Characterization of cyclic triphosphenium ions in solution,and the crystal and molecular structure of 1,1,3,3‐tetraphenyl‐tetrahydro‐1,2,3‐triphosphenium hexachlorostannate

The preparation in solution of a number of cyclic triphosphenium ions and their identification by ³¹P NMR spectroscopy are described; the crystal and molecular structure of the six‐membered cationic ring (as its hexachlorostannate) has been determined. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:226–231, 2000     

Cooperative use of cytochrome cd1 nitrite reductase and its redox partner cytochrome c552 to improve the selectivity of nitrite biosensing

In this work, a novel enzymatic biosensor for determination of nitrites constructed on an electrochemical transducing platform is proposed. The sensor is based on cytochrome-cd(1) (cyt-cd(1)) nitrite reductase from Marinobacter hydrocarbonoclasticus strain 617 as biological recognition element, and its putative physiological redox partner cytochrome-c(552) (cyt-c(552)), as electron mediator. The proteins were co-immobilized using a photopolymerizable polyvinyl alcohol (PVA) derivative, onto carbon paste screen printed electrodes (CPSPEs); the optimal modification conditions were 100 μM cyt-cd(1)/100 μM cyt-c(552) and 50% PVA, after a 48 h polymerization time. Electrochemical characterization of the mediator was carried out by cyclic voltammetry. The one-electron exchange between cyt-c(552) and the working electrode is a quasi-reversible process, without mass transport limitations. The formal potential of the mediator is 254±2 mV vs NHE and the intermolecular electron transfer rate constant between cytochromes c(552) and cd(1) is 9.9×10(3)M(-1)s(-1). The analytical parameters of the biosensor response to nitrite as assessed by amperometric measurements were: linear range from 10 to 200 μM; detection and quantification limits of 7 and 24 μM, respectively; sensitivity of 2.49±0.08 Amol(-1)cm(2) μM(-1). Catalytic profiles in the presence of possible interfering species were also investigated. The interference from competitive enzymatic reduction of dissolved oxygen could be overcome by tuning the cyclic voltammograms for faster sweep rates.     

Functional plasticity and allosteric regulation of α-ketoglutarate decarboxylase in central mycobacterial metabolism

The α-ketoglutarate dehydrogenase (KDH) complex is a major regulatory point of aerobic energy metabolism. Mycobacterium tuberculosis was reported to lack KDH activity, and the putative KDH E1o component, α-ketoglutarate decarboxylase (KGD), was instead assigned as a decarboxylase or carboligase. Here, we show that this protein does in fact sustain KDH activity, as well as the additional two reactions, and these multifunctional properties are shared by the Escherichia coli homolog, SucA. We also show that the mycobacterial enzyme is finely regulated by an additional acyltransferase-like domain and by?the action of acetyl-CoA, a powerful allosteric activator able to enhance the concerted protein motions observed during catalysis. Our results uncover the functional plasticity of a crucial node in bacterial metabolism, which may be important for M. tuberculosis during host infection.     

Extraordinarily robust polyproline type I peptoid helices generated via the incorporation of α-chiral aromatic N-1-naphthylethyl side chains

Peptoids, or oligomers of N-substituted glycines, are a class of foldamers that have shown extraordinary functional potential since their inception nearly two decades ago. However, the generation of well-defined peptoid secondary structures remains a difficult task. This challenge is due, in part, to the lack of a thorough understanding of peptoid sequence-structure relationships and, consequently, an incomplete understanding of the peptoid folding process. We seek to delineate sequence-structure relationships through the systematic study of noncovalent interactions in peptoids and the design of novel amide side chains capable of such interactions. Herein, we report the synthesis and detailed structural analysis of a series of (S)-N-(1-naphthylethyl)glycine (Ns1npe) peptoid homo-oligomers by X-ray crystallography, NMR spectroscopy, and circular dichroism (CD) spectroscopy. Four of these peptoids were found to adopt well-defined structures in the solid state, with dihedral angles similar to those observed in polyproline type I (PPI) peptide helices and in peptoids with α-chiral side chains. The X-ray crystal structure of a representative Ns1npe tetramer revealed an all cis-amide helix, with approximately three residues per turn, and a helical pitch of approximately 6.0 ?. 2D-NMR analysis of the length-dependent Ns1npe series showed that these peptoids have very high overall backbone amide K(cis/trans) values in acetonitrile, indicative of conformationally homogeneous structures in solution. Additionally, CD spectroscopy studies of the Ns1npe homo-oligomers in acetonitrile and methanol revealed a striking length-dependent increase in ellipticity per amide. These Ns1npe helices represent the most robust peptoid helices to be reported, and the incorporation of (S)-N-(1-naphthylethyl)glycines provides a new approach for the generation of stable helical structure in this important class of foldamers.     

Synthèse énantiospécifique des acides (+)-(2R)- et (−)-(2S)-éthyl-6-dihydro-3,4-méthyl-2-oxo-4–2H pyranecarboxylique-5

Enantiospecific Synthesis of (+)-(2R)- and (?)-(2S)-6-Ethyl-3,4-dihydro-2-methyl-4-oxo-2H-pyran-5-carboxylic Acid The two enantiomers (?)-(2S)- and (+)-(2R)-6-ethyl-3,4-dihydro-2-methyl-4-oxo-2H-pyran-5-carboxylic acid ((S)- and (R)- 7 ) have been synthesized from (+)-(3S) and (?)-(3R)-3-hydroxybutanoates, respectively (Scheme 1). By reduction and decarboxylation, the tetrahydro-2H-pyranols (2R, 4R, 6S)- and (2S, 4S, 6R)- 13 , respectively, were obtained with an enantiomeric excess of ≥ 93%.     

Electronic properties of Ru(II) complexes bound to a bisphenolate bridge with low lying pi* orbitals

The synthesis and a detailed investigation into the electronic properties of mononuclear and dinuclear Ru(II) complexes of the ligand bis(2-hydroxyphenyl)-2,5-dihydropyrazine (H(2)BHD) is described. In these complexes the Ru(II) moieties are bound through O,N coordination to an anionic phenolate and the pyrazine bridge. Relatively few reports are available on the dinuclear complexes bridged across a phenolate and this study provides an opportunity to examine the impact of reduced oxygen donor ligands on metal-metal communication. The results presented here reveal some very unusual behavior whereby the apparent location of the LUMO changes between the mononuclear and dinuclear complexes. The lowest energy optical transition appears to involve the peripheral bipyridine ligand as acceptor in the mononuclear complex, whereas this ligand is not involved in the lowest energy optical transition in the dinuclear complex. The origin of this difference is not clear, however, significant changes in the electronic properties of the mononuclear complex are observed on coordination of the second metal, reflected in significant alterations in the electrochemistry of the bridge and metals as well as changes in the optical spectroscopy. The BHD(2-) bridge is shown to support weakly coupled class II behavior according to the Robin and Day classification, reflected in a K(c) of 335.     

Characterization of complicated new polymorphs of chlorothalonil by X-ray diffraction and computer crystal structure prediction

A simultaneous experimental and computational search for polymorphs of chlorothalonil (2,4,5,6-tetrachloro-1,3-benzenedicarbonitrile) has been conducted, leading to the first characterization of forms 2 and 3. The crystal structure prediction study, using a specifically developed anisotropic atom-atom potential for chlorothalonil, gave as the global minimum in the lattice energy a structure that was readily refined against powder diffraction data to the known form 1 (P2(1)/a). The structure of form 2 was solved and refined from powder diffraction data, giving a disordered structure in the Rm (166) space group (Z = 3). It could also be refined against a P1 ordered model, starting from a low-energy hypothetical sheet structure found in the computational search. This shows that the disorder could be associated with the stacking of ordered sheets. The disordered structure for form 2 was later confirmed by single-crystal X-ray diffraction. The structure of form 3, determined from single-crystal diffraction, contains three independent molecules in the asymmetric unit in P2(1) (4) (Z = 6). Powder diffraction showed that this single-herringbone structure was similar to two low-energy structures found in the search. Further analysis confirmed that form 3 has a similar lattice energy and contains elements from both these predicted structures, which can be considered as good approximations to the form 3 structure.     

Ion exchange of Pb(2+), Cu(2+), Fe(3+), and Cr(3+) on natural clinoptilolite: selectivity determination and influence of acidity on metal uptake

In the present study ion exchange of Pb(2+), Cu(2+), Fe(3+), and Cr(3+) on natural Greek clinoptilolite was examined in terms of selectivity toward the above heavy metals in single- and multicomponent solutions in batch systems. Also examined are the influence of clinoptilolite on solution acidity and the effect of acidity on the ion exchange process. Clinoptilolite increases solution acidity due to the exchange of H(+) cations with the cations initially present in its structure. H(+) cations should be considered as competitive ones in ion exchange processes, and consequently ion exchange of metals is favored at high acidity values. Cu(2+) and Cr(3+) are the most sensitive cations with respect to acidity. Selectivity determination demonstrates that the selectivity at total concentration 0.01 N and acidity 2 in both single- and multicomponent solutions is following the order Pb(2+)>Fe(3+)>Cr(3+) > or =Cu(2+). This order is set since the first days of equilibration. However, Cu(2+) shows remarkable changes in selectivity and generally its uptake and selectivity are increasing with time. On the other hand selectivity in single metal solutions where acidity is not adjusted is following the order Pb(2+)>Cr(3+)>Fe(3+) congruent with Cu(2+).