Design of efficient ambipolar host materials for organic blue electrophosphorescence: theoretical characterization of hosts based on carbazole derivatives

Density functional theory calculations were carried out to investigate the electronic structures of representative ambipolar hosts for blue electroluminescence, based on two carbazole end groups and meta-terphenyl (mTP)-like bridges. The bridge molecular segments include mTP, 2,6-bisphenylpyridine, 3,5-bisphenylpyridine, and 2,6-bisphenylpyrimidine. While the ionization potentials and electron affinities of these molecules are mainly determined by their hole- and electron-transport subunits, respectively, each subunit impacts the electronic properties of the other upon their binding, mainly in an inductive way. Importantly, the lowest triplet state of the hosts is determined to be confined into the mTP-like bridges since these are the subunits with lowest individual triplet energy. Extension of the phenyl-based π-conjugated system via meta linkages is found to be effective in modulating the electron affinity value while maintaining a high triplet energy.     

Nonribosomal peptide synthesis in animals: the cyclodipeptide synthase of Nematostella

Cyclodipeptide synthases (CDPSs) are small enzymes structurally related to class-I aminoacyl-tRNA synthetases (aaRSs). They divert aminoacylated tRNAs from their canonical role in ribosomal protein synthesis, for cyclodipeptide formation. All the CDPSs experimentally characterized to date are?bacterial. We show here that a predicted CDPS from the sea anemone Nematostella vectensis is an active CDPS catalyzing the formation of various cyclodipeptides, preferentially containing tryptophan. Our findings demonstrate that eukaryotes encode active CDPSs and suggest that all CDPSs have?a similar aminoacyl-tRNA synthetase-like architecture and ping-pong mechanism. They also raise questions about the biological roles of the cyclodipeptides produced in bacteria and eukaryotes.     

Water behavior in mesoporous materials as studied by NMR relaxometry

Water in mesoporous materials possessing a two-dimensional hexagonal structure has been studied by the variation of its NMR longitudinal relaxation time T(1) as a function of the static magnetic field value, or equivalently of the NMR measurement frequency. This technique, dubbed relaxometry, has been applied from 5 kHz (measurement frequency) up to 400 MHz with various instruments including a variable-field spectrometer operating between 8 and 90 MHz. Moreover, the range 0-5 kHz could be investigated by transverse relaxation, T(2) denoting the corresponding relaxation time, and relaxation in the rotating frame, T(1ρ) denoting the corresponding relaxation time. Measurements of proton relaxation rates (inverse of relaxation times) have been performed with H(2)O and HOD (residual protons of heavy water) at water volumes of 80%, 60%, and 40% relative to the porous volume. Comparison between H(2)O and HOD shows clearly that, above 1 MHz where both sets of data are superposed, relaxation is purely intermolecular and due to paramagnetic relaxation (dipolar interactions of water protons with unpaired electrons of paramagnetic entities). Below 1 MHz, it is possible to subtract the intermolecular contribution (given by HOD data) from H(2)O data so that one is left with intramolecular relaxation which is solely due to water reorientational motions. The analysis of these low-frequency data (in terms of Lorentzian functions) reveals two types of water within the pores: one interacting strongly with the surface and the other corresponding to a second layer. High-frequency data, which arise from paramagnetic relaxation, exhibit again two types of water. Due to their correlation times, one type is assigned to relatively free water within the pores while the other type corresponds to bulk (interparticular) water. Their proportions, given as a function of the volume fraction, are consistent with the above assignments.     

L-arginine binding to human inducible nitric oxide synthase: an antisymmetric funnel route toward isoform-specific inhibitors?

Nitric oxide (NO) is an important signaling molecule produced by a family of enzymes called nitric oxide synthases (NOS). Because NO is involved in various pathological conditions, the development of potent and isoform-selective NOS inhibitors is an important challenge. In the present study, the dimer of oxygenase domain of human iNOS (iNOSoxy) complexed to its natural substrate L-arginine (L-Arg) and both heme and tetrahydro-L-biopterin (BH4) cofactors was studied through multiple molecular dynamics simulations. Starting from the X-ray structure available for that complex (PDB: 1NSI ), a 16 ns equilibration trajectory was first obtained. Twelve dynamics of slow extraction of L-Arg out from the iNOSoxy active site were then performed. The steered molecular dynamics (SMD) approach was used starting from three different points of the reference trajectory for a total simulation time of 35 ns. A probable unbinding/binding pathway of L-Arg was characterized. It was suggested that a driving force directed the substrate toward the heme pocket. Key intermediate steps/residues along the access route to the active site were identified along this "funnel shape" pathway and compared to existing data. A quasi-normal mode analysis performed on the SMD data suggested that large collective motions of the protein may be involved in L-Arg binding and that opening the route to the active site in one monomer promoted an inverse, closing motion in the second monomer. Finally, our findings might help to rationalize the design of human iNOS isoform competitive inhibitors.     

Extensions of system signatures to dependent lifetimes: Explicit expressions and interpretations

The concept of system signature was introduced by Samaniego for systems whose components have i.i.d. lifetimes. We consider its extension to the continuous dependent case and give an explicit expression for this extension as a difference of weighted means of the structure function values. We then derive a formula for the computation of the coefficients of these weighted means in the special case of independent continuous lifetimes. Finally, we interpret this extended concept of signature through a natural least squares approximation problem.     

Mono- and dicarbonyl-bridged tricyclic heterocyclic acceptors: synthesis and electronic properties

A series of trialkylsilyl-substituted 2,2'-dithiophene, 4,4'-di-n-hexyl-2,2'-dithiophene, 5,5'-dithiazole, and 2,2'-diselenophene with carbonyl (2a-d) and α-dicarbonyl bridges (3a-d) were prepared from readily available dihalides, using double lithiation followed by trapping with N,N-dimethylcarbamoyl chloride or diethyl oxalate (or N,N-dimethylpiperazine-2,3-dione), respectively. Cyclic voltammetry reveals that the first half-wave reduction potentials for this series of compounds span a wide range, from -1.87 to -0.97 V vs the ferrocene/ferrocenium couple at 0 V (0.1 M (n)Bu(4)NPF(6) in THF). A significant increase of the first half-wave reduction potential (by 0.50-0.67 V) was observed on substitution of the monocarbonyl bridge with α-dicarbonyl. Adiabatic electron affinity (AEA, gas phase) trends determined via density functional theory (DFT) calculations are in good agreement with the electrochemical reduction potentials. UV-vis absorption spectra across the series show a weak absorption band in the visible range, corresponding to the HOMO→LUMO transition within a one-electron picture, followed by a more intense, high-energy transition(s). Single-crystal X-ray structural analyses reveal molecular packing features that balance the interplay of the presence of the bulky substituents, intermolecular π-stacking interactions, and S···O intermolecular contacts, all of which affect the DFT-evaluated intermolecular electronic couplings and effective charge-carrier masses for the crystals of the tricyclic cores.     

Isolation of C-glycosyl xanthones from Coffea pseudozanguebariae and their location

The biochemical composition of leaves from Coffea pseudozanguebariae, a wild caffeine-free coffee species, was determined. Two phenolic compounds were extracted from leaves, separated and characterized. Their structures were elucidated by mass spectrometry, and 1D and 2D NMR spectroscopy and were shown to be mangiferin (1) and isomangiferin (2), which were the main polyphenol products. Multiphoton fluorescence imaging was performed to visualize polyphenol distribution in leaf cross sections. Consistent biochemical analysis cell imaging techniques on leaves revealed yellow fluorescence in the epidermis and parenchyma cells corresponding to xanthone compounds.     

Determination of the enantiomeric excess of α-hydroxy acids

This work describes a convenient and accurate method for optical purity determination of α-hydroxy acids. Their derivatization with commercially available valine methyl ester affords diastereoisomers easily separable by HPLC using achiral C₁₈ columns.