Die Reaktion von Trithiazylchlorid mit Titantetrachlorid Die Kristallstruktur von (S4N5)2[Ti2Cl10]

Reaction of Trithiazyl Chloride with Titanium Tetrachloride. Crystal Structure of (S₄N₅)₂[Ti₂Cl₁₀] In the reaction of trithiazyl chloride with titanium tetrachloride, chlorine is abstracted and the brown-yellow adduct TiCl₄(N₂S₂) is obtained. In this compound — according to its i.r. spectrum — a N₂S₂ ring is bonded to the titanium via the N atoms, thus forming a polymer. As a by-product, brown crystalline (S₄N₅)₂[Ti₂Cl₁₀] forms. Its crystal structure was determined and refined with X-ray diffraction data (R = 0.042 for 812 reflexions). It crystallizes in the monoclinic space group P2₁/c with two formula units per unit cell. The lattice constants are a = 670, b = 1 633, c = 1108 pm, β = 97.24°. The structure consists of S₄N₅^⊕ cations, which are nearly equal to those in [S₄N₅]Cl, and [Ti₂Cl₁₀]^2? anions, which are nearly identical with those in (PCl₄)₂[Ti₂Cl₁₀].     

Die Reaktionen von Eisen(III)-chlorid mit Trithiazylchlorid. Die Kristallstruktur von [S4N4Cl]+[FeCl4]⊖

Reactions of Iron Trichloride with Trithyazyl Chloride. Crystal Structure of [S₄N₄Cl]⁺[FeCl₄]^? Iron trichloride reacts with (NSCl)₃ yielding S₄N₄[FeCl₄]₂, S₃N₃Cl₂[FeCl₄] or S₄N₄Cl[FeCl₄], depending on the reaction conditions. The i.r. spectra prove the presence of [FeCl₄]^? ions for all three compounds. The ⁵⁷Fe-Mössbauer spectra show a slight quadrupole splitting at 80 K for S₃N₃Cl₂[FeCl₄] (ΔE^Q = 0.42 mm · s^?1) and S₄N₄Cl[FeCl₄] (ΔE^Q = 0.23 mm · s^?1), which indicates a slight deformation of the FeCl₄^? tetrahedra. The crystal structure of S₄N₄Cl[FeCl₄] was determined and refined with X-ray diffraction data (2549 independent reflexions, R = 0.026). S₄N₄Cl[FeCl₄] crystallizes in the triclinic space group P1 with two formula units per unit cell. The lattice constants are a = 712, b = 911, c = 1006 pm, α = 76.5°, β = 83.8° and γ = 80.5°. The structure consists of the so far unknown [S₄N₄Cl]^⊕ cations and slightly deformed FeCl₄^? ions. The [S₄N₄Cl]^⊕ ion consists of a S₄N₄ ring built up of two nearly planar S₃N₂ fragments having a dihedral angle of 136°. The average SN bond length is 157 pm, the SCI bond length 214 pm.     

Critical evaluation of additivity schemes for estimating heats of atomization

Additivity schemes for estimating heats of formation and heats of atomization of alkanes and halo- alkanes are scrutinized. A least squares analysis of the heats of atomization of alkanes leads to a revision of the values of the parameters. The improvement in the estimated heats of atomization over the values obtained with parameters previously suggested amounts to at least 100%. The reasons for the deviations from the experimental values are discussed. These parameters can be transfered to haloalkanes. Additional parameters necessary for estimating heats of atomization of haloalkanes are also obtained through least squares analyses.     

Acidity functions of aqueous fluorinated acid solutions by differential pulse polarography

The acidity functions of aqueous trifluoroacetic and trifluoromethanesulphonic acid mixtures, and aqueous hexafluoropropane-2, 2-diol solutions, have been determined by differential pulse polarography. The apparent shift of the half-wave potential for the ferrocene—ferricinium couple, as the solvent composition is changed, is used to indicate the change in potential of a glass electrode; acidity is measured as the H_GF acidity function. The densities of two of these solvent systems as a function of composition are also reported. Trifluoromethanesulphonic acid—water mixtures represent the strongest aqueous acid solvent system so far studied.     

Protein prosthesis: a nonnatural residue accelerates folding and increases stability

Nonnatural residues can endow proteins with desirable properties. Here, replacing a proline residue that has a cis peptide bond in native ribonuclease A with 5,5-dimethyl-l-proline is shown to accelerate protein folding by 6-fold and enhance conformational stability by DeltaTm = 2.8 +/- 0.3 degrees C while having no effect on enzymatic activity. The rational use of this and other prosthetic segments could enable chemotherapeutic proteins to survive longer in vivo or retain activity after oral administration.     

Core of projective dimension one modules

The core of a projective dimension one module is computed explicitly in terms of Fitting ideals. In particular, our formula recovers previous work by R. Mohan on integrally closed torsionfree modules over a two-dimensional regular local ring. The first author was partially supported by a Faculty Research Fellowship from the University of Kentucky. The second and third authors gratefully acknowledge partial support from the NSF.Mathematics Subject Classification (2000):Primary 13A30, 13B21; Secondary 13C99     

Susceptibility corrections in solid-state NMR experiments with oriented membrane samples. Part I: applications

Chemical shift referencing of solid-state NMR experiments on oriented membranes has to compensate for bulk magnetic susceptibility effects that are associated with the non-spherical sample shape, as described in the accompanying paper [J. Magn. Reson. 164 (2003) 115-127]. The resulting frequency deviations can be on the order of 10 ppm, which is serious for nuclei with a narrow chemical shift anisotropy such as 1H or 13C, and in some cases even 19F. Two referencing schemes are proposed here to compensate for these effects: A flat (0.4 mm) glass container with an isotropic reference molecule dissolved in a thin film of liquid is stacked on top of the oriented membrane sample. Alternatively, the intrinsic proton signal of the hydrated lipid can be used for chemical shift referencing. Further aspects related to magnetic susceptibility are discussed, such as air gaps in susceptibility-matched probeheads, the benefits of shimming, and limitations in the accuracy of orientational constraints. A biological application is illustrated by a series of experiments on the antimicrobial peptide PGLa, aimed at understanding its concentration-dependent membranolytic effect. To address a wide range of molar peptide/lipid ratios between 1:3000 and 1:8, multilayers of hydrated DMPC containing a 19F-labeled peptide were oriented between stacked glass plates. Maintaining an approximately constant amount of peptide gives rise to thick samples (18 plates) at low, and thin samples (3 plates) at high peptide/lipid ratio. Accurate referencing was critical to reveal a small but significant change over 5 ppm in the anisotropic chemical shift of the 19F label on the peptide, indicative of a change in the orientation and/or dynamics of PGLa in the membrane.     

Dynamics of competitive evolution on a smooth landscape

We study competitive DNA sequence evolution directed by in vitro protein binding. The steady-state dynamics of this process is well described by a shape-preserving pulse which decelerates and eventually reaches equilibrium. We explain this dynamical behavior within a continuum mean-field framework. Analytical results obtained on the motion of the pulse agree with simulations. Furthermore, finite population correction to the mean-field results are found to be insignificant.