Show Abstract ▼

Abstract: 6,6′,13,13′- tetrahydro-6,6′-bipentacene (HBP), the intermediate molecule connecting pentacene to previously observed peripentacene and extended pentacene oligomers through the formation of a carbon-carbon bond, is synthesised and crystallographically characterised. Heating pentacene to 300 ºC under vacuum for 200 hours results in pale golden crystals of HBP and amorphous material containing pentacene oligomers, offering experimental evidence that pentacene preferentially dimerises at the 6,6′- position. Continued heating of HBP results in co-crystals of 6,13-dihydrogenated pentacene (HP) and pentacene and further amorphous pentacene oligomers. The amorphous material consists of layered carbonaceous species with a graphenic nature, as determined by Raman spectroscopy and electron microscopy, and suggests HBP as an intermediate to hydrogenated pentacene species and pentacene oligomers, such as peripentacene, of interest in organic electronics.

Open Access

Show Abstract ▼

Abstract: On cooling to 229 K at ambient pressure, 2,2,2-trifluoroethanol freezes to form a crystal structure characterised by hydrogen-bonded molecular chains in the orthorhombic space group Pca21 (form 1). On compression to 0.22 GPa at room temperature, however, liquid 2,2,2-trifluoroethanol crystallises in a structure adopting triclinic P symmetry (form 2). Although the form 1 and form 2 polymorphs are both characterised by the formation of hydrogen-bonded chains, the arrangement of the molecules within the chains are significantly different with the high-pressure form appearing to adopt a more strained configuration. The orientation of the molecules in form 2 is such that the hydrogen bonds in neighbouring chains are in close proximity with a bridging interchain O⋯O contact distance that is of the same order as the O⋯O distances found for the hydrogen bonds. The resulting close pairing of neighbouring catemeric chains produces molecular pillars in the structure of form 2.

Show Abstract ▼

Abstract: Functionalization of graphene on Ir(111) is a promising route to modify graphene by chemical means in a controlled fashion at the nanoscale. Yet, the nature of such functionalized sp3 nanodots remains unknown. Density functional theory (DFT) calculations alone cannot differentiate between two plausible structures, namely true graphane and substrate stabilized graphane-like nanodots. These two structures, however, interact dramatically differently with the underlying substrate. Discriminating which type of nanodots forms on the surface is thus of paramount importance for the applications of such prepared nanostructures. By comparing X-ray standing wave measurements against theoretical model structures obtained by DFT calculations we are able to exclude the formation of true graphane nanodots and clearly show the formation graphane-like nanodots.

Open Access

Show Abstract ▼

Abstract: Coagulation factor XII (FXII) is a key initiator of the contact pathway, which contributes to inflammatory pathways. FXII circulates as a zymogen, which when auto-activated forms factor XIIa (FXIIa). Here, the production of the recombinant FXIIa protease domain (βFXIIaHis) with yields of ∼1–2 mg per litre of insect-cell culture is reported. A second construct utilized an N-terminal maltose-binding protein (MBP) fusion (MBP-βFXIIaHis). Crystal structures were determined of MBP-βFXIIaHis in complex with the inhibitor D-Phe-Pro-Arg chloromethyl ketone (PPACK) and of βFXIIaHis in isolation. The βFXIIaHis structure revealed that the S2 and S1 pockets were occupied by Thr and Arg residues, respectively, from an adjacent molecule in the crystal. The Thr-Arg sequence mimics the P2–P1 FXIIa cleavage-site residues present in the natural substrates prekallikrein and FXII, and Pro-Arg (from PPACK) mimics the factor XI cleavage site. A comparison of the βFXIIaHis structure with the available crystal structure of the zymogen-like FXII protease revealed large conformational changes centred around the S1 pocket and an alternate conformation for the 99-loop, Tyr99 and the S2 pocket. Further comparison with activated protease structures of factors IXa and Xa, which also have the Tyr99 residue, reveals that a more open form of the S2 pocket only occurs in the presence of a substrate mimetic. The FXIIa inhibitors EcTI and infestin-4 have Pro-Arg and Phe-Arg P2–P1 sequences, respectively, and the interactions that these inhibitors make with βFXIIa are also described. These structural studies of βFXIIa provide insight into substrate and inhibitor recognition and establish a scaffold for the structure-guided drug design of novel antithrombotic and anti-inflammatory agents.

Show Abstract ▼

Abstract: In this contribution, we introduce a simple approach to quickly estimate the environment-induced crack velocity (CV) as a function of the calculated applied stress intensity factor (K) developed during the slow strain rate testing of aluminum alloys exposed to aqueous or humid air-type environments. The CV-K behavior for a commercial aluminum-magnesium alloy, AA5083-H131, sensitized and pre-exposed to a 0.6 m NaCl solution has been estimated from slow strain rate test data. The predicted threshold K and crack velocities match recently published data for the same alloy in similarly sensitized conditions where the CV-K data were obtained using state-of-the-art fracture mechanics-based testing.