Chiral separation of diastereomeric flavanone-7-O-glycosides in citrus by capillary electrophoresis

The 2S- and 2R-diastereomers of major flavanone-7-O-glycosides found in sweet orange (Citrus sinensis), mandarine (Citrus deliciosa), grapefruit (Citrus paradisi), lemon (Citrus limon), and sour or bitter orange juice (Citrus aurantium) were separated for the first time by chiral capillary electrophoresis (CE) employing various buffers with combined chiral selectors. Native cyclodextrins (CDs), neutral and charged CD derivatives were examined as chiral additives to the background electrolyte (BGE). Separation efficiency has not proved satisfactory with one single CD as chiral selector in the buffer, a full and simultaneous separation could often be achieved only by using combined buffer with two different CDs. Chiral separation of major flavanones in sweet orange, mandarine and grapefruit juices raised more difficulties than in lemon and sour orange juices as narirutin will not readily build complexes with most CDs. Diastereomeric flavanones of mature and immature grapefruits were compared and some differences were found: naringin showed different diastereomeric ratio and 2S-prunin appeared only in immature grapefruit. Marmalade was also examined by chiral CE. Its major flavanones corresponded to flavanone pattern of mixed sour and sweet oranges.     

New insights into the multiple minima problem

Minima distribution of thorough conformational searches of three peptides of different length ranging from five to nine residues, were compared with the density of states of a flexible molecule derived from the rotational isomeric approximation. It is observed that minima distributions generated from the conformational searches exhibit the same characteristics as the density of states derived from the rotational isomeric model: an asymmetric distribution with a maximum. These results together with a more profound understanding of the characteristics of the energy landscapes of polypeptides, provide new insights into the multiple minima problem. The implications in devising more robust conformational search strategies are discussed.     

Development of a new flow-through bulk optode for the determination of manganese(III)

A flow-through bulk optode based on the use of 1-(2-pyridylazo)-2-naphthol (PAN) immobilized in a plasticized poly(vinyl chloride) membrane entrapped in a cellulose support, in conjuntion with the flow injection analysis technique, is proposed for the determination of manganese(II). The calibration graph obtained at 570 nm was linear in the range 0.27-27.5 mg L(-1) (5 x 10(-6)-5 x 10(-4) M) Mn(II) with a detection limit of 0.18 mg L(-1). The coefficients of variation of the sensor response for 5.5 mg L(-1) of Mn(II) were +/-0.22% for consecutive measurements (n = 10), +/-0.48% between days (n = 5) and +/-0.38% between different membranes (n = 6). The sensor was readily regenerated with the carrier acetic acid/acetate buffer of pH 4.5. The method was applied to the determination of manganese in steels, waters and lemon tree leaves.     

14-helical folding in a cyclobutane-containing beta-tetrapeptide

The efficient synthesis of tetrapeptide 5 containing, in alternation, cyclobutane and beta-alanine residues is described. NMR experiments both at low temperature in CDCl(3) and at 298 K in DMSO-d(6) solutions show the contribution of a strong hydrogen bond in the folded major conformation of 5. Temperature coefficients and diffusion times point out a hydrogen bond involving the NH proton from the cyclobutane residue 1 whereas NOEs manifest the high rigidity of the central fragment of the molecule and are compatible with a 14-membered macrocycle. Theoretical calculations predict a most stable folded conformation corresponding to a 14-helix stabilized by a hydrogen bond between NH(10) in the first residue and OC(25) in the third residue. This structure remains unaltered during the molecular dynamics simulation at 298 K in chloroform. All these results provide evidence for a 14-helical folding and reveal the ability of cis-2-aminocyclobutane carboxylic acid residues to promote folded conformations when incorporated into beta-peptides.     

Regioselective enzymatic acylation of methyl shikimate. Influence of acyl chain length and solvent polarity on enzyme specificity

Candida antarctica lipase A (CAL-A) selectively catalyzes the acylation at the secondary C-4 hydroxyl group of methyl shikimate (2), which possesses three secondary hydroxyl groups, the C-3 allylic one being chemically more reactive. The effect both of the acyl group of the acylating agents and of the solvent polarity has been studied. The selectivity of CAL-A is almost complete with acyl donors that possess short chains. However, when acyl donors have longer chains, better results are obtained by C. antarctica lipase B (CAL-B).     

First synthesis of beta-keto sulfoxides by a palladium-catalyzed carbonylative Suzuki reaction

[reaction: see text] An unprecedented palladium-catalyzed three-component cross-coupling reaction between alpha-bromo sulfoxide, carbon monoxide, and aromatic boronic acids provides a new and efficient approach to the synthesis of beta-ketosulfoxides. The reaction takes place under mild conditions with a wide range of variously substituted aryl and heteroaryl boronic acids. The carbonylative cross-coupling reaction is strongly favored over competing direct cross-coupling and homocoupling processes, except with boronic acids carrying strong electron-withdrawing substituents.     

Novel fused D-A dyad and A-D-A triad incorporating tetrathiafulvalene and p-benzoquinone

Novel fused donor-acceptor dyad (TTF-Q or D-A) and acceptor-donor-acceptor triad (Q-TTF-Q or A-D-A) incorporating the donor tetrathiafulvalene (TTF) and the acceptor p-benzoquinone (Q) have been synthesized. The solution UV-vis spectra of these molecules display a low-energy absorption band that is attributed to an intramolecular charge transfer between both antagonistic units. The presence of reversible oxidation and reduction waves for the donor and acceptor moieties was shown by cyclic voltammetry, in agreement with the ratio TTF/quinone(s) units. The successive generation from these compounds of the cation radical and anion radical obtained upon (electro)chemical oxidation and reduction, respectively, was monitored by optical and ESR spectroscopies. The anion radical Q-TTF-Q(-.) triad was demonstrated to be a class II mixed-valence system with the existence of a temperature-dependent intramolecular electron transfer. The crystallographic tendency of these fused systems to overlap in mixed stacks of alternating A-D-A units is also discussed.     

Kramers-Kronig relations and the barrier interaction time problem

It is shown that the two characteristic interaction times and for classical electromagnetic waves with an arbitrarily shaped barrier are not independent quantities, but are connected by Kramers-Kronig relations for the real and imaginary components of a causal magnitude. The corresponding macroscopic sum rule for the complex time is also derived. An analogy between the interaction time problem and an electrical circuit with capacitive and conducting frequency dependent components is established. Received: 21 July 1998