electrophoresis, spectrofluorometric study of the 1–8 aniline naphtalene sulfonate (ANS)–κ-CN binding and circular dichroism (CD) spectroscopy. The aggregation process to form aggregates like micelles or submicelles and the structural characteristics of these aggregates were pH dependent. Far-UV CD showed that the aggregates obtained by heating presented changes in the κ-CN secondary structure. Near-UV CD spectra showed a certain degree of tertiary organization in the Tyr environment for the protein heated or unheated, only at pH 5.2. ANS binding at both pH was quite different and depends on the self-association process. Heating produced exposition of hydrophobic binding sites only at pH 7.2, including those in the neighborhood of the κ-CN Trp residue.     

Hexafluorotitanate salts containing organic cations: use as a reaction medium and precursor to the synthesis of titanium dioxide

The straightforward modification of commercial hexafluorotitanic acid with organic derivatives containing a tetraalkylammonium cation produced stable precursor solutions with only a small fraction of water, which were then used as a reaction medium for the synthesis of nanocrystalline TiO(2).     

Toward an understanding of the acceleration of Diels-Alder reactions by a pseudo-intramolecular process achieved by molecular recognition. A DFT study

The pseudo-intramolecular Diels-Alder (DA) reaction between a 2-substituted furan (1) and a N-maleimide derivative (2) has been analyzed using DFT methods. Formation of two hydrogen bonds between the appendages on furan and maleimide derivatives favors thermodynamically the formation of a molecular complex (MC1) through an efficient molecular recognition process. The large enthalpy stabilization associated with the molecular recognition overcomes the unfavorable activation entropy associated with the bimolecular process. As a consequence, the subsequent DA reaction is clearly accelerated through a pseudo-intramolecular process.     

Highly enantioselective synthesis of beta-amino alcohols: a catalytic version

Highly enantioselective rearrangement of beta-amino alcohols was realized by using a catalytic amount of trifluoroacetic anhydride.     

Synthesis of pyrrolo- and pyrido[1,2-a]xanthene [1,9-de]azepines: a study of the azepine ring construction

Pentacyclic pyrrolo- and pyrido[1,2-a]xanthene[1,9-de]azepines were synthesized in various oxidation states by assembling the azepine ring following two strategies: 7-endo-trig cyclization of the aryl radical derived from a gamma-methylene lactam and cyclodehydration of aldehydes. Other strategies examined (Heck reaction and intramolecular acylation) did not afford azepines, but six-membered nitrogenated rings.     

Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.     

Measurement of the Po concentration in air using makrofol-de detectors

The measurement of the ²¹⁴Po concentration in air with Makrofol-DE detectors is useful to estimate the long-term averaged equilibrium factors indoors. To differentiate α-particles emitted by ²¹⁴Po from those emitted by ²¹⁸Po and ²²²Rn, the detector must register only α-particles with energies between 6.2 and 7.5 MeV. The required energy response is obtained only if a removed layer of about 43 μm is achieved in a chemical etching of the detector. The methodology used to determine the etching conditions is described in this paper. The optimum conditions found are: a) chemical etching for 6 h at a temperature of 40°C, using 7.5 M KOH mixed with 50% ethanol as an etchant, and b) electrochemical etching for 1 h at a frequency of 3 kHz and an electric field strength of 34 kV cm⁻¹. Several dosimeters have exposed during 2 months in dwellings located in the Barcelona area, Spain. A ²¹⁴Po averaged concentration of (13.6 ± 8.6) Bq m⁻³ was obtained.     

Understanding the stereo‐ and regioselectivities of the polar Diels–Alder reactions between 2‐acetyl‐1,4‐benzoquinone and methyl substituted 1,3‐butadienes: a DFT study

The polar Diels–Alder (DA) reactions of 2‐acetyl‐1,4‐benzoquinone (acBQ) with methyl substituted 1,3‐butadienes have been studied using DFT methods at the B3LYP/6‐31G(d) level of theory. These reactions are characterized by a nucleophilic attack of the unsubstituted ends of the 1,3‐dienes to the β conjugated position of the acBQ followed by ring‐closure. The reactions present a total regioselectivity and large endo selectivity. The analysis based on the global electrophilicity of the reagents at the ground state, and the natural bond orbital (NBO) population analysis at the transition states correctly explain the polar nature of these cycloadditions. The large electrophilic character of acBQ is responsible for the acceleration observed in these polar DA reactions. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface tension and adsorption behavior of mixtures of diacyl glycerol arginine-based surfactants with DPPC and DMPC phospholipids

The binary surface interactions of some novel cationic diacyl glycerol arginine-based surfactants with model phospholipids, which are often used as model membrane components, are studied at 25 °C in aqueous solutions of 0.1 M sodium chloride. The surfactants are 1,2-dimyristoyl-rac-glycero-3-O-(N^α-acetyl-l-arginine) hydrochloride (1414RAc) and 1,2-dilauroyl-rac-glycero-3-O-(N^α-acetyl-l-arginine) hydrochloride (1212RAc), and they are important as potential antimicrobial agents which are biodegradable and with less toxicity than other cationic surfactants. The phospholipids are 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC). The equilibrium and dynamic surface tension of each surfactant, each phospholipid, and some of their binary mixtures are studied using the bubble surfactometry method at constant area or pulsating area conditions. In addition, the surface densities of pure and mixed monolayers of these compounds at the air/water interface are probed with infrared reflection–absorption spectroscopy (IRRAS). Steady state and dynamic surface tension synergism, or antisynergism in one case, and synergistic adsorption effects are detected for the mixed dispersions studied. The enhanced adsorption detected with IRRAS, and the enhanced dynamic and steady state surface tension lowering indicate strong miscibility and net attractive interactions between the compounds in the adsorbed mixed monolayers.