Group contraction and the nine Cayley-Klein geometries

A one-to-one correspondence between the nine Cayley-Klein geometries and the so-called kinematic groups is presented. As is well known, the kinematic groups are related to each other through group contraction. The pattern of contraction is explained by relating each kinematic group to a specific Cayley-Klein geometry. The very meaning of group contraction is deeply rooted in the relationship between the nine geometries. Lie algebras of those geometries are explicitly constructed.In part from the Tesina de LicenciaturaM. A. Fernandez Sanjuan, Tesina de Licenciatura: Contracciones anisötropas del grupo de Poincar, Universidad de Valladolid, 1981.     

Amperometric multidetection with composite enzyme electrodes

The use of composite biosensors for multianalyte detection strategies is discussed. Graphite–Teflon rigid composite biosensors offer the possibility of coimmobilization of several enzymes by simple physical inclusion in the bulk of the electrode matrix with no covalent linkages. A novel trienzyme graphite–Teflon–glucose oxidase (GOD)–alcohol oxidase (AOD)–peroxidase (HRP)–ferrocene bisosensor yielded amperometric steady-state currents similar to those obtained with graphite–Teflon–GOD–HRP–ferrocene and graphite–Teflon–AOD–HRP–ferrocene electrodes for the same concentration of glucose and ethanol, respectively. The performance of the trienzyme biosensor for multianalyte detection was evaluated with the simultaneous determination of glucose and ethanol after separation by HPLC, in samples of sweet wine. The simultaneous analysis of several analytes in the same sample should imply that, with an adequate dilution, the concentration levels of the analytes can be included within the ranges of linearity of the corresponding calibration plots. The use of two composite biosensors in a parallel configuration, so that different analytes can be simultaneously detected with no need of chromatographic separation, is also discussed. The usefulness of this approach was evaluated by the simultaneous analysis of glucose and ethanol in sweet wine, and of glucose and lactic acid in red wine.     

Determination of the uronic acid composition of seaweed dietary fibre by HPLC

A high-performance liquid chromatographic (HPLC) method is described for determination of the ratio of beta-d-mannuronic acid to alpha-l-guluronic acid (M/G ratio) in dietary fibre of edible seaweeds. Total dietary fibre (TDF) content was determined gravimetrically. The TDF fraction was hydrolysed with 12 m and 1 m H(2)SO(4), then neutralized with AG 4 x 4 resin. The uronic acids were separated in a Tracer Extrasil SAX 5 micro m column (25 cm x 4 mm) at 35 degrees C, with 2 mm KH(2)PO(4) containing 5% methanol as mobile phase at a fl ow rate of 1.5 mL/min. The detection wavelength was UV 210 nm. The chromatographic identifications of beta-d-mannuronic acid and alpha-l-guluronic acid were confirmed by liquid chromatography-mass spectrometry (LC-MS). The method precision was 1.4% for beta-d-mannuronic acid and 3.5% for alpha-l-guluronic acid. The method was used to determine M/G ratio in canned seaweeds (Saccorhiza polyschides and Himanthalia elongata) and in dried seaweeds (H. elongata, Laminaria ochroleuca, Undaria pinnatifida, Palmaria sp. and Porphyra sp.).     

Characterisation of flavonols in broccoli (Brassica oleracea L. var. italica) by liquid chromatography-uV diode-array detection-electrospray ionisation mass spectrometry

The flavonoid composition of broccoli inflorescences has been studied by LC/UV-DAD/ESI-MSn. A large number of hydroxycinnamic acid esters of kaempferol and quercetin glucosides has been characterised. The structures of the flavonoid glycosides were analysed after alkaline hydrolysis, and were identified as 3-sophoroside/sophorotrioside-7-glucoside/sophoroside of kaempferol, quercetin and isorhamnetin (this last found in trace amount). These complex quercetin and isorhamnetin glucosides have not been previously characterised in nature. In addition, several less complex glucosides based on the same aglycones have been identified. The effect of sugar substitution and acylation on chromatographic mobility and ESI ionisation and fragmentation are discussed.     

Synthesis and photoluminescent properties of 1,1'-binaphthyl-based chiral phenylenevinylene dendrimers

New chiral, soluble binaphthyl derivatives that incorporate stilbenoid dendrons at the 6,6'-positions have been prepared. The synthesis of the new enantiopure dendrimers was performed in a convergent manner by Horner-Wadsworth-Emmons (HWE) reaction of the appropriately functionalized 1,1'-binaphthyl derivative (R)-1 and the appropriate dendrons (R)(2n)G(n)-CHO. Different electroactive units were incorporated in the peripheral positions of the dendrons in order to tune both the optical and electrochemical behavior of these systems. Fluorescence measurements on the chiral dendrimers reveal a strong emission with maxima between 409 and 508 nm depending upon the substitution pattern. Finally, the redox properties of the dendrimers were determined by cyclic voltammetry, showing the influence of the functional groups at the peripheral positions of the dendrimer on the redox behavior of these systems.     

A convergent approach to huperzine A and analogues

We describe a concise and convergent synthesis of (rac)-5-methoxy-6-azatricyclco[7.3.1.0(2,7)]trideca-2(7),3,5,11-tetraen-13-ol, which has the basic ring system of huperzine A, a potent inhibitor of acetylcholinesterase. We also describe the synthesis of the novel system 5-methoxy-6-azatricyclo[7.2.2.0(2,7)]trideca-2(7),3,5-trien-10-one and a series of related systems.     

Structural and energetic aspects of the protonation of phenol,catechol, resorcinol,and hydroquinone

The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol.     

Molecular dynamics simulations of the dinuclear zinc-beta-lactamase from Bacteroides fragilis complexed with imipenem

Herein, we present results from MD simulations of the Michaelis complex formed between the dizinc beta-lactamase from B. fragilis and imipenem. We considered two catalytically important configurations, which differ in the presence or absence of a hydroxide bridge connecting the two zinc ions in the active site. The structural and dynamical effects induced by substrate binding, the specific roles of the conserved residues and the zinc-bound water molecules, the near attack conformers of the Michaelis complex, and so forth, are discussed in detail. The relative stability of the two configurations was estimated from QM linear scaling calculations on the enzyme-substrate complex combined with Poisson-Boltzmann electrostatic calculations and normal mode calculations. Importantly, we find that the two configurations have similar energies, indicating that these two structures could readily be interchanged, thereby facilitating catalysis. The configuration with the hydroxide bound to the two zinc ions is predicted to be the resting form of the enzyme, while the configuration without the bridge is the reactive form that was found to place the hydroxide in position to attack the carbonyl of the beta-lactam ring. Thus, we propose that the enzyme initiates catalysis by converting from the hydroxide bridge form into the configuration that lacks the hydroxide bridge. This interconversion increases the nucleophilicity of the hydroxide ion and exposes it to the beta-lactam carbonyl, which ultimately facilitates nucleophilic attack. The implications of the observed modes of binding, the possible influence of mutating the Lys184 and Asn193 residues on substrate binding, and the reaction mechanism are also discussed in detail.     

Sensitive determination of diquat by a kinetic method using the stopped-flow mixing technique

A simple, sensitive, selective, fast and inexpensive assay for the determination of diquat is proposed. The method is based on the reduction of the herbicide to a strongly fluorescent monocation radical with sodium dithionite. The initial rate of this reaction is directly proportional to the diquat concentration. The stopped-flow mixing technique was used because the kinetic data can be obtained in only 7 s, meaning that the method can be automated. The calibration graph is linear over the range 5-500 ng ml-1 and the precision (RSD) is close to 1.2%. The applicability of the method was demonstrated by determining the herbicide in different kinds of samples.     

Thermal oxidation of cellulose investigated by chemiluminescence. The effect of magnesium and calcium carbonates and of different pHs

Whatman cellulose impregnated with calcium and magnesium carbonates was oxidized and chemiluminescence accompanying this oxidation has been measured. It was shown that magnesium ions in deacidified cellulose pulp promote the light emission significantly. On the other hand, acid papers give the more pronounced light signals at temperatures close to ambient. The relation between chemiluminescence runs and paper degradation has been implicated, with an attempt to extrapolate half-lives of paper samples to conditions close to ambient. To cite this article: J. Rychlý et al., C. R. Chimie 9 (2006).