Dislocation-phonon interaction

The recent result that uniform screw dislocation motion can be sustained by an applied stress s < 10^?5 initiates a study of the influence of intrinsic, low energy atomic vibrations on the dislocation. This dislocationphonon interaction is examined for two types of interatomic potentials in a crystal with an anisotropic slip system. For realistic interatomic potentials, we find that, in the presence of an appropriate phonon, uniform motion can occur at velocities v?0.50 (in units of the speed of sound) with zero externally applied stress, or even against the direction of motion favored by such a stress.     

On the theory of nuclear heavy-ion direct transfer reactions

We review the distorted-wave approach to direct transfer reactions and draw attention to some of the shortcomings of current theories. We show that a reformulated form of the distorted-wave Born approximation (DWBA) for transfer can lead to important simplifications of the theory, which are valid for nuclear heavy-ion induced reactions at energies ? 10 MeV/nucleon. In particular, in the semiclassical limit, it leads to a new and simple formula for the transfer t-matrix which includes all the essential physics while offering several important advantages over standard “full-recoil finite-range” DWBA. One such advantage is that the new formula is more transparent in that it is amenable to interpretation and analytical manipulation. At high-energy it is shown to reduce to one earlier deduced using eikonal-DWBA.The conditions for the validity of the new theory are discussed in detail. They are shown to be generally well satisfied for small-mass transfer between heavy-ions at energies at or above those which particularly favour transfer (? 10 MeV/nucleon for transfer of valence nucleons). The restriction to small mass is not due to any recoil approximation; in fact, it is only a necessary restriction at certain energies. The theory treats recoil exactly.Consideration of the optimum dynamical conditions for transfer leads to a set of matching conditions. The presence of hitherto neglected absorption, arising from dynamical effects of poor matching, is suggested and qualitatively discussed. Conditions under which such absorption may be neglected are derived. Results of numerical calculations are presented showing that the theory is capable of good agreement with standard full-recoil finite-range DWBA, and that it is capable of giving at least as good an account of experimental data for nucleon-transfer between heavy-ions at energies ～10 MeV/nucleon.     

Molecular recognition. Self-assembly of molecular trigonal prisms and their host-guest adducts

Two supramolecular trigonal prisms, each bearing three molecular clefts are shown to form 1:6 and 1:7 host-guest complexes with 9-methylanthracene and one of the prisms forms a 1:2 host-guest complex with a tritopic tri-anthracene guest that registers with the recognition sites of the host.     

Semiclassical treatment of quadrupole shape effects in the elastic scattering of heavy ions below the Coulomb barrier

A perturbative semiclassical approximation for the elastics-matrix is used to derive simple and accurate formulae describing the effects of a nuclear quadrupole deformation on the elastic scattering of aligned nuclei. Expressions are derived for the second rank tensor components of the analysing power, the ratios of which turn out to be simple trigonometric functions of the scattering angle in agreement with experimental observations. The approximations are discussed in some detail and higher order corrections are derived. In an appendix we derive a semiclassical first-order perturbation formula describing the effect of a non-central complex perturbation in the presence of a non-perturbatively treated central term. Our formula is in disagreement with some earlier published formulae which fail to treat the real part of the perturbation correctly.     

Optically non-linear organic materials

A large number of non-linear organic materials have been investigated by second-harmonic generation (SHG) in powders. The materials studied include several urea derivatives, sterically perturbed anilines and pyridines, and substituted stilbenes. Several compounds have been found with both a very large second-order non-linearity and a high optical damage threshold. Many compounds, even though known to possess large molecular hyperpolarizabilities, showed no detectable SHG, indicating a centrosymmetric crystal point group. The results are interpreted in terms of bond configuration, charge transfer and crystal structure. The use of good non-linear organic materials in integrated non-linear optics is discussed.     

Partitioning the loss in vancomycin binding affinity for D-Ala-D-Lac into lost H-bond and repulsive lone pair contributions

The binding affinity of 4, which incorporates a methylene (CH2) in place of the key linking amide of Ac2-l-Lys-d-Ala-d-Ala, for vancomycin was compared with that of Ac2-l-Lys-d-Ala-d-Ala (3) and Ac2-l-Lys-d-Ala-d-Lac (5). The vancomycin affinity for 4 was approximately 10-fold less than that of 3, but 100-fold greater than that of 5. This suggests that the reduced binding affinity of 5 (4.1 kcal/mol) may be attributed to both the loss of a key H-bond (1.5 kcal/mol) and a destabilizing lone pair/lone pair electrostatic interaction introduced with the ester oxygen of 5 (2.6 kcal/mol) with the latter, not the H-bond, being responsible for the largest share of the 1000-fold reduction.     

Increasing the sensitivity of Listeria monocytogenes assays: evaluation using ELISA and amperometric detection

An immunosensor for the detection of Listeria monocytogenes was developed. ELISA and amperometric studies were run in parallel to develop a more sensitive and rapid assay for the bacterium. Conditions for the immunosensor were primarily characterised using ELISA. A direct sandwich assay was employed and the affinities of two polyclonal (goat and rabbit) and one monoclonal (mouse) anti-L. monocytogenes antibodies were compared using this format. Owing to low sensitivity being obtained with all antibodies, biotin-avidin amplification and an indirect sandwich assay were employed. The system was then transferred to screen-printed electrodes (SPEs), the primary antibody being immobilised by cross-linking with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide, and the mode of detection being amperometric. Various parameters (limit of detection, working range, incubation time, cross-reactivity) of the systems were characterised. The effect of direct incubation in milk is also discussed. The final immunosensor had a working range of 1 x 10(6)-1 x 10(3) cells ml-1 and a detection limit of 9 x 10(2) cells ml-1. The assay took about 3.5 h to complete.     

Liquid chromatography with electrospray ion-trap mass spectrometry for the determination of anatoxins in cyanobacteria and drinking water

Anatoxin-a (AN) and homoanatoxin-a (HMAN) are potent neurotoxins produced by a number of cyanobacterial species. A new, sensitive liquid chromatography/multiple tandem mass spectrometry (LC/MS(n)) method has been developed for the determination of these neurotoxins. The LC system was coupled, via an electrospray ionisation (ESI) source, to an ion-trap mass spectrometer in positive ion mode. The [M+H](+) ions at m/z 166 (anatoxin-a) and m/z 180 (homoanatoxin-a) were used as the precursor ions for multiple MS experiments. MS(2)bond;MS(4) spectra displayed major fragment ions at m/z 149 (AN), 163 (HMAN), assigned to [Mbond;NH(3)+H](+); m/z 131 (AN), 145 (HMAN), assigned to [Mbond;NH(3)bond;H(2)O+H](+), and m/z 91 [C(7)H(7)](+). Although the chromatographic separation of these neurotoxins is problematic, reversed-phase LC, using a C(18) Luna column, proved successful. Calibration data for anatoxin-a using spiked water samples (10 mL) in LC/MS(n) modes were: LC/MS (25-1000 microg/L), r(2) = 0.998; LC/MS(2) (5-1000(microg/L), r(2) = 0.9993; LC/MS(3) (2.5-1000 microg/L), r(2) = 0.9997. Reproducibility data (% RSD, N = 3) for each LC/MS(n) mode ranged between 2.0 at 500 microg/L and 7.0 at 10 microg/L. The detection limit (S/N = 3) for AN was better than 0.03 ng (on-column) for LC/MS(3) which corresponded to 0.6 microg/L.     

The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO₂3Al₂O₃1.5P₂O₅3CaO2CaF₂, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca₅(PO₄)₃F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The ²⁹Si, ²⁷Al and ³¹P NMR spectra for the glasses with different strontium contents were identical. The ¹⁹F spectra indicated the presence of F-Ca(n) and Al-F-Ca(n) species in the calcium glasses and in the strontium glasses F-Sr(n) and Al-F-Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium-strontium apatite and the fully strontium substituted glass to strontium fluorapatite.     

Silicone-protein surfactants; stability of water-in-silicone oil emulsions

Small scale water-in-silicone oil emulsions were readily prepared using high speed mixers. Two surfactant systems were studied: a comb-type silicone-polyether surfactant, and a surfactant system employing a mixture of the surface active protein human serum albumin (HSA, in the internal phase) and an alkoxysilane-modified silicone TES-PDMS in the silicone oil (continuous phase). Little difference in particle sizes was noted between the two surfactant types for a given mixing protocol, but dual-blade turbulent mixing led to relatively monodisperse particles of approximately 2–5 m in diameter while high speed Dremel mixers led to bimodal particle distributions. Prior to spontaneous demulsification of the latter emulsions stabilized by HSA/TES-PDMS (the 3225C emulsions remain stable), they proved very difficult to break. The addition of dibutyltin dilaurate to the HSA/TES-PDMS-stabilized emulsions led to catastrophic collapse of the emulsion and formation of a silicone elastomer at the bulk water/oil interface. This makes unlikely the possibility that silicone elastomers, formed by protein-catalyzed crosslinking of the alkoxysilane in albumin/TES-PDMS-stabilized emulsions, are involved in stabilizing the emulsion. The nature of the stabilization of the interface is discussed.