Reaktionen von Zinnchloriden mit Polysulfiden. Die Kristallstrukturen von (PPh4)2[SnCl2(S6)2], (PPh4)2[Sn4Cl4S5(S3)O] und (PPh4)2[SnCl6] · S8 · 2CH3CN

Reaction of Tin Chlorides with Polysulfides. Crystal Structures of (PPh₄)₂[SnCl₂(S₆)₂], (PPh₄)₂[Sn₄Cl₄S₅(S₃)O], and (PPh₄)₂[SnCl₆] · S₈ · 2CH₃CN . The reaction of PPh₄[SnCl₃] with Na₂S₄ in acetonitrile in the presence of small amounts of water yields (PPh₄)₂[Sn₄Cl₄S₅(S₃)O] and minor amounts of (PPh₄)₂[SnCl₂(S₆)₂], PPh₄Cl · 2S₈ and (PPh₄)₂[SnCl₆]. SnCl₄ is partially reduced by (PPh₄)₂S_x, PPh₄[SnCl₃] and (PPh₄)₂[SnCl₆] · S₈ · 2CH₃CN being produced. According to the X-ray crystal structure determination the [Sn₄Cl₄S₅(S₃)O]^2?-ion consists of an O atom that is coordinated by four Sn atoms which in turn are liked with one another by five single S atoms and one S₃ group. In the [SnCl₂(S₆)₂]^2?-ion the Sn atom is octahedrally coordinated by two Cl atoms in trans arrangement and by two chelating S₆ groups. Octahedral [SnCl₆]^2? ions and S₈ molecules in the crown conformation are present in (PPh₄)₄[SnCl₆] · S₈ · 2CH₃CN.     

Intramolecular charge transfer with the planarized 4-aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)

Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap DeltaE(S(1),S(2)), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to micro(e)(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at -70 degrees C, the forward ICT rate constant (1.3 x 10(11) s(-1)) is much smaller than that of the back reaction (5.9 x 10(9) s(-1)), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when DeltaE(S(1),S(2)) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing DeltaE(S(1),S(2)) relative to that of DMABN.     

Kinetics of swelling of polyether-modified poly(acrylic acid) microgels with permanent and degradable cross-links

Spherical particles of 50-100 mum size composed of poly(acrylic acid) networks covalently bonded to Pluronic polyether copolymers were tested for swelling in aqueous media. The microgels were cross-linked either by permanent ethylene glycol dimethacrylate (EGDMA) cross-links alone or by EDGMA together with reversible disulfide or biodegradable azoaromatic cross-links. Optimum conditions for a rapid, diffusion-limited swelling of the pH- and temperature-sensitive microgels with nondegradable cross-links were found. The microgels cross-linked by disulfide groups and equilibrium-swollen in the buffer solution exhibited degradation-limited kinetics of swelling under physiological conditions, with a first-order reaction constant, k(1), linearly proportional to the concentration of reducing agents such as dithiotreitol and tris(2-carboxyethyl)phosphine (TCEP). A severalfold faster swelling in the presence of more powerful reducing agent, TCEP, was observed, indicating the chemical specificity of the microgel swelling. The reoxidation of the thiol groups into disulfide cross-links by sodium hypochlorite led to the restoration of the microgels' diameter measured prior to the reduction-reoxidation cycle, which confirms the shape memory of the microgels. Enzymatically degradable azoaromatic cross-links enabled slow microgel swelling due to degradation of the cross-links by azoreductases from the rat intestinal cecum. The low rate of swelling of the Pluronic-containing microgels can enable sustained drug release in colon-specific drug delivery.     

EC–SPE–stripline-NMR analysis of reactive products: a feasibility study

Flow-through electrochemical conversion (EC) of drug-like molecules was hyphenated to miniaturized nuclear magnetic resonance spectroscopy (NMR) via on-line solid-phase extraction (SPE). After EC of the prominent p38α mitogen-activated protein kinase inhibitor BIRB796 into its reactive products, the SPE step provided preconcentration of the EC products and solvent exchange for NMR analysis. The acquisition of NMR spectra of the mass-limited samples was achieved in a stripline probe with a detection volume of 150 nL offering superior mass sensitivity. This hyphenated EC–SPE–stripline-NMR setup enabled the detection of the reactive products using only minute amounts of substrate. Furthermore, the integration of conversion and detection into one flow setup counteracts incorrect assessments caused by the degradation of reactive products. However, apparent interferences of the NMR magnetic field with the EC, leading to a low product yield, so far demanded relatively long signal averaging. A critical assessment of what is and what is not (yet) possible with this approach is presented, for example in terms of structure elucidation and the estimation of concentrations. Additionally, promising routes for further improvement of EC–SPE–stripline-NMR are discussed.     

Deuterated Disaccharides for the Investigation of Protein-Carbohydrate Interactions-Application of Bioaffinity-and STD-NMR

ABSTRACT

NMR spectroscopic analysis of carbohydrates often suffers from severe overlap of resonance signals, especially in ¹H NMR spectra. Therefore, we synthesized four 2,3,4-trideuterio-α-L-fucose containing disaccharides, α-L-Fuc-(1→6)-β-D-GlcNAc-OMe 1, α-L-Fuc-(1→4)-β-D-GlcNAc-OMe 2, α-L-Fuc-(1→3)-β-D-GlcNAc-OMe 3, and α-L-Fuc-(1→2)-β-D-Gal-OMe 4. Compounds 1 to 4 are well suited to be subjected to NMR conformational analysis because their ¹H NMR spectra show almost no overlap of signals. The deuterated disaccharides 1 to 4 will therefore be used as NMR probes for the exploration of fucose-binding proteins. With a mixture of the corresponding non-deuterated disaccharides it is demonstrated that recently developed parallel NMR screening protocols, Bio-Affinity NMR and STD-NMR, deliver fast and robust tools to assay the compounds synthesized for protein-binding affinity.