Md-simulation of α-keratin intermediate filaments

In recent years powerful computer systems have become readily accessible to simulate complex chemical problems. Based on the primary structure of the intermediate filament monomer unit of wool, small sequences are selected. Their molecular dynamic behaviour is simulated, in order to investigate the secondary and tertiary structure as well as their stability. The simulations are carried out for a helical segment and a linker segment, selecting the ideal α-helix as start conformation. In vacuum all simulations show an unstable α-helix due to shifts of the intrahelical hydrogen bonds. So a new helical structure with a larger helix diameter is formed. However in simulations with surrounding water the α-helix remains stable throughout the simulation time. Up to now it has not been possible to dectect any fundamental difference in the molecular dynamic behaviour of the helical and the linker segment.     

Modeling of the transition temperature for the helical denaturation of α-keratin intermediate filaments

Simulations of the stability of the secondary and tertiary structure of the α-keratin intermediate filament (IF) monomeric unit of wool are reported. Based on the assumed secondary structure three segments of the primary structure were selected: 1A, L12, and a part of 2B. Starting with an ideal α-helical conformation for each IF-segment, molecular dynamics simulations were carried out on the atomistic level at various temperatures in vaccum using the CFF91 force field. In either simulation the expected destabilization of the helical structure with increasing simulation temperature was observed. By use of different procedures of analysis, transition temperatures for the α-helical denaturation were determined that are significantly higher for the supposedly α-helical segments 1A and 2B than for the linker segment L12. The different stabilities of segments 1A and L12 were further verified through simulations in water environment that show the linker segment to be non-helical at room temperature. The lower transition temperature of segment L12 confirms the expectation that its amino acid sequence leads to increased conformational flexibility. The mobility of the water molecules surrounding the IF-segment is found to be significantly decreased by protein/water interactions.     

The x-ray crystal and molecular structure of chloro-α,β,γ,δ-tetraphenylporphinatopyridinegallium(III)

Crystals of the title compound C₄₉H₃₃N₅ClGa·1/2C₅H₅N·1/2C₅H₁₂ [Ga(py)(Cl)(TPP)]·1/2(py)·1/2(n-pen) are monoclinic, P2₁/n, a = 13.162(2), b = 23.422(6), c, = 14.677(2) Å, β = 101.47(1)°, and Z = 4. The crystal structure refined to R = 0.056 for 2249 observed reflections. The coordination polyhedron of the gallium atom is an octahedron, and the distances between the central metal and axial ligands are Ga-Cl = 2.328(1) and Ga-py = 2.274(3) Å. The gallium atom is displaced slightly out of the porphyrin plane towards Cl, 0.14 Å from the 4N plane and 0.16 Å from the mean porphinato plane, with an average Ga-N distance of 2.01 Å. Although the complex is isostructural with the Mn and Co analogs, it is the first reported structure of a monomeric hexacoordinate gallium(III) porphyrin.     

Crystal and molecular structure of 3-hydroxy-5-phenylisoxazole (α form)

The crystal structure of the α form of 3-hydroxy-5-phenylisoxazole has been determined from three-dimensional X-ray data. The crystals are monoclinic, space group P2₁/n with four molecules in the unit cell. The cell dimensions are: a = 10.41(2); b = 3.89(1); c = 19.14(4) Å; β = 96.1° (3). The structure was refined by the block-diagonal least-squares method to a final R value of 0.080 for 1111 observed reflections. The molecular dimensions are in good overall agreement with those in β form, while the packing is markedly different. Conjugation and planarity in the molecule are discussed.     

A novel synthesis of α-hydroxy- and α,α′-dihydroxyketone from α-iodo and α,α′-diiodo ketone using photoirradiation

A novel reaction of α-iodo ketone (α-iodocycloalkanone, α-iodo-β-alkoxy ester, and α-iodoacyclicketone) with irradiation under a high-pressure mercury lamp gave the corresponding α-hydroxyketone in good yields. In the case of α,α′-diiodo ketone, α,α′-dihydroxyketone which little has been reported until now was obtained. This reaction affords a new, clean and convenient synthetic method for α-hydroxy- and α,α′-dihydroxyketone.     

Ring-opening polymerization of α,β-disubstituted oxiranes by α-methoxyphenylmethylium hexachloroantimonate

α-Methoxyphenylmethylium hexachloroantimonate was used as a novel initiator for the polymerization of α,β-disubstituted oxiranes such as cyclohexene oxide (CHO) and 2-butene oxide (trans and cis) (2-BO) at ?78°C with dichloromethane or dichloromethane-toluene mixtures as solvents. The CHO polymerization mixture became turbid and the polymer precipitated in dichloromethane. The CHO polymerization proceed quantitatively in dichloromethane–toluene mixtures. The molecular weight distribution of polyCHO obtained was bimodal regardless of the solvent used. The polymerization of trans-2-BO was heterogeneous in both dichloromethane and dichloromethane–toluene mixture. The polymerization mixtures of cis-2-BO were transparent but reached a limit yield which was less than the polymer yield of trans-2-BO. Furthermore, the microstructure of the poly2-BOs were analyzed by Vandenberg's method and the results confirmed Vandenberg's finding that inversion of configuration occurs in the propagation step.     

Microwave-assisted synthesis of α-hydroxy ketone and α-diketone and pyrazine derivatives from α-halo and α,α′-dibromo ketone

A novel reaction of α-halo ketone (α-bromo and α-chloro ketone) with irradiation under microwave gave the corresponding α-hydroxyketone and pyrazine derivative in good yields. In the case of α,α′-dibromo ketone, α-diketone was obtained. This reaction affords a new, clean and convenient synthetic method for α-hydroxyketone, α-diketone, α-chloro ketone and pyrazine derivative.     

Studies in organic mass spectrometry-XIII: Investigation of electron-induced isomerisation of αβ- and βγ-unsaturated esters

The mass spectral fragmentation of methyl esters of α,β- and β,γ-unsaturated carboxylic acids has been studied. Deuterium labelling, metastable ion analysis and high resolution mass spectrometry have been utilised to elucidate the mechanism of a number of fragmentations and to check possible double bond migration prior to fragmentation. Some breakdown modes were found to occur through double bond migration. Additional support was provided by application of the ‘Metastable Ion Characteristics’ method. Partial isomerisation of molecular ions must therefore be accepted.