Absolute asymmetric photoreactions of aliphatic amino acids by circularly polarized synchrotron radiation: critically pH-dependent photobehavior

The absolute asymmetric photoreaction (AAP) of racemic aliphatic amino acids, such as alanine (Ala) and leucine (Leu), by left- and right-handed circularly polarized light (l- and r-CPL) irradiation was investigated in aqueous solutions at various pHs, by using the Onuki-type polarizing undulator installed in an electron storage ring. The magnitude of the optical purity (op) generated and the enantiomer-enriching mechanism operative in the AAP were found to be entirely dependent on the ionic state (and thus pH) of the amino/carboxylic acid moieties. At pH 1, the op of Ala and Leu determined by circular dichroism (CD) spectral measurement gradually developed with CPL irradiation, according to Kagan's equation. In contrast, irradiation at pH 7 gave op's much smaller than the theoretical values predicted by Kagan's equation. However, it turned out that the photodecomposition at pH 7 produces the corresponding alpha-hydroxycarboxylic acids stereoselectively, the CD sign of which is just opposite to that of the remaining amino acid, thus affording the apparently small op. It is concluded that, irrespective of solution pH, the AAP of amino acid proceeds upon CPL irradiation. At pH 1, the photodecomposition of valine, Leu, and isoleucine occurs via a Norrish type II mechanism, which is also applicable to other amino acids possessing a gamma-hydrogen. In the case of amino acids lacking a gamma-hydrogen, such as glycine and Ala, the photodecomposition mechanism is a photodeamination/hydroxylation and a Norrish type I reaction. At pH 7, the main photoproducts were ammonia and alpha-hydroxycarboxylic acids that were produced via photodeamination.     

Supramolecular chirogenesis with bis-chlorin versus bis-porphyrin hosts: peculiarities of chirality induction and modulation of optical activity

[structure: see text] The complexation behavior, chirality induction and inversion in the achiral host, a racemic mixture of ethane-bridged bis(zinc octaethylchlorin) (1), and optical activity modulation in the chiral hosts, enantiopure 1(R) and 1(S), upon interaction with chiral and achiral amine guests have been investigated by means of the UV-vis, CD, and (1)H NMR techniques and compared with the corresponding spectral data of the bis-porphyrin analogue. It was found that the chirogenesis pathway is strongly dependent upon the structures of both major components (hosts and guests) of these supramolecular systems. Particularly, the distinct orientation of electronic transitions in the chlorin chromophores arisen from the reduced pyrrole ring, which makes it radically different from that of the porphyrin chromophores, and the size of the guest's substituents lead to the remarkable phenomenon of chirality induction-inversion in racemic 1 originating from the process of asymmetry transfer from enantiopure guests of the same homologous type and absolute configuration. This surprising chirogenic behavior is found to be in a sharp contrast to that observed in the analogous porphyrin-based systems. Furthermore, these structural and electronic phenomena also account for the effective optical activity quenching of enantiopure 1(R) and 1(S) upon interaction with chiral and with achiral amines, which results in formation of supramolecular complexes of opposite chirality.     

The study of chiral discrimination of organophosphonate derivatives on pirkle type chiral stationary phase by molecular modeling

Molecular modeling and molecular dynamics (MD) have been used to study the chiral discrimination and interaction energy of organophosphonate in N-(3,5-dinitrobenzoyl)-S-leucine chiral stationary phase (CSP). The elution order of the enantiomers can be predicted from the interaction energy. Quantitative structure-retention relationship (QSRR) has also been used as an alternative method to confirm the elution order of enantiomers. Molecular mechanics (MM), molecular dynamics and QSRR proved to be useful methods to study chiral discrimination.     

Degradation of epoxy polymers: 3—Photo-degradation of bisphenol-A diglycidyl ether

The principal characteristics and products of photo-degradation of a commercial epoxy resin, bisphenol-A diglycidyl ether, have been studied. Comparison with the results of thermal degradation shows that many of the products are identical, especially the phenols and derivatives of phenol. The principal differences are the appearance of hydrogen in the photo, but not in the thermal, reaction and a series of aromatic hydrocarbons, including xylenes and/or ethyl benzene, styrene, isopropyl benzene and isopropenyl benzene, which are completely absent from the products of thermal degradation. Mechanisms are outlined.     

First Synthesis of Caerulomycin C

The first synthesis of caerulomycin C (1), an antibiotic produced by Streptomyces caeruleus, is reported. This molecule, which exhibits a 2,3,4,6-tetrasubstituted pyridine structure, was prepared from 3,4-dimethoxypyridine in a five-step sequence. The methodology involves metalation, transmetalation, aromatic cross-coupling, and halogen migration reactions.     

A Constantly Evolving O.R. Team

The aim of this paper is to present the activities of the Operational Research team of the Paris Transport Authority (the R.A.T.P.), but without going into the technical details of methods and results. The paper is in three parts:

• - general information on the O.R. team and its environment;
• - the history of its activities;
• -its present and future tendencies.

     

Deprotonation of thiophenes using lithium magnesates

Thiophene was regioselectively deprotonated at C2 on treatment with 1/3 equiv of Bu₃MgLi in THF at room temperature. The lithium arylmagnesate formed was either trapped with electrophiles or cross-coupled in a ‘one-pot’ procedure with aryl halides under palladium catalysis. 2-Chlorothiophene and 2-methoxythiophene were similarly deprotonated at C5 under the same reaction conditions. The enhancement of the reactivity of the base using TMEDA was evidenced using ¹H NMR spectroscopy.     

Protonation thermochemistry of selected hydroxy- and methoxycarbonyl molecules

The gas-phase basicities of a representative set of hydroxy- and methoxycarbonyl compounds (hydroxyacetone, 1, 3-hydroxybutanone, 2, 3-hydroxy-3-methylbutanone, 3, 1-hydroxy-2-butanone, 4, 4-hydroxy-2-butanone, 5, 5-hydroxy-2-pentanone, 6, methoxyacetone, 7, 3-methoxy-2-butanone, 8, 4-methoxy-2-butanone, 9, and 5-methoxy-2-pentanone, 10) were experimentally determined by the equilibrium method using Fourier transform ion cyclotron resonance and high-pressure mass spectrometry techniques. The latter method allows the measurement of proton transfer equilibrium constants at various temperatures and thus the estimate of both the proton affinities and the protonation entropies of the relevant species. Quantum chemical calculations at the G3 and the B3LYP/6-311+G(3df,2p)//6-31G(d) levels of theory were undertaken in order to find the most stable structures of the neutrals 1-10 and their protonated forms. Conformational and vibrational analyses have been done with the aim of obtaining a theoretical estimate of the protonation entropies.     

Capillary zone electrophoresis of metal-binding proteins in formic acid with UV- and mass spectrometric detection using cationic transient capillary isotachophoresis for preconcentration

A capillary zone electrophoresis (CZE) method with preceding cationic transient capillary isotachophoresis (tCITP-CZE) was developed for uncoated fused-silica capillaries to analyze metal-binding proteins (MBPs) of clinical relevance. UV detection was followed by mass spectrometry (MS). Optimization was done with model proteins of properties similar to relevant human MBPs. Using 1.0 mol x L(-1) formic acid (pH 1.78) as electrolyte resulted in up to 165000 plates m(-1) in CZE and 230000 plates m(-1) in combination with tCITP and analysis time was less than 5 min in uncoupled mode. Cationic tCITP with 125 mmol x L(-1) ammonium formate, buffered to pH 4.00, as leading electrolyte improved sample loadability considerably in comparison with sample stacking without impairing resolution. Following systematic optimization of the electrospray ionization process (ESI) the coupled system ((tCITP)-CZE-UV-ESI-MS) was tested with protein model mixtures and human MBPs. Repeatability of migration times was < 0.64% in pure CZE mode and in tCITP-CZE mode and < 0.83% in CZE-ESI-MS coupled mode. Mass accuracy was < 0.015%. Limits of detection were found to be in the range 50-160 fmol.