Search PubMed⌕ Search

Biomedical subjects

J A Howard

Publications and source records attributed to J A Howard.

At least 37 records · Page 2Linked to original sources
PubMed2001

New molecular complexes of fullerenes C60 and C70 with tetraphenylporphyrins [M(tpp)], in which M=H2, Mn, Co, Cu, Zn, and FeCl.

New molecular complexes of fullerenes C60 and C70 with tetraphenylporphyrins [M(tpp)] in which M-H2, MnII, CoII, CuII, ZnII and Fe(III)Cl, have been synthesised. Crystal structures of two C60 complexes with H2TPP, which differ only in the number of benzene solvated molecules, and C60 and C70 complexes with [Cu(tpp)] have been studied. The fullerene molecules form a honeycomb motif in H2TPP.2C60. 3C6H6, puckered graphite-like layers in H2TPP.2C60.4C6H6, zigzag chains in [Cu(tpp)].C70.1.5C7H8.0.5C2HCl3 and columns in [Cu(tpp)]2.C60. H2TPP has van der Waals contacts with C60 through nitrogen atoms and phenyl groups. Copper atoms of the [Cu(tpp)] molecules are weakly coordinated with C70, but form no shortened contacts with C60. The formation of molecular complexes with fullerenes affects the ESR spectra of [M(tpp)] (M = Mn, Co and Cu). [Mn(tpp)] in the complex with C70 lowers its spin state from S = 5/2 to S = 1/2, whereas [Co(tpp)] and [Cu(tpp)] change the constants of hyperfine interaction. ESR, IR, UV-visible and X-ray photoelectron spectroscopic data show no noticeable charge transfer from the porphyrinate to the fullerene molecules.

Journal Article↗
PubMed2001

Influence of hydrogen bonding on the second harmonic generation effect: neutron diffraction study of 4-nitro-4'-methylbenzylidene aniline.

A neutron diffraction study of the non-linear optical (NLO) material 4-nitro-4'-methylbenzylidene aniline (NMBA) is presented. NMBA exhibits a large macroscopic second-order NLO susceptibility, chi((2)), and this study shows that hydrogen bonding is, in part, responsible for this. No hydrogen bonding was reported in the X-ray study [Ponomarev et al. (1977). Sov. Phys. Crystallogr. 22, 223-225], whereas the present work shows that C-H.X hydrogen bonds (where X = N, O or pi) direct the nature of the three-dimensional lattice. C-H.X (X = N or O) hydrogen bonds are common; however, C-H.pi hydrogen-bond motifs are relatively rare. Such intermolecular interactions help extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Molecular planarity, coupled with the favourable nature of the hydrogen bonds, results in parallel stacking of molecules in both the a and c crystallographic directions with extremely close interplanar spacings. Such a combination of influential hydrogen-bonding characteristics accounts, in part, for the large second-order NLO output of the material since the phenomenon is so critically dependent upon the nature of the charge transfer.

Journal Article↗
PubMed2001

General method for the description, visualization and comparison of metal coordination spheres: geometrical preferences, deformations and interconversion pathways.

The coordination sphere geometry of metal atoms (M) in their complexes with organic and inorganic ligands (L) is often compared with the geometry of archetypal forms for the appropriate coordination number, n in ML(n) species, by use of the k = n( n- 1)/2 L-M-L valence angles subtended at the metal centre. Here, a Euclidean dissimilarity metric, R(c)(x), is introduced as a one-dimensional comparator of these k-dimensional valence-angle spaces. The computational procedure for R(c)(x), where x is an appropriate archetypal form (e.g. an octahedron in ML(6) species), takes account of the atomic permutational symmetry inherent in ML(n) systems when no distinction is made between the individual ligand atoms. It is this permutational symmetry, of order n!, that precludes the routine application of multivariate analytical techniques, such as principal component analysis (PCA), to valence angle data for all but the lowest metal coordination numbers. It is shown that histograms of R(c)(x) values and, particularly, scatterplots of R(c)(x) values computed with respect to two or more different appropriate archetypal forms (e.g. tetrahedral and square-planar four-coordinations), provide information-rich visualizations of the observed geometrical preferences of metal coordination spheres retrieved from, e.g. the Cambridge Structural Database. These mappings reveal the highly populated clusters of similar geometries, together with the pathways that map their geometrical interconversions. Application of R(c)(x) analysis to the geometry of four- and seven-coordination spheres provides information that is at least comparable to, and in some cases is more complete than, that obtained by PCA.

Journal Article↗
PubMed2001

Molecular saddles. 7. New 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene cyclophanes: synthesis, redox properties, and x-ray crystal structures of neutral species and a dication salt.

We report the synthesis of cyclophanes 18-20 by ester-forming macrocyclization reactions of diols 15 and 16 with 1,4-benzenedicarbonyl chloride. Compounds 18 and 19 display a two-electron, quasireversible oxidation wave in the cyclic voltammogram to yield the dication species at E(ox)(pa) 0.52 and 0.47 V, respectively (vs Ag/AgCl in acetonitrile), whereas the 2 + 2 product 20 undergoes a single four-electron oxidation process at E(o)(x)(pa) 0.51 V. X-ray crystal structures are reported for compounds 18-20 and the dication salt 18(2+)(I(3)(-))(2).(I(2))(0.5). For comparative purposes, the structures are also reported for the precursor diol 15 and its dication salt 15(2+)(ClO(4)(-))(2), which was obtained by electrocrystallization. In the neutral cyclophanes 18-20, the 9,10-bis(1,3-dithiol-2-ylidene)-9, 10-dihydroanthracene moieties adopt a saddle-shaped conformation. The overall measure of folding, the dihedral angle (theta) between the S(1)C(16)C(17)S(2) and S(5)C(21)C(22)S(6) planes, is similar in 15 and 18 (87.6 degrees and 83.7 degrees, respectively) whereas this angle is significantly narrower in 19 (61.1 degrees ), illustrating the flexibility of the saddle conformation and its dependence on the packing. Dimeric molecule 20 contains two saddle moieties with very similar conformations, theta = 73.4 degrees and 73.1 degrees. The structures of dication salts 15(2+)(ClO(4)(-))(2) and 18(2+)(I(3)(-))(2).(I(2))(0.5) reveal that a dramatic conformational change accompanies oxidation of the donor with the dithiolium rings planar and nearly perpendicular to the mean plane of the anthracene moieties. A notable feature of 18(2+) is that the bridge enforces a fold of 22 degrees along the C(9).C(10) vector of the anthracene unit. In 15(2+) there is no fold about this axis, instead the anthracene moiety is slightly twisted with the two (planar) outer rings forming an angle of 7 degrees.

Journal Article↗
PubMed2001

Plant-based vaccines: unique advantages.

Numerous studies have shown that viral epitopes and subunits of bacterial toxins can be expressed and correctly processed in transgenic plants. The recombinant proteins induce immune responses and have several benefits over current vaccine technologies, including increased safety, economy, stability, versatility and efficacy. Antigens expressed in corn are particularly advantageous since the seed can be produced in vast quantities and shipped over long distances at ambient temperature, potentially allowing global vaccination. We have expressed the B-subunit of Escherichia coli heat-labile enterotoxin and the spike protein of swine transmissible gastroenteritis virus at high levels in corn, and demonstrate that these antigens delivered in the seed elicit protective immune responses.

Animals↗
PubMed2001

Crystal chemistry and physical properties of superconducting and semiconducting charge transfer salts of the type (BEDT-TTF)(4)[A(I)M(III)(C2O4)3]*PhCN (A(I) = H30,NH4,K; M(III) = Cr, Fe, Co, Al; BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene.

Synthesis, structure determination by single-crystal X-ray diffraction, and physical properties are reported and compared for superconducting and semiconducting molecular charge-transfer salts with stoichiometry (BEDT-TTF)(4)[A(I)M(III)(C(2)O(4))(3)].PhCN, where A(I) = H(3)O, NH(4), K; M(III) = Cr, Fe, Co, Al; BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene. Attempts to substitute M(III) with Ti, Ru, Rh, or Gd are also described. New compounds with M = Co and Al are prepared and detailed structural comparisons are made across the whole series. Compounds with A = H(3)O(+) and M = Cr, Fe are monoclinic (space group C2/c), at 150, 120 K a = 10.240(1) A, 10.232(12) A; b = 19.965(1) A, 20.04(3) A; c = 34.905(1) A, 34.97(2) A; beta = 93.69(1) degrees, 93.25(11) degrees, respectively, both with Z = 4. These salts are metallic at room temperature, becoming superconducting at 5.5(5) or 8.5(5) K, respectively. A polymorph with A = H(3)O(+) and M = Cr is orthorhombic (Pbcn) with a = 10.371(2) A, b = 19.518(3) A, c = 35.646(3) A, and Z = 4 at 150 K. When A = NH(4)(+), M = Fe, Co, Al, the compounds are also orthorhombic (Pbcn), with a = 10.370(5) A, 10.340(1) A, 10.318(7) A; b = 19.588(12) A, 19.502(1) A, 19.460(4) A; c = 35.790(8) A, 35.768(1) A, 35.808(8) A at 150 K, respectively, with Z = 4. All of the Pbcn phases are semiconducting with activation energies between 0.15 and 0.22 eV. For those compounds which are thought to contain H(3)O(+), Raman spectroscopy or C=C and C-S bond lengths of the BEDT-TTF molecules confirm the presence of H(3)O(+) rather than H(2)O. In the monoclinic compounds the BEDT-TTF molecules adopt a beta' ' packing motif while in the orthorhombic phases (BEDT-TTF)(2) dimers are surrounded by monomers. Raman spectra and bond length analysis for the latter confirm that each molecule of the dimer has a charge of +1 while the remaining donors are neutral. All of the compounds contain approximately hexagonal honeycomb layers of [AM(C(2)O(4))(3)] and PhCN, with the solvent occupying a cavity bounded by [M(C(2)O(4))(3)](3-) and A. In the monoclinic series each layer contains one enantiomeric conformation of the chiral [M(C(2)O(4))(3)](3-) anions with alternate layers having opposite chirality, whereas in the orthorhombic series the enantiomers form chains within each layer. Analysis of the supramolecular organization at the interface between the cation and anion layers shows that this difference is responsible for the two different BEDT-TTF packing motifs, as a consequence of weak H-bonding interactions between the terminal ethylene groups in the donor and the [M(C(2)O(4))(3)](3-) oxygen atoms.

Journal Article↗
PubMed2001

Molecular saddles. 4. Redox-active cyclophanes by bridging the 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene system: synthesis, electrochemistry, and X-ray crystal structures of neutral species and a dication salt.

We report the synthesis of a new series of cyclophanes 11a-d by ester-forming macrocyclization reactions of diol 9 with the dicarbonyl chloride derivatives of benzene, thiophene, ferrocene, and diphenyl ether, 10a-d, respectively. Compounds 11a-d display a two-electron, quasireversible oxidation wave in the cyclic voltammogram to yield the dication species at Eoxpa = 0.70-0.72 V (for 11a-c) and 0.47 V (for 11d) (vs Ag/AgCl in acetonitrile). The raised oxidation potentials for 11a-c reflect the reduced stability of the twisted dication structure within the steric constraints of the smaller cyclophanes. Consistent with this, the value of delta E (defined as Eoxpa - Eoxpc) decreases (i.e., reversibility of the oxidation process increases) in the sequence 11d > 11c > 11a > 11b as the bridging chain becomes shorter. X-ray crystal structures are reported for compounds 11a-d and the dication salt 11d2+(I3-)2.(CH2Cl2)2.25. For 11a-d the typical saddle-shaped conformation of the 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene moiety is observed, with the strain imposed by the cyclophane ring being accommodated in the structure of the bridging unit. In the dication 11d2+ both dithiolium rings are planar and the anthracene unit is essentially aromatic, with the conformation of the bridge basically the same as in neutral 11d.

Journal Article↗
PubMed2001

The 1:1 adduct of 2,5-dihydroxy-1,4-benzoquinone with 4,4'-bipyridine.

2,5-Dihydroxy-1,4-benzoquinone (DHBQ) and 4,4'-bipyridine (BPY) crystallize in a 1:1 ratio as a neutral molecular adduct, C(6)H(4)O(4).C(10)H(8)N(2), in space group C2/c, with half of each molecule in the asymmetric unit. The molecules are linked by a strong O--H...N hydrogen bond [O...N 2.6323 (15) A] and a weak C--H...O hydrogen bond [C...O 3.2082 (17) A] to form infinite stacks of parallel one-dimensional hydrogen-bonded ribbons. The two rings of the bipyridine are twisted at 28.3 degrees with respect to each other, and the benzoquinone ring is inclined at an angle of 18.3 degrees with respect to the plane of the neighbouring pyridine ring. The 4,4'-bipyridine molecule lies on a twofold axis and the benzoquinone molecule lies across an inversion centre.

Journal Article↗
PubMed2001

Automatic detection of molecular symmetry in the Cambridge Structural Database.

A method for the detection of approximate molecular symmetry in crystal structures has been developed. The point-group symmetry is assigned to each molecule and the relevant symmetry elements can be visualized, superimposed on the molecule. The method has been validated against reference structures with exact symmetry subjected to small random perturbation.

Journal Article↗
PubMed2000

Intramolecular hydrogen bonds: common motifs, probabilities of formation and implications for supramolecular organization.

A systematic survey of the Cambridge Structural Database (CSD) has identified all intramolecular hydrogen-bonded ring motifs comprising less than 20 atoms with N and O donors and acceptors. The probabilities of formation P(m) of the 50 most common motifs, which chiefly comprise five- and six-membered rings, have been derived by considering the number of intramolecular motifs which could possibly form. The most probable motifs (P(m) > 85%) are planar conjugated six-membered rings with a propensity for resonance-assisted hydrogen bonding and these form the shortest contacts, whilst saturated six-membered rings typically have P(m) < 10%. The influence of intramolecular-motif formation on intermolecular hydrogen-bond formation has been assessed for a planar conjugated model substructure, showing that a donor-H is considerably less likely to form an intermolecular bond if it forms an intramolecular one. On the other hand, the involvement of a carbonyl acceptor in an intramolecular bond does not significantly affect its ability to act as an intermolecular acceptor and thus carbonyl acceptors display a substantially higher inclination for bifurcation if one hydrogen bond is intramolecular.

Journal Article↗
PubMed2000

A highly functionalized ferrocenylpyrazolo[2,3-a]pyridine.

The crystal structure of [2-(4-bromophenyl)-4-cyano-5-ferrocenylpyrazolo[2,3-a]pyridin-7-yl]acetonitrile, C(26)H(17)N(4)FeBr or [Fe(C(5)H(5))(C(21)H(12)BrN(4))], shows that the pyrazolopyridine ring system (PP), the bromophenyl ring (BP) and the cyclopentadiene ring (Cp) are nearly planar. The PP ring system is twisted out of the plane of the BP and Cp rings by about 20 degrees.

Journal Article↗
PubMed2000

A thermal spin transition in [Co(bpy)3][LiCr(ox)3] (ox = C2O4(2-); bpy = 2,2'-bipyridine).

In the three-dimensional oxalate network structures [M(II)(bpy)3][M(I)-M(III)(ox)3] (ox= C2O4(2-); bpy = 2,2'-bipyridine) the negatively charged oxalate backbone provides perfect cavities for tris-bipyridyl complex cations. The size of the cavity can be adjusted by variation of the metal ions of the oxalate backbone. In [Co(bpy)3][NaCr(ox)3], the [Co(bpy)3]2 + complex is in its usual 4T1(t2g5e(g)2) high-spin ground state. Substituting Na+ by Li+ reduces the size of the cavity. The resulting chemical pressure destabilises the high-spin state of [Co(bpy)3]2+ to such an extent that the 2E(t2g6e(g)1) low-spin state becomes the actual ground state. As a result. [Co(bpy)3][LiCr(ox)3] becomes a spin-crossover system, as shown by temperature-dependent magnetic susceptibility measurements and single-crystal optical spectroscopy, as well as by an X-ray structure determination at 290 and 10 K.

Journal Article↗
PubMed2000

Transgenic avidin maize is resistant to storage insect pests.

Avidin is a glycoprotein found in chicken egg white, that sequesters the vitamin biotin. Here we show that when present in maize at levels of > or =100 p.p.m., avidin is toxic to and prevents development of insects that damage grains during storage. Insect toxicity is caused by a biotin deficiency, as shown by prevention of toxicity with biotin supplementation. The avidin maize is not, however, toxic to mice when administered as the sole component of their diet for 21 days. These dates suggest that avidin expression in food or feed grain crops can be used as a biopesticide against a spectrum of stored-produce insect pests.

Animals↗
PubMed2000

Tris(2-methylphenyl)phosphonium tetrachloroborate.

The title compound, C(21)H(22)P(+).BCl(4)(-), is the first structurally characterized example of the [HP(o-tolyl)(3)](+) cation, presented here with BCl(4)(-) as the counter-ion. The cation has a near-tetrahedral P atom and the BCl(4)(-) anion is near-tetrahedral at boron. There are no unusually short cation-anion contacts.

Journal Article↗
PubMed2000

Racemic 1,2-diphenylbut-3-yn-2-ol.

Molecules of the title compound, C(16)H(14)O, are chiral and crystallize in space group P-4 with Z' = 2, and with one R and one S molecule in the asymmetric unit. The conformations of the phenyl rings in the two independent molecules differ slightly. Supramolecular organization in the crystal is via tetrameric O-H. H(O) hydrogen-bonded synthons formed separately by each conformer. These tetrameric synthons stack along the c axis via C[triple-bond]C-H.O(H) hydrogen bonds. The only link between the conformer stacks is provided by weaker C(methylene)-H and C(phenyl)-H interactions with pi(arene) density.

Journal Article↗