5-phenyl-1,2,3,4-thiatriazole-3-oxide: A new class of heteroaromatic N-oxides

Oxidation of 5-phenyl-1,2,3,4-thiatriazole (1) with peroxytrifluoroacetic acid yields 5-phenyl-1,2,3,4-thiatriazole-3-oxide (2) a representative of a new class of heteroaromatic N-oxides. The structure is based on the mass spectral fragmentation of 2 and the isotope labelled with ¹⁵N at position 2. IR and ESCA measurements are consistent with this assignment. The thermal, photochemical, and chemical properties of the oxide are discussed.     

Microwave-assisted decarboxylation of bicyclic 2-pyridone scaffolds and identification of Abeta-peptide aggregation inhibitors

A reagent-free microwave-assisted decarboxylation procedure for carboxylic acid functionalized bicyclic 2-pyridones has been developed. This new method, based on microwave heating at 220 degrees C for 600 seconds in N-methyl pyrrolidone (NMP), proved to be practical and very efficient, resulting in decarboxylated 2-pyridones in near-quantitative yields. The decarboxylated products and the intermediate 2-pyridones in the form of carboxylic acid methyl esters and carboxylic acids were screened for their effect on Abeta-peptide aggregation. Two out of the 21 2-pyridones described in this study inhibited amyloid formation of the Alzheimer Abeta(1-40) peptide. The effect was seen even at a 4 : 1 ratio of 2-pyridone and monomeric Abeta-peptide.     

Impact of azaproline on amide cis-trans isomerism: conformational analyses and NMR studies of model peptides including TRH analogues

The beta-turn is a well-studied motif in both proteins and peptides. Four residues, making almost a complete 180 degree-turn in the direction of the peptide chain, define the beta-turn. Several types of the beta-turn are defined according to Phi and Psi torsional angles of the backbone for residues i + 1 and i + 2. One special type of beta-turn, the type VI-turn, usually contains a proline with a cis-amide bond at residue i + 2. In an aza-amino acid, the alpha-carbon of the amino acid is changed to nitrogen. Peptides containing azaproline (azPro) have been shown to prefer the type VI beta-turn both in crystals and in organic solvents by NMR studies. MC/MD simulations using the GB/SA solvation model for water explored the conformational preferences of azPro-containing peptides in aqueous systems. An increase in the conformational preference for the cis-amide conformer of azPro was clearly seen, but the increased stability was relatively minor with respect to the trans-conformer as compared to previous suggestions. To test the validity of the calculations in view of the experimental data from crystal structures and NMR in organic solvents, [azPro(3)]-TRH and [Phe(2), azPro(3)]-TRH were synthesized, and their conformational preferences were determined by NMR in polar solvents as well as the impact of the azPro substitution on their biological activities.     

Crystal structure refinements of the κ phases in the Hf-Mo-Se and Hf-Mo-Ge systems

The crystal structures of κ-(Hf-Mo-Se) and κ-(Hf-Mo-Ge) were determined using X-ray single-crystal diffractometry. Both structures crystallize in the space groupP6₃/mmc (No. 194) with the unit-cell dimensionsa = 8.6995(4)A?,c = 8.6234(7)A? for κ-(Hf-Mo-Se) anda = 8.6394(4)A?,c = 8.6827(5)A? for κ-(Hf-Mo-Ge). The structures have been refined onF² toR(F²) values of 0.0784 and 0.0661, respectively. κ-(Hf-Mo-Se) and κ-(Hf-Mo-Ge) are isostructural with κ-(Hf-Mo-P) with a variable degree of hafnium substitution on the molybdenum sites of the structure. In κ-(Hf-Mo-Ge) germanium vacancies were found to occur on the trigonal prismatic 2c site and the phase was also found to contain oxygen on the octahedral 6g site. According to the final refinements the compositions are Hf_9+xMo_4?xSe withx = 0.40(2) and Hf_9+xMo_4?xGe_yO₂ withx = 0.25(2),y = 0.88(1), andz = 1.47(12).     

Application of acyl cyanophosphorane methodology to the synthesis of protease inhibitors: poststatin, eurystatin, phebestin, probestin and bestatin

Full details are given for the syntheses of the protease inhibitors, poststatin and eurystatin by the acyl cyanophosphorane coupling procedure used for the formation of α-keto amides. We have also extended this methodology to the syntheses of the related α-hydroxy amide natural products, phebestin, probestin and bestatin. The key step in the latter synthetic sequences involved diastereomeric selectivity in the reduction of the α-keto precursor to the corresponding α-hydroxy amide by the use of zinc borohydride.     

A pharmacophore model for NK2 antagonist comprising compounds from several structurally diverse classes

A neurokinin 2 (NK2) antagonist pharmacophore model has been developed on the basis of five non-peptide antagonists from several structurally diverse classes. To evaluate the pharmacophore model, another 20 antagonists were fitted to the model. By use of exhaustive conformational analysis (MMFFs force field and the GB/SA hydration model) and least-squares molecular superimposition studies, 23 of the 25 antagonists were fitted to the model in a low energy conformation with a low RMS value. The pharmacophore model is described by four pharmacophore elements: Three hydrophobic groups and a hydrogen bond donor represented as a vector. The hydrophobic groups are generally aromatic rings, but this is not a requirement. The antagonists bind in an extended conformation with two aromatic rings in a parallel displaced and tilted conformation. The model was able to explain the enantioselectivity of SR48968 and GR159897.     

Mass spectral fragmentation patterns of heterocycles. VII . Reinvestigation of fundamental processes in phenothiazines

Electron impact induced fragmentation patterns of simple phenothiazines have been reinvestigated using metastable ion studies, exact mass measurements and deuterated derivatives. Secondary fragmentation processes involving ions m/e 198, 171, 167, 166, 154, 140 and 139 have been clarified. Mechanisms for the release of sulfur (SH· and CSH·) nitrogen (HCN and H₂CN·) containing fragments from phenothiazine molecular ion are proposed based on the deuterium content of the daughter ions obtained from 1,9-dideutenophenothiazine. A revised mechanism for the expulsion of ketene from 10-acetylphenothiazine is suggested based on the fragmentation pattern of the 1,9-dideuterioderivative. The composition of m/e 140 was determined by high resolution measurement to be C₁₀H₆N and not C₁₁H₇ as previously reported.     

A sensitive and accurate automatic coulometric detector system for enzymatically produced carbon dioxide

A system for the determination of carbon dioxide liberated in an enzyme reactor is described. The properties of the system are illustrated by the determination of urea. Enzymatically generated carbon dioxide is expelled by boiling, dried and determined coulometrically. The lowest concentration of urea which can be determined with a relative standard deviation of 5% is 1 μM. Compared with other methods based on immobilized enzymes for the determination of compounds as carbon dioxide, this sensitivity constitutes an improvement of about two orders of magnitude. Possible application to sea water is discussed.     

Interactions of Humicola insolens cutinase with an anionic surfactant studied by small-angle neutron scattering and isothermal titration calorimetry

The interaction of cutinase from Humicula insolens (HiC) and sodium dodecyl sulfate (SDS) has been investigated by small-angle neutron scattering (SANS) and isothermal titration calorimetry (ITC). The concerted interpretation of structural and thermodynamic information for identical systems proved valuable in attempts to elucidate the complex modes of protein-detergent interaction. Particularly so at the experimental temperature 22 degrees C, where the formation of SDS micelles is athermal (deltaH = 0), and the effects of protein-detergent interactions stand out clearly in the thermograms. It was found that the effect of SDS on cutinase depended strongly on the sample composition. Thus, addition of SDS corresponding to a molar ratio, n(s) = n(SDS)/n(HiC) of about 10, was associated with the formation of HiC/SDS aggregates, which include more than one protein molecule. The SANS results suggested that on the average such adducts contained two HiC, and the ITC traces showed that they form and break down slowly. At slightly higher SDS concentrations (n(s) = 10-25) these "dimers" dissociated, and the protein denatured. The denaturation showed the characteristic positive enthalpy change, but the SDS denatured state of HiC was unusually compact with a radius of gyration close to that of the native conformation. Further titration with SDS was associated with exothermic binding to the denatured protein until the saturation point at about n(s) = 90. At this point, the free monomer concentration was 2.2 mM and the binding number was approximately 40 SDS/HiC. Interestingly, this degree of SDS binding (approximately 0.5 g of SDS/g of HiC) is less than half the amount bound to typical water-soluble proteins.     

Vibrational Analysis of a Molecular Heme-Copper Assembly with a Nearly Linear Fe(III)-CN-Cu(II) Bridge: Insight into Cyanide Binding to Fully Oxidized Cytochrome c Oxidase

The complete vibrational analysis of [(1-MeIm)Fe(OEP)-CN-Cu(Me(6)tren)](2+) (1), which has been constructed as a model for the cyanide-ligated binuclear center in the respiratory protein cytochrome c oxidase, has been carried out. The resonance Raman spectra (lambda(exc) = 647 nm) and the mid-infrared spectra display three cyanide isotope-dependent vibrational modes. Two vibrations showed monotonic decreases with increasing mass of the cyanide ligand (2182-2137-2146-2101 cm(-)(1) and 535-526-526-520 cm(-)(1), respectively, for the (12)C(14)N-(13)C(14)N-(12)C(15)N-(13)C(15)N isotopomers), and could thus be assigned to the C&tbd1;N and Fe-CN-Cu stretching vibrations, respectively. The third vibration, detected with resonance Raman, showed a zigzag-type behavior (495-487-493-485 cm(-)(1) with the set of isotopomers above) with the frequency being more sensitive to (13)C labeling of the cyanide ligand than with (15)N labeling. This pattern of isotopic dependence is characteristic of a bending vibration. Additionally, with the same laser excitation frequency, the C&tbd1;N stretching mode was observed, which is the first time that this vibration has been detected in the resonance Raman spectrum of a synthetic heme-cyanide complex. The normal coordinate analysis showed marked differences between bridged and unbridged heme-cyanide complexes. Internal coordinates that are orthogonal in unbridged systems are significantly mixed in the bridged model, despite the overall linearity of the Fe-CN-Cu moiety. These measurements strengthen the proposal that cyanide bridges the two metal atoms in the cyanide-ligated, oxidized binuclear center of cytochrome c oxidase. A quantitative consideration of the vibrational characteristics of cyanide bound to the resting enzyme, in light of our model compound results, strongly suggests that the binuclear center is flexible and can undergo structural rearrangement to accommodate exogenous ligands. This is likely to be of mechanistic importance in both dioxygen reduction and proton translocation.