Control of the evolution of iron peroxide intermediate in superoxide reductase from Desulfoarculus baarsii. Involvement of lysine 48 in protonation

Superoxide reductase is a nonheme iron metalloenzyme that detoxifies superoxide anion radicals O(2)(?-) in some microorganisms. Its catalytic mechanism was previously proposed to involve a single ferric iron (hydro)peroxo intermediate, which is protonated to form the reaction product H(2)O(2). Here, we show by pulse radiolysis that the mutation of the well-conserved lysine 48 into isoleucine in the SOR from Desulfoarculus baarsii dramatically affects its reaction with O(2)(?-). Although the first reaction intermediate and its decay are not affected by the mutation, H(2)O(2) is no longer the reaction product. In addition, in contrast to the wild-type SOR, the lysine mutant catalyzes a two-electron oxidation of an olefin into epoxide in the presence of H(2)O(2), suggesting the formation of iron-oxo intermediate species in this mutant. In agreement with the recent X-ray structures of the peroxide intermediates trapped in a SOR crystal, these data support the involvement of lysine 48 in the specific protonation of the proximal oxygen of the peroxide intermediate to generate H(2)O(2), thus avoiding formation of iron-oxo species, as is observed in cytochrome P450. In addition, we proposed that the first reaction intermediate observed by pulse radiolysis is a ferrous-iron superoxo species, in agreement with TD-DFT calculations of the absorption spectrum of this intermediate. A new reaction scheme for the catalytical mechanism of SOR with O(2)(?-) is presented in which ferrous iron-superoxo and ferric hydroperoxide species are reaction intermediates, and the lysine 48 plays a key role in the control of the evolution of iron peroxide intermediate to form H(2)O(2).     

Energetic distribution of the electrons from a 200 keV beam in polyurethane layers : EGS4 calculation and FTIR analysis

The aim of this study is to characterize the chemical transformation of a polymer resulting from irradiation by a 200 keV electron beam. Thermoplastic PU polyetherurethane material was used and irradiation was performed with applied electron fluences in the range of 10¹⁴–10¹⁷ electron cm⁻² at 77 K.

The chemical changes have been observed by FTIR analysis of irradiated layers. A NH bond evolution study has allowed us to follow polymer degradation versus depth and fluence. The results have been compared to a simulation of electronic energy loss and to the energy spectrum of the generated electrons in the polymer using EGS4 code.     

Phosphorescent Cationic Heterodinuclear IrIII/MI Complexes (M=CuI,AuI) with a Hybrid Janus-Type N-Heterocyclic Carbene Bridge

A novel class of phosphorescent cationic heterobimetallic Ir^III/M^I complexes, where M^I=Cu^I ( 4 ) and Au^I ( 5 ), is reported. The two metal centers are connected by the hybrid bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesAcac) ligand that combines both a chelating acetylacetonato-like and a monodentate N-heterocyclic carbene site coordinated onto an Ir^III and a M^I center, respectively. Complexes 4 and 5 have been prepared straightforwardly by a stepwise site-selective metalation with the zwitterionic [(IPr)M^I(IMesAcac)] metalloproligand (IPr=1,3-(2,6-diisopropylphenyl)-2H-imidazol-2-ylidene) and they have been fully characterized by spectroscopic, electrochemical, and computational investigation. Complexes 4 and 5 display intense red emission arising from a low-energy excited state that is located onto the “Ir(C^N)” moiety featuring an admixed triplet ligand-centered/metal-to-ligand charge transfer (³IL/¹MLCT) character. Comparison with the benchmark mononuclear complexes reveals negligible electronic coupling between the two distal metal centers at the electronic ground state. The bimetallic systems display enhanced photophysical properties in comparison with the parental congeners. Noteworthy, similar non-radiative rate constants have been determined along with a two-fold increase of radiative rate, yielding brightly red-emitting cyclometalating Ir^III complexes. This finding is ascribed to the increased MLCT character of the emitting state in complexes 4 and 5 due to the smaller energy gap between the ³IL and ¹MLCT manifolds, which mix via spin–orbit coupling.     

Crystal growth and characterisation of new organic nonlinear optical materials: 6-cyano and 6-nitro-2,2-dimethyl-3,4-epoxychroman

Two chiral organic nonlinear optical materials, (3S,4S)-(−)-6-cyano-2,2-dimethyl-3,4-epoxychroman (1) and (3R,4R)-(+)-6-nitro-2,2-dimethyl-3,4-epoxychroman (2), have been grown into single crystals of cm³-size. Although both compounds crystallise in the orthorhombic P2₁2₁2₁ space group, they are not isomorphous and their crystal packings are quite different. Angle tuned type II phase-matched second harmonic generation between 0.8 and 1.064 μm has been evidenced, with effective nonlinear coefficients d_eff of 1 and 5 pm/V at 0.96 μm for 1 and 2, respectively. These values are in agreement with those estimated in the oriented gas model approximation using EFISH first order hyperpolarisability values (β₀=2.6 and 4.0×10⁻³⁰ esu for 1 and 2, respectively).     

Modeling the fission track etching process in apatite: Segmentation or crystallography influence

In this study, two factors which can influence fission track etching in apatite are considered: track segmentation (induced by thermal annealing) and variable radial etching speed (due to the reagent diffusion during the etching process).

During the latent track annealing, two distinguishable steps can be identified by measuring track lengths or diameters. A length reduction is firstly observed, followed by a segmentation process which leads to the emergence of disrupted regions (gaps).

At present time, electron microscopy studies on fission tracks in apatite show profiles which lead to hypotheses of a variable radial etching speed versus depth. These variations can be interpreted in terms of acid diffusion along the track. Moreover, the existence of several bulk etching speeds related to crystallographic orientation is approached.

Taking into account these different points, a software program, integrating parameters as original track orientation and depth, number of gaps, etc., is developed in order to model the track profile evolution during the etching process. Comparison with experiments in Durango apatite (Mexico) are also undertaken.     

The synthesis of [Fe(CO)3(η1-dppm) {Sn(n-Bu)2}]2 by silicon-tin exchange,its crystal structure and its use as a metallodiphosphine ligand

The reaction of the siloxyl containing ferrate [Fe(CO)₃( ¹-dppm){Si(OMe)₃}]^–,1 (dppm = Ph₂PCH₂PPh₂) with Sn(OAc)₂(n-Bu)₂ has yielded the new dimeric complex [Fe(CO)₃( ¹-dppm){µ-Sn(n-Bu)₂}]₂, 3 in 89% yield. Compound3 was characterized crystallographically and was found to be a centrosym-etrical molecule with a rhomboidal Fe₂Sn₂ cluster at the center. Each iron atom contains me ¹-dppm ligand. Compound3 was found to react with [Pd(dmba) (µ-Cl)]₂ (dmba=dimethylbenzylamine) to yield the new complexmer-[Fe(CO)₃{Sn(n-Bu)₂}(µ-dppm)Pd(dmba)Cl]₂, 4 by attachment of a palla-dium grouping to each of the uncoordinated phosphorus atoms in 3. Crystal data for 3: space groupP ,a=11.399(2) Å, 6=15.98(3) Å, c=10.869(3) Å, =94.10(2)°.=100.56(2)°, =69.35(1)°,Z=2, 3533 reflections,R=0.034.     

Potent inhibitors of the hepatitis C virus NS3 protease: design and synthesis of macrocyclic substrate-based beta-strand mimics

The virally encoded NS3 protease is essential to the life cycle of the hepatitis C virus (HCV), an important human pathogen causing chronic hepatitis, cirrhosis of the liver, and hepatocellular carcinoma. The design and synthesis of 15-membered ring beta-strand mimics which are capable of inhibiting the interactions between the HCV NS3 protease enzyme and its polyprotein substrate will be described. The binding interactions between a macrocyclic ligand and the enzyme were explored by NMR and molecular dynamics, and a model of the ligand/enzyme complex was developed.     

Synthesis of Secondary α-Methylene γ-Lactams

N-trimethylsilyl aldimines, on reaction with the isolated organozinc derivative of ethyl α-(bromomethyl) acrylate, afford secondary α-methylene γ-lactams with quite quantitative yields.     

Synthesis, structures, optical properties, and TD-DFT studies of donor-π-conjugated dipicolinic acid/ester/amide ligands

A series of 2,6- and 4-functionalized dipicolinic acid, ester, or amide featuring π-conjugated substituents such as donor-(phenyl or fluorenyl)-acetylene groups have been synthesized. Four crystallographic structures are reported. The influence of the substituent position, the nature of the donor group, and the conjugated backbone as well as the role of the pyridinic side arms on the absorption and emission properties are studied and discussed on the basis of TD-DFT calculations.