A new incorporation mechanism for trivalent actinides into bioapatite: a TRLFS and EXAFS study

One of the most toxic byproducts of nuclear power and weapons production is the transuranics, which have a high radiotoxicity and long biological half-life due to their tendency to accumulate in the skeletal system. This accumulation is inhomogeneous and has been associated with the chemical properties and structure of the bone material rather than its location or function. This suggests a chemical driving force to incorporation and requires an atomic scale mechanistic understanding of the incorporation process. Here we propose a new incorporation mechanism for trivalent actinides and lanthanides into synthetic and biologically produced hydroxyapatite. Time-resolved laser fluorescence spectroscopy and extended X-ray absorption fine structure have been used to demonstrate that trivalent actinides and lanthanides incorporate into the amorphous grain boundaries of apatite. This incorporation site can be used to explain patterns in uptake and distribution of radionuclides in the mammalian skeletal system.     

InAs nanowire growth on oxide-masked 〈111〉 silicon

Here we investigate the growth of InAs nanowires on 〈111〉 Si substrates masked by SiO_x using metal–organic chemical vapor deposition. We study 〈111〉 (axial) and 〈1?10〉 (radial) growth of InAs NWs by varying growth duration, temperature, group-III molar flows, V/III ratio, mask material, mask opening size, and inter-wire distance. We find that growth takes place without an In droplet and the process evolves through three successive phases: nucleation of an InAs cluster, followed by two distinct nanowire growth phases. These two growth phases have different axial and radial growth rates, which originate in a transition from having In supply dominated by the open Si area in the first phase towards an In supply from the vapor/oxide mask in the second growth phase. The linear relation found between nanowire length and diameter vs. time in the last growth phase indicates that 〈111〉 growth is not surface diffusion limited as is usually the case for catalyzed growth. A high yield of vertical nanowires is obtained if group-III flow is above and V/III ratio below threshold values, in addition to having an arsenic-terminated Si surface. Furthermore, we observe that 〈111〉 and 〈1?10〉 growth is surface kinetically limited below 520 °C and 540 °C, respectively, with activation energies of 20 and 6.5 kcal/mol. This difference in activation energies limits the selectivity of the 〈111〉 to 〈1?10〉 growth to 25:1 under optimized conditions, which must be considered when fabricating axially modulated structures. However, we find that by placing wires in large arrays it is possible to completely stop the 〈1?10〉 growth rate in favor of the 〈111〉 growth rate.     

Every scattered space is subcompact

We prove that every scattered space is hereditarily subcompact and any finite union of subcompact spaces is subcompact. It is a long-standing open problem whether every ?ech-complete space is subcompact. Moreover, it is not even known whether the complement of every countable subset of a compact space is subcompact. We prove that this is the case for linearly ordered compact spaces as well as for ω  -monolithic compact spaces. We also establish a general result for Tychonoff products of discrete spaces which implies that dense _Gδ$G_{\delta }$-subsets of Cantor cubes are subcompact.     

Temperature dependent solid-state proton migration in dimethylurea-oxalic acid complexes

The phenomenon of solid-state proton migration within molecular complexes containing short hydrogen bonds is investigated in two dimethylurea-oxalic acid complexes. Extensive characterisation by both X-ray and neutron diffraction shows that proton migration along the hydrogen bond can be induced in these complexes as a function of temperature. This emphasises the subtle features of the hydrogen bond potential well in such short hydrogen bonded complexes, both intrinsically and in the effect of the local crystalline environment. Based on these findings, the synthesis and analysis of a series of solid-state molecular complexes is shown to be a potential route to designing materials with tuneable proton migration effects.     

Stereoselective synthesis of polyhydroxylated aminocyclohexanes

The stereoselective synthesis of a series of di- and tri-hydroxylated aminocyclohexane derivatives has been developed. A one-pot, two step tandem process involving an Overman rearrangement and a ring closing metathesis reaction has been utilised for the asymmetric synthesis of (1S)-1-(2',2',2'-trichloromethylcarbonylamino)cyclohexa-2-ene. Oxidation of this cyclohexene derivative was then studied leading to the preparation of two diol analogues in excellent stereoselectivity. (1S)-1-(2',2',2'-trichloromethylcarbonylamino)cyclohexa-2-ene was then converted to a novel allylic alcohol via a 4,5-dihydro-1,3-oxazole. Functionalisation of this allylic alcohol by Upjohn dihydroxylation conditions or by a directed epoxidation/hydrolysis sequence of reactions allowed the synthesis of two dihydroconduramines in excellent stereoselectivity. The stereochemical assignment of all compounds prepared was confirmed by NOE experiments or X-ray structure determination.     

Concise synthesis of the C-1-C-12 fragment of amphidinolides T1-T5

The C-1-C-12 segment of the amphidinolides T1-T5 has been synthesised in an efficient manner. The key transformations are highly diastereoselective rearrangement of an oxonium ylide, or metal-bound ylide equivalent, produced by intramolecular reaction of a copper carbenoid with an allylic ether, and macrocyclic fragment coupling by one-pot ring-closing metathesis and hydrogenation.     

A one-pot, reductive amination/6-endo-trig cyclisation for the stereoselective synthesis of 6-substituted-4-oxopipecolic acids

The first stereoselective synthesis of 2,6-trans-6-substituted-4-oxo-L-pipecolic acids using a tandem reductive amination/6-endo-trig cyclisation process is described. The sequential reduction and cyclisation mediated by sodium cyanoborohydride allowed the preparation of a series of highly functionalised 6-alkyl and 6-aryl analogues.     

Synthesis of a novel heptacoordinated Fe(III) dinuclear complex: experimental and theoretical study of the magnetic properties

A new functionalized bis-pyrazol-pyridine ligand has been prepared by reaction with hydrazine of the corresponding bis-β-diketone precursor, also unprecedented. The aerobic reaction of this ligand with ferrous thiocyanate in the presence of ascorbic or oxalic acid affords the dinuclear complex of seven-coordinate Fe(III), [Fe?(H?L2)?(ox)(NCS)?] (1), as revealed by single crystal X-ray diffraction. This may represent an entry into a new family of [Fe?] compounds with heptacoordinate metal centres. The capacity of this unusual chromophore to undergo magnetic super-exchange was investigated by means of bulk magnetization and DFT calculations. Both approaches confirmed the presence of antiferromagnetic interactions within the molecule. The theoretical investigation has served to describe the magnetic orbitals of Fe(III) in this unusual coordination geometry, as well as the exchange mechanism. A brief review of the scarce number of iron heptacoordinate complexes reported in the literature is also included and discussed.     

A new method for the covalent attachment of DNA to a surface for single-molecule studies

Attachments between DNA and a surface or bead are often necessary for single-molecule studies of DNA and DNA-protein interactions. In single-molecule mechanical studies using optical or magnetic tweezers, such attachments must be able to withstand the applied forces. Here we present a new method for covalently attaching DNA to a glass surface, which uses N-hydroxysuccinimide (NHS) modified PEG that is suitable for high-force single-molecule mechanical studies. A glass surface is coated with silane-PEG-NHS and DNA is covalently linked through a reaction between the NHS group and an amine modified nucleotide that has been incorporated into the DNA. After DNA attachment, non-reacted NHS groups are hydrolyzed leaving a PEG-covered surface which has the added benefit of reducing non-specific surface interactions. This method permits specific binding of the DNA to the surface through a covalent bond. At the DNA end not attached to the surface, we attach a streptavidin-coated polystyrene bead and measure force-versus-extension using an optical trap. We show that our method allows a tethered DNA molecule to be pulled through its overstretching transition (> 60pN) multiple times. We anticipate this simple yet powerful method will be useful for many researchers.