Experimental and computational study of the conrotatory ring opening of various 3-chloro-2-azetines

A combined experimental and theoretical study is presented on 2-azetines, a class of azaheterocyclic compounds, which are difficult to access but have shown a unique reactivity as strained cyclic enamines. New highly substituted 2-azetines bearing aryl substituents at the 2- and 4-position were synthesized from 3,3-dichloroazetidines. Whereas 2-aryl-3,3-dichloroazetidines gave stable 2-aryl-3-chloro-2-azetines upon treatment with sodium hydride in DMSO, 2,4-diaryl-3,3-dichloroazetidines showed a remarkably different reactivity in that they afforded benzimidoyl-substituted alkynes under similar mild treatment with base. The formation of the alkynes involves electrocyclic ring opening of intermediate 2,4-diaryl-3-chloro-2-azetines and elimination of hydrogen chloride. Ab initio theoretical calculations confirmed the experimental findings and demonstrated that the 4-aryl substituent is responsible for this remarkably enhanced reactivity of 2-azetines toward electrocyclic conrotatory ring opening by a significant decrease in reaction barrier of about 30 kJ/mol. This activation effect by an aryl group in the allylic position toward electrocyclic ring opening of unsaturated four-membered rings is of general importance since a similar increased reactivity of 4-aryloxetes, 4-arylthiete-1,1-dioxides, and 3-arylcyclobutenes has been reported in literature as well.     

Aza-beta3-cyclotetrapeptides

The cyclization of aza-beta(3)-tetrapeptides gives access to new CTP (cyclotetrapeptide) analogues. These stereocontrolled templates are assembled without any asymmetric synthesis. X-ray crystallographic structure and NMR analysis show that the macrocyclic scaffold is characterized by a fully cooperative intramolecular H-bond network, in sharp contrast with the nanotubular assemblies observed for beta(3)-cyclotetrapeptides. This folding property reduces considerably the polarity of aza-beta(3)-tetrapeptides and should be useful in addressing intracellular targets.     

Optimized and far-red-emitting variants of fluorescent protein eqFP611

Fluorescent proteins (FPs) emitting in the far-red region of the spectrum are highly advantageous for whole-body imaging applications because scattering and absorption of long-wavelength light is markedly reduced in tissue. We characterized variants of the red fluorescent protein eqFP611 with bright fluorescence emission shifted up to 639 nm. The additional red shift is caused by a trans-cis isomerization of the chromophore. The equilibrium between the trans and cis conformations is strongly influenced by amino acid residues 143 and 158. Pseudo monomeric tags were obtained by further genetic engineering. For the red chromophores of eqFP611 variants, molar extinction coefficients of up to approximately 150,000 were determined by an approach that is not affected by the presence of molecules with nonfunctional red chromophores. The bright fluorescence makes the red-shifted eqFP611 variants promising lead structures for the development of near-infrared fluorescent markers. The red fluorescent proteins performed well in cell biological applications, including two-photon imaging.     

Can electrospray ionization help the characterization of iron gall inks? Investigation on the interactions of gallic acid with iron ions in aqueous solutions

In the present work, an investigation has been conducted by electrospray ionization (ESI) experiments to characterize the structures of iron gall ink complexes in solution. Simple mono and polyphenolic acid molecules added to iron sulfate salts were chosen to model the recipes of ink composition. Theoretical calculations have been used (1) to determine the stability of the ionic complexes generated in the gas phase, (2) to explain which structures are more likely generated in the electrospray ion source, and (3) to determine which mechanisms are likely involved in their formation. Fragmentation pathways of the derived structures have also been investigated and rationalized to facilitate the interpretation of the data obtained under collisionally induced dissociation (CID) conditions.The present study confirms the assumption that ESI experiments with ions that are preformed in solution must be considered carefully. As a matter of fact, the study of ion formation mechanisms in the ion source is necessary to establish relationships between the ion structures in the condensed phase and the gas phase.     

One-electron-transfer reactions of polychlorinated ethylenes: concerted and stepwise cleavages

Reaction barriers were calculated by using ab initio electronic structure methods for the reductive dechlorination of the polychlorinated ethylenes: C2Cl4, C2HCl3, trans-1,2-C2H2Cl2, cis-1,2-C2H2Cl2, 1,1-C2H2Cl2 and C2H3Cl. Concerted and stepwise cleavages of R-Cl bonds were considered. Stepwise cleavages yielded lower activation barriers than concerted cleavages for the reduction of C2Cl4, C2HCl3, and trans-1,2-C2H2Cl2 for strong reducing agents. However, for typical ranges of reducing strength concerted cleavages were found to be favored. Both gas-phase and aqueous-phase calculations predicted C2Cl4 to have the lowest reaction barrier. Additionally, the reduction of C2HCl3 was predicted to show selectivity toward formation of cis-1,2-C2HCl2* over the formation of trans-1,2-C2HCl2*, and 1,1-C2HCl2* radicals.     

Convergence of classes of high-order semi-Lagrangian schemes for the Vlasov-Poisson system

In this paper we present some classes of high-order semi-Lagran- gian schemes for solving the periodic one-dimensional Vlasov-Poisson system in phase-space on uniform grids. We prove that the distribution function and the electric field converge in the norm with a rate of

where is the degree of the polynomial reconstruction, and and are respectively the time and the phase-space discretization parameters.

     

Synthesis, EPR spectrum and X-ray structure of tetra-n-butylammonium bis(benzene-1,2-dithiolato(2−)-κS,S′)platinate(III)

The new salt, tetra-n-butylammonium bis(benzene-1,2-dithiolato(2−)-κ²S,S′)platinate(III), [NBu₄][Pt(C₆H₄S₂)₂] (1), has been synthesized in ethanol/water, and fully characterized by single crystal X-ray structure determination. The central platinum in the complex ion [Pt(bdt)₂]⁻ is tetracoordinated by the S atoms of the bdt²⁻ ligands (bdt²⁻ is benzene-1,2-dithiolate) in a square-planar geometry. The well-resolved frozen solution EPR spectrum exhibits rhombic symmetry. The room temperature effective magnetic moment (μ_eff = 1.80 Bohr magneton) is in line with this spectrum and strongly supports the Pt(III) oxidation state in 1. This observation is in excellent agreement with previous results reported on closely related Ni(III), Pd(III) and Pt(III) species.     

Separation of casein micelles from whey proteins by high shear microfiltration of skim milk using rotating ceramic membranes and organic membranes in a rotating disk module

This paper investigates the microfiltration of skim milk in order to separate caseins micelles from two whey proteins, α-lactalbumin (α-La) and β-lactoglobulin (β-Lg), using a modified dynamic filtration pilot (MSD) consisting in 6 ceramic 9-cm diameter membrane disks of 0.2 μm pores, rotating around a shaft inside cylindrical housing. A comparison was made with another dynamic filtration module consisting in a disk rotating near a fixed PVDF 15.5 cm diameter membrane with 0.15 μm pores. Maximum permeate fluxes were 120 L h⁻¹ m⁻² with the MSD module at 1930 rpm and at 40 °C, and 210 L h⁻¹ m⁻² at 2500 rpm and 45 °C, with the rotating disk module. Casein rejection was around 99% at high speed for both membranes. α-La transmission decreased with increasing transmembrane pressure (TMP) from 75% to 60% for ceramic membranes and from 25% to 10% for the PVDF one. β-Lg transmissions were lower, ranging from 23% to 15% for ceramic membranes and from 20% to 5% for the PVDF one. In a concentration test with the PVDF membrane at 2000 rpm, the flux decayed from 200 L h⁻¹ m⁻² at initial concentration to 80 L h⁻¹ m⁻² at VRR = 3.2 and 22.1% of the initial α-La mass was recovered in the permeate, against 8.1% for β-Lg. Permeate fluxes in the mass transfer limited regime (J_lim) of the MSD and rotating disk module operated at various speeds were well correlated by the equation J_lim = 17.13 V_av where V_av denoted the disk azimuthal velocity averaged over the membrane area. Measurements of J_lim, taken from Ref. [G. Samuelsson, P. Dejlmek, G. Tragardh, M. Paulsson, Minimizing whey protein retention in crossflow microfiltration of skim milk. Int. Dairy J. 7 (1997) 237–242] during MF of skim milk using tubular ceramic membranes at velocities from 1.5 to 8 m s⁻¹ with permeate co-current recirculation were found to obey the same correlation.     

Synthesis and light gas separations in cross-linked gemini room temperature ionic liquid polymer membranes

Cross-linkable gemini room temperature ionic liquids (GRTILs) were synthesized and photo-cross-linked into thin films. The resultant polymer membranes were tested for their permeabilities to CO₂, N₂, CH₄ and H₂. Permeabilities for each gas were found to be much lower when compared to previously reported poly(RTIL) membranes, mainly as a result of highly restricted diffusion. Separation factors were similar to previously studied poly(RTIL) membranes. CH₄ and N₂ fluxes were small enough to consider these membranes as “barrier” films to the transport of those gases. Poly(GRTILs) may have use in applications where flow of those gases is not desirable.