The Structure of Gas Phase Tin Oxide Ions Generated by Laser Ablation: A Combined Fourier Transform Mass Spectrometry and Density Functional Theory Study

The radical ion series (SnO)⁺ _2-6, (SnO)^– _2-6, (SnO)_0-5Sn⁺ and (SnO)_1-6O^– have been generated by the high power laser ablation of SnO and SnO₂ targets positioned inside an ICR cell. In all ablation spectra obtained, and for any particular size Sn _x core, the tin-rich clusters (SnO) _x Sn⁺ were more abundant than the corresponding oxygen-equivalent clusters (SnO)⁺ _x , while the oxygen-rich clusters (SnO) _x O^– were always more abundant than the oxygen-equivalent clusters (SnO)^– _x . High yields of the ions (SnO)_1,3Sn⁺, (SnO)_3,6O^– and (SnO)^– ₆ suggest high stabilities for these species. Low energy CID studies revealed that loss of neutral (SnO) _x units is the preferred, and for most ions investigated the exclusive, dissociation pathway. Global minima for the smaller cations and anions are proposed on the basis of local density functional theory (DFT) calculations. Calculated dissociation energies for the neutral and charged clusters were found to compare well with effusion cell and FTICR results. DFT also predicts that, for any cluster with the same size Sn _x core, IE(SnO) _x x Sn and EA(SnO) _x O>EA(SnO) _x . A correlation between ion abundances and DFT heats of formation is evident, and the ground state geometries provide insight into the evolution of structural versus size trends. Without assistance from the calculations, erroneous conclusions regarding the structures of the experimentally-sampled clusters might have been drawn from the low energy CID results.     

Immobilization of several multienzyme systems on porous glass beads

Equimolar concentrations of malate dehydrogenase (EC 1.1.1.37) and fumarase (EC 4.2.1.2) and equimolar concentrations of malate dehydrogenase and citrate synthase (EC 4.1.3.7) were simultaneously immobilized to alkylamine porous silica beads with gluteraldehyde. The activity of each enzyme in the two-enzyme immobilized systems was determined and exact concentrations of the free nonimmobilized enzymes were prepared. The activities of the coupled free and coupled immobilized systems were measured, and it was observed that there was a 10-fold enhancement in the catalysis of the immobilized enzymes.     

Role of measurement uncertainty in conformity assessment in legal metrology and trade

There is increasing interest in the role of measurement uncertainty in conformity assessment, particularly where measurements are in support of fair trade, health & safety and the environment. A lack of clarity, however, in treating measurement uncertainties and in setting specification limits for compliance may sometimes lead to drastic consequences. Examples concerning conformity assessment, particularly in the measurement of trace quantities (Cd in plastics and antibiotic trace residues in shrimps), are amongst cases identified in the ongoing EU project MetroTrade, which can be significant impediments to international trade. This paper considers the present status of the role of measurement uncertainty in conformity testing, covering both recent work in international standardisation and physical metrology (e.g. OIML recommendations on weighing). One conclusion is that, while much has been achieved, there still exists some work to be done in order to facilitate acceptance of conformity from both customer and supplier and avoid perceived barriers to trade and serious international trade disputes.Presented at the MetroTrade Workshop on Traceability and Measurement Uncertainty in Testing, 30–31 January 2003, Berlin, GermanyAn erratum to this article can be found at     

Ab-Initio MRD-CI calculations for breaking a chemical bond in a molecule in a crystal or other solid environment. II.H3C—NO2 decomposition of nitromethane in a nitromethane crystal with voids

Recently we extended our strategy for MRD-CI (multireference double excitation-configuration interaction) calculations based on localized/local orbitals and an “effective” CI Hamiltonian for molecular decompositions of large molecules to breaking a chemical bond in a molecule in a crystal or other solid environment. Our technique involves solving a quantum chemical ab-initio SCF explicitly for a system of a reference molecule surrounded by a number of other molecules in the multipole environment of more distant neighbors. The resulting canonical molecular orbitals are then localized and the localized occupied and virtual orbitals in the region of interest are included explicitly in the MRD-CI with the remainder of the occupied localized orbitals being folded into an “effective” CI Hamiltonian. The MRD-CI calculations are carried out for breaking a bond in the reference molecule. This method is completely general. The space treated explicitly quantum chemically and the surrounding space can have voids, defects, deformations, dislocations, impurities, dopants, edges and surfaces, boundaries, etc. We previously applied this procedure successfully to the H₃C? NO₂ bond dissociation of nitromethane in a nitromethane crystal with extensive testing of the number of molecules that have to be included explicitly in the SCF and how many molecules have to be represented by more distant multipoles. The results indicated that it took more energy to dissociate the H₃C? NO₂ bond when the nitromethane molecule was in the crystal than it did to dissociate that bond in the free nitromethane molecule. In this present study we have investigated the effect of voids (both in the nitromethane molecules treated explicitly in the SCF and those in the environment represented by multipoles) on the calculated H₃C? NO₂ bond dissociation energies.     

Glycosyl disulfides: novel glycosylating reagents with flexible aglycon alteration

[reaction: see text] Glycosyl disulfides have been shown for the first time to be effective glycosyl donors. Glucosylation and galactosylation of a panel of representative alcohol acceptors allowed the formation of 28 simple glycosides, disaccharides, and glycoamino acids in yields of up to 90%. As well as providing a novel class of effective glycosyl donors, the ability to easily alter the nature of the aglycon and the ability to differently activate donors that differ only in their aglycon simply through altering conditions lends glycosyl disulfide donors to their use in latent-active reactivity tuning strategies.     

Ab initio and molecular mechanics calculations on imine derivatives: A study of the rotational barriers and the development of MM2 parameters

The application of molecular mechanics methods for the study of structures has become a standard approach to conformational analysis. The MM2 force field has been extended to include imines. In general, a diverse group of aliphatic and aromatic imine structures can now be treated. The rotational energy profiles and barriers to N-substituted imines about the C_sp2? C_sp2 single bonds adjacent to C?N functional group were calculated via ab initio MO theory. Information obtained from the quantum mechanical calculations at the 3-21G, 6-31G*, and MP4/6-31G* //6-31G* levels was used both to study the phenomena involved and to parameterize MM2. The syn-anti isomerization was also studied, and the mechanistic pathways have been evaluated. In cases where the comparison with experimental data can be made, the agreements are good.     

New peptidomimetic polymers for antifouling surfaces

Exposure of therapeutic and diagnostic medical devices to biological fluids is often accompanied by interfacial adsorption of proteins, cells, and microorganisms. Biofouling of surfaces can lead to compromised device performance or increased cost and in some cases may be life-threatening to the patient. Although numerous antifouling polymer coatings have enjoyed short-term success in preventing protein and cell adsorption on surfaces, none have proven ideal for conferring long-term biofouling resistance. Here we describe a new biomimetic antifouling N-substituted glycine polymer (peptoid) containing a C-terminal peptide anchor derived from residues found in mussel adhesive proteins for robust attachment of the polymer onto surfaces. The methoxyethyl side chain of the peptoid portion of the polymer was chosen for its chemical resemblance to the repeat unit of the known antifouling polymer poly(ethylene glycol) (PEG), whereas the composition of the 5-mer anchoring peptide was chosen to directly mimic the DOPA- and Lys-rich sequence of a known mussel adhesive protein. Surfaces modified with this biomimetic peptide-peptoid conjugate exhibited dramatic reduction of serum protein adsorption and resistance to mammalian cell attachment for over 5 months in an in vitro assay. These new synthetic peptide based antifouling polymers may provide long-term control of surface biofouling in the physiologic, marine, and industrial environments.     

Suicide Inactivation of Dioldehydrase by 2‐Chloroacetaldehyde: Formation of the ‘cis‐Ethanesemidione’ Radical,and the Role of a Monovalent Cation

Dioldehydrase is an adenosylcobalamin‐dependent enzyme that catalyzes the dehydration of (R)‐ or (S)‐propane‐1,2‐diol to propanal. The reaction proceeds by a radical mechanism initiated by the homolytic scission of the covalent Co? C(5′) bond in the coenzyme to form cob(II)alamin and the 5‐deoxyadenosyl radical as transient intermediates. Dioldehydrase is subject to ‘suicide inactivation’ by substrate/product analogs. Inactivation by 2‐chloroacetaldehyde converts the inactivator into the ‘cis‐ethanesemidione’ radical. A mechanism for this process includes reaction of chloroacetaldehyde in the reverse of the normal catalytic process to a rearranged radical that eliminates HCl. K⁺ and other monovalent cations of similar size, including Tl⁺, are required for dioldehydrase activity and for suicide inactivation by glycolaldehyde or 2‐chloroacetaldehyde. A K⁺ ion is bound to propane‐1,2‐diol in dioldehydrase. Both EPR and pulsed‐EPR experiments show that the magnetic nuclei of thallous ions (²⁰³Tl⁺, ²⁰⁵Tl⁺) do not interact with the unpaired electron in the cis‐ethanesemidione radical at the active site of dioldehydrase. Pulsed‐EPR experiments implicate a ¹⁴NH group, possibly of His¹⁴³, interacting with the radical at the active site.     

Total internal reflection Raman spectroscopy of barley leaf epicuticular waxes in vivo

The use of total internal reflection (TIR)-Raman spectroscopy to examine cuticular wax layers in vivo at the surface of barley leaves (Hordeum vulgare L. cultivar Pastoral) is demonstrated. The limited penetration depth (40 nm) of the evanescent wave compared to the thickness of the wax layer eliminates problems arising from fluorescence from underlying pigments. Epicuticular wax crystals are observed to be more crystalline than the amorphous wax layer, which is analysed after removal of the wax crystals by cellulose acetate stripping. Carotenoids are detected as trace species in the cuticular waxes.     

Characterization of polycyclic aromatic hydrocarbons in shale oil by chromatography and fluorescence densitometry

Polycyclic aromatic hydrocarbons (6-, 5-, 4- and 3-ring) were characterized in several shale oil samples. Open-column, dry-column and thin-layer chromatography were used in the separation steps. Visible fluorescence from the separated components on chromatoplates was measured directly to obtain fluorescence profiles of the distribution on the chromatoplates. The methods developed are inexpensive, reproducible, and allow rapid characterization of polycyclic aromatic hydrocarbons in shale oil samples.