Enhanced photochemical [6π] electrocyclization within the lipophilic protein binding site

Photocyclization of N-methyldiphenylamine to N-methylcarbazole is achieved within the microenvironment provided by site I of serum albumins. Quantum yield determinations, combined with transient absorption spectroscopic detection of the dihydrocarbazole intermediate, demonstrate that protein encapsulation provides a subtle control of the kinetic parameters, leading to optimized efficiencies.     

Influence of self-assembly of amphiphilic imidazolium ionic liquids on their host–guest complexes with cucurbit[n]urils

Two symmetric amphiphilic imidazolium ionic liquids having ω-undecenyl chains form supramolecular complexes with CB[7] and CB[8] in water as revealed by ¹H NMR spectroscopy and MALDI-MS. Binding constants in the range 10⁴ to 10⁵ M^?1 were estimated from the conductivity measurements for the 1:1 complexes of these imidazolium ionic liquids with CB[7] and CB[8]. Radical initiated polymerization of these host–guest complexes at concentrations above the critical self-assembly concentration of imidazolium ionic liquids to form liposomes, destroys completely (CB[7]) or partially (CB[8]) the host–guest ionic liquid@CB[n] complex; this behaviour was proved by titration with acridine orange tricyclic dye, of CB[n]s in the colloidal solutions of the liposomes before and after performing dialysis to remove free CB[n]s. Thus, the increase in the fluorescence emission of acridine orange by CB[7] is not observed if the polymerized ionic liquid@CB[7] complex is submitted to dialysis to remove uncomplexed CB[7]. Analogous study by titration of absorbance change of acridine orange solutions caused by CB[8], reveals only a partial destruction of the host–guest complex by self-assembly of amphiphilic ionic liquid above the critical self-assembly concentration. The results obtained have been rationalized considering that the driving force for the formation of supramolecular ionic liquid@CB[n] complexes is a hydrophobic interaction between the apolar alkenyl chain and the cucurbituril interior cavity and that these hydrophobic interactions are disturbed when self-assembly leading to liposomes occurs.     

Computational Model of the Complex between GR113808 and the 5-HT4 Receptor Guided by Site-Directed Mutagenesis and the Crystal Structure of Rhodopsin

A computational model of the transmembrane domain of the human 5-HT₄ receptor complexed with the GR113808 antagonist was constructed from the crystal structure of rhodopsin and the putative residues of the ligand-binding site, experimentally determined by site-directed mutagenesis. The recognition mode of GR113808 consist of: (i) the ionic interaction between the protonated amine and Asp^3.32; (ii) the hydrogen bond between the carbonylic oxygen and Ser^5.43; (iii) the hydrogen bond between the ether oxygen and Asn^6.55; (iv) the hydrogen bond between the C-H groups adjacent to the protonated piperidine nitrogen and the electrons of Phe^6.51; and (v) the - aromatic-aromatic interaction between the indole ring and Phe^6.52.This computational model offers structural indications about the role of Asp^3.32, Ser^5.43, Phe^6.51, Phe^6.52, and Asn^6.55 in the experimental binding affinities. Asp^3.32Asn mutation does not affect the binding of GR113808 because the loss of binding affinity from an ion pair to a charged hydrogen bond is compensated by the larger energetical penalty of Asp to disrupt its side chain environment in the ligand-free form, and the larger interaction between Phe^6.51 and the piperidine ring of the ligand in the mutant receptor. In the Phe^6.52Val mutant the indole ring of the ligand replaces the interaction with Phe^6.52 by a similarly intense interaction with Tyr^5.38, with no significant effect in the binding of GR113808. The mutation of Asn^6.55 to Leu replaces the hydrogen bond of the ether oxygen of the ligand from Asn^6.55 to Cys^5.42, with a decrease of binding affinity that approximately equals the free energy difference between the SHO and NHO hydrogen bonds.Because these residues are also present in the other members of the neurotransmitter family of G protein-coupled receptors, these findings will also serve for our understanding of the binding of related ligands to their cognate receptors.     

Merging Gradient-Based Methods to Improve Benchtop NMR Spectroscopy: A New Tool for Flow Reaction Optimization

Emerging low cost, compact NMR spectrometers that can be connected in-line to a flow reactor are suited to study reaction mixtures. The main limitation of such spectrometers arises from their lower magnetic field inducing a reduced sensitivity and a weaker spectral resolution. For enhancing the spectral resolution, the merging of Pure-Shift methods recognized for line narrowing with solvent elimination schemes was implemented in the context of mixtures containing protonated solvents. One more step was achieved to further enhance the resolution power on compact systems, thanks to multiple elimination schemes prior to Pure-Shift pulse sequence elements. For the first time, we were able to remove up to 6 protonated solvent signals simultaneously by dividing their intensity by 500 to 1700 with a concomitant spectral resolution enhancement for signals of interest from 9 to 12 as compared to the standard 1D ¹H. Then, the potential of this new approach was shown on the flow synthesis of a complex benzoxanthenone structure.     

Epicatechin,procyanidins, and phenolic microbial metabolites after cocoa intake in humans and rats

Proanthocyanidins, flavonoids exhibiting cardiovascular protection, constitute a major fraction of the flavonoid ingested in the human diet. Although they are poorly absorbed, they are metabolized by the intestinal microbiota into various phenolic acids. An analytical method, based on an optimized 96-well plate solid-phase extraction (SPE) procedure and liquid chromatography tandem mass spectrometry (SPE-LC-MS/MS) for the analysis of 19 phenolic microbial metabolites and monomeric and dimeric flavanols in urine samples, was developed and validated. Human urine samples were obtained before and after ingestion of an acute consumption of 40 g of soluble cocoa powder and rat urines before and after the prolonged administration (2 weeks) of different diets composed of natural cocoa powder. The mean recovery of analytes using the new SPE-LC-MS/MS method ranged from 87% to 109%. Accuracy ranged from 87.5% to 113.8%, and precision met acceptance criteria (<15% relative standard deviation). Procyanidin B2 has been detected and quantified for the first time in human and rat urine after cocoa consumption. Changes in human and rat urinary levels of microbial phenolic acids and flavanols were in the range of 0.001–59.43 nmol/mg creatinine and of 0.004–181.56 nmol/mg creatinine, respectively. Major advantages of the method developed include reduction of laboratory work in the sample preparation step by the use of 96-well SPE plates and the sensitive measurement of a large number of metabolites in a very short run time, which makes it ideal for use in epidemiological studies. ^a INGENIO-CONSOLIDER Program, Fun-c-food CSD2007-063 ^b CIBER 06003 Physiopathology of Obesity and Nutrition (CIBEROBN), and RETICS RD06/0045/0003, Instituto de Salud Carlos III     

Simple modeling of the physical sample dispersion process in rectangular meso (micro) channels with pressure-driven flows

The present paper reports the modeling and characterization of the physical sample dispersion process observed in rectangular microchannels when pressure-driven pumping is used. To explain experimental results provided by the silicon fluidic device constructed, two different mathematical models were tested. The first one is based on the diffusion–convection model, and the second one is based on the combination of ideal reactors. The silicon designed and constructed chip includes a microfluidic manifold with four inlet–outlet ports and a monolithically integrated optical flow cell. The microchannels, the optical flow cell, and the input–output ports were micromachined on a silicon wafer and then sealed with Pyrex glass anodically bonded. Optical windows were integrated in the chip, allowing simple absorbance–transmission measurements. Pressure-driven flows through fluidic channels were controlled via three-way solenoid valves and provided by an automatic microburette operating in aspiration mode. Experimentally obtained results demonstrate that the physical sample dispersion process can be easily modeled as a combination of a continuous stirred tank reactor and a plug-flow reactor.     

Spin-state splittings of iron(II) complexes with trispyrazolyl ligands

We report a computational study at the OPBE/TZP level on the chemical bonding and spin ground-states of mono-nuclear iron(II) complexes with trispyrazolylborate and trispyrazolylmethane ligands. We are in particular interested in how substitution patterns on the pyrazolyl-rings influence the spin-state splittings, and how they can be rationalized in terms of electronic and steric effects. One of the main observations of this study is the large similarity of the covalent metal–ligand interactions for both the borate and methane ligands. Furthermore, we find that the spin-state preference of an individual transition-metal (TM) complex does not always concur with that of an ensemble of TM-complexes in the solid-state. Finally, although the presence of methyl groups at the 3-position of the pyrazolyl groups leads to ligand–ligand repulsion, it is actually the loss of metal–ligand bonding interactions that is mainly responsible for shifts in spin-state preferences.     

The search for stationary points on a quantum mechanical/molecular mechanical potential-energy surface

 We propose a methodology to locate stationary points on a quantum mechanical/molecular mechanical potential-energy surface. This algorithm is based on a suitable approximation of an initial full Hessian matrix, either a modified Broyden–Fletcher–Goldfarg–Shanno or a Powell update formula for the location of, respectively, a minimum or a transition state, and the so-called rational function optimization. The latter avoids the Hessian matrix inversion required by a quasi-Newton–Raphson method. Some examples are presented and analyzed. Received: 16 July 2001 / Accepted: 9 October 2001 / Published online: 9 January 2002