Influence of peak measurement parameters on the quality of chiral electrophoretic separations

The effect of nine peak measurement/analysis parameters on chiral capillary electrophoresis (CE), followed by UV detection, was studied. The parameters pertained UV-detection (detection wavelength, reference wavelength, and wavelength bandwidths), signal processing (data acquisition rate, type and amount of filtering) and peak detection (detection threshold and peak width). The influence of these factors on the chiral separation of dimethindene enantiomers was studied at two different concentrations (i.e., at high and low signal-to-noise (S/N) ratio) by the means of experimental designs. The electropherogram characteristics considered were the resolution between the two enantiomers, the peak areas, and the S/N ratio. A D-optimal design was first used as screening design to identify the most critical parameters. Afterwards, a modelling of the different responses as a function of these critical parameters was performed based on the results of a face-centered central composite design. The results showed that the signal-processing parameters should be carefully selected when developing a CE separation since very important variations in the separation, the S/N ratio and the peak area of the substances can occur by setting these parameters at different levels. The detection wavelength should also be carefully chosen for optimal peak area measurement. The role of these parameters becomes more important with decreasing concentration of the analytes (i.e., low S/N ratio). This study showed that the peak measurement/analysis parameters should be optimized as the chemical and physical parameters of a method. They also should always be well specified in order to allow a good transfer of a method from one instrument to another.     

Möbius aromaticity in [12]annulene: cis-trans isomerization via twist-coupled bond shifting

Density functional and coupled cluster calculations show that facile thermal configuration change in [12]annulene occurs via a twist-coupled bond-shifting mechanism. The transition state for this process is highly aromatic with M?bius topology. At the CCSD(T)/cc-pVDZ//BH&HLYP/6-311+G** level, the isomerization of tri-trans-[12]annulene 1a (CTCTCT) to its di-trans isomer 2 (CCCTCT) via such a mechanism has a barrier of 18.0 kcal/mol, in good agreement with earlier experiments. Two other aromatic M?bius bond-shifting transition states were located that result in configuration change for other [12]annulene conformers. This mechanism contrasts sharply with diradical configuration change for acyclic polyenes and with planar bond-shifting mechanisms generally assumed for annulenes. This constitutes evidence that neutral M?bius aromatic annulenes play a role in the dynamic processes of neutral [4n]annulenes.     

CdS nanoclusters stabilized by thiolate ligands: A mini-review

One of the most exciting frontiers in materials chemistry in recent years is the optoelectronics of quantum-confined semiconductor nanoclusters. These nanoclusters are 10–200 A in diameter, and in this size regime exhibit extra-ordinarily interesting quantum mechanical effects. Cadmium sulfide is a popular semiconductor for these studies, and reviewed here is the synthesis and charac-terization of such CdS nanoclusters, with emphasis on how chemical control of the surface by thiolates influences product formation and properties. Also described are the syntheses and structures of true molecular clusters of CdS capped with thiolate ligands.     

Playing with podands based on cone-shaped cavities. How can a cavity influence the properties of an appended metal centre?

The potential of molecules that combine the properties of a conical cavity with those of a covalently-linked transition-metal centre is highlighted through the assessment of cyclodextrin- and calixarene-derived podands ("cavitand" ligands) in coordination chemistry and catalysis. Metallocavitands with coordination sites directed towards the interior of the generic cavity provide interesting systems for studying host-guest complexation processes, their enhanced strength of metal-ion binding allowing for regioselective catalysis in a confined environment, and stabilisation of coordination complexes of unusual forms. Where cavitands have exo-oriented podand arms, the intrinsic dynamics of the cavity can dramatically modify metal chelation behaviour and the catalytic properties of the complexes. Such functionalised cavities are also useful as metal-ion transporters.     

Conducting polymers containing in-chain metal centers: electropolymerization of oligothienyl-substituted {M(tpy)2} complexes and in situ conductivity studies, M = Os(II), Ru(II)

The electropolymerization of a series of Ru and Os bis-terpyridine complexes that form rodlike polymers with bithienyl, quaterthienyl, or hexathienyl bridges has been studied. Absorption spectroscopy, scanning electron microscopy, and cyclic voltammetry have been used to characterize the monomers and resulting polymer films. The absolute dc conductivity of the quaterthienyl-bridged {Ru(tpy)2} and {Os(tpy)2} polymers is unusually large and independent of the identity of the metal center at 1.6 x 10(-3) S cm(-1). The maximum conductivity occurs at the formal potential of each redox process, which typically is observed for systems where redox conduction is the dominant charge transport mechanism. Significantly, the dc conductivity of the metal-based redox couple observed in these polymers is 2 orders of magnitude higher thanthat of a comparable nonconjugated system.     

Probing the formation of bicyclo[4.2.0]octan-1-ols

Reaction of lithium enolates of simple ketones with (+/-)-phenyl vinyl sulfoxide has potential for the convergent construction of complex fused ring systems containing a bicyclo[n.2.0]alkan-1-ol. The formation of sulfinylbicyclo[4.2.0]octan-1-ols 1-3 from the lithium enolate of cyclohexanone with (+/-)-phenyl vinyl sulfoxide or (R)-(+)-p-tolyl vinyl sulfoxide 18 was used to probe the mode of this novel cyclization reaction. Using phenyl vinyl sulfoxide, variations in the reaction lighting and solvent were investigated, in conjunction with radical trapping (TEMPO) and isotope labeling (deuterium) experiments. Cyclization to form sulfinylbicyclooctanols 1-3 is likely to proceed via an intermediate that ring closes to the bicycloalkanol anion 11 and was presently favored by the use of solvents such as THF or DME.     

Diversity-generation from an allenoate-enone coupling: syntheses of azepines and pyrimidones from common precursors

Amine-catalyzed coupling reactions of allenoate esters and α,β-unsaturated carbonyls lead to a diverse range of α,α′-disubstituted allenoates. With appropriately substituted monomers, intermolecular reactions can lead to pyrimidone products. Alternatively, with amine substituted allenoates, a 7-endo-dig cyclization can be carried out such that a divergent pathway is observed that leads to azepine scaffolds.     

Binding site optimisation for artificial enzymes by diffusion NMR of small molecules

A binding site optimisation protocol for the design of artificial enzymes based on "small molecule-small molecule" binding studies by diffusion NMR is presented. Since the reaction chosen was the hydrolysis of ester 1 ([4-(4-carboxy-1-oxobutyl)-aminobenzyl]-phenethyl ester), an analogous phosphonate ester 2 ([4-(4-carboxy-1-oxobutyl)-aminobenzyl]-phosphonic phenethyl ester) was selected as a suitable transition state analogue (TSA). The key objective of the NMR studies was to find a unit with functional groups capable of binding to the acidic sites of the TSA. Nine dipeptides, mainly with basic and hydroxyl groups, were used and their affinity to the TSA was studied by measuring the change in the diffusion coefficient, D(pep), upon binding by pulse field gradient NMR. The value of D(pep) at 298 K in D(2)O at pD 5, 7 and 10 was measured both in free solution, and mixtures containing one dipeptide and the TSA. As both components are low molecular weight species with M < 500, a TSA-to-dipeptide ratio of 10:1 was used to detect significant changes in D(pep). The results revealed that dipeptides with basic residues show higher affinity to the TSA than those with hydroxyl or aliphatic side chains in aqueous solutions. The dipeptide showing the most significant relative change in D(pep) was H-Arg-Arg-OH, and the binding constant was estimated to be 86 L M(-1) by measuring D(pep) at varying concentrations of the TSA. In addition, binding of the TSA to a new water-soluble polymer with a polyallylamine backbone and randomly distributed Arg-Arg binding sites was examined, and the binding constant was estimated to be > or =1500 L M(-1). As confirmed by further catalytic activity tests, polymers containing Arg-Arg as a binding site are capable of significant rate accelerations in the hydrolysis of ester 1.