Anomalous grazing incidence small-angle x-ray scattering studies of platinum nanoparticles formed by cluster deposition

The size evolution of platinum nanoparticles formed on a SiO2/Si(111) substrate as a function of the level of surface coverage with deposited clusters has been investigated. The anisotropic shapes of sub-nanometer-size nanoparticles are changed to isotropic on the amorphous substrate as their sizes increased. Using anomalous grazing incidence small-angle x-ray scattering (AGISAXS), the scattering from nanoparticles on the surface of a substrate is well separated from that of surface roughness and fluorescence. We show that AGISAXS is a very effective method to subtract the background and can provide unbiased information about particle sizes of less than 1 nm.     

Nordesferriferrithiocin. Comparative Coordination Chemistry of a Prospective Therapeutic Iron Chelating Agent(1)

Nordesferriferrithiocin, NDFFTH(2), is a derivative of the siderophore desferriferrithiocin, DFFTH(2), in which the methyl group is substituted by a hydrogen atom. Both compounds show high oral activity as possible drugs for the treatment of iron overload. While DFFTH(2) is significantly toxic, NDFFTH(2) exhibits a lower toxicity and offers a much better therapeutic window than other orally active iron chelators. In this study, complexes of DFFTH(2) and NDFFTH(2) with various trivalent metals have been synthesized and characterized. Five isomers (the maximum possible) have been observed in the case of [Co(DFFT)(2)](-) in solution, as proved by (1)H-NMR measurements. Although normally labile, complexes of Al(3+) ([Al(DFFT)(2)](-)) have been separated by HPLC. In general, DFFTH(2) forms kinetically inert complexes whereas complexes of NDFFTH(2) tend to isomerize quickly in solution, as indicated by CD spectroscopy of separated HPLC fractions of [Cr(NDFFT)(2)](-). The most stable isomers of the aluminum complexes of both ligands have been characterized by X-ray crystallography; K[Al(DFFT)(2)] crystallizes from methanol/diethyl ether in the orthorhombic space group P2(1)2(1)2 with a = 11.238(3) ?, b = 31.719(11) ?, c = 7.684(2) ?, V = 2739.2(24) ?(3), and Z = 4. This isomer has the mer-(N,O-Lambda)(S,S) configuration, while K[Al(NDFFT)(2)] crystallizes from methanol/diethyl ether in the space group P6(1) (a = 21.269(8) ?, c = 9.643(3) ?, V = 3777.8(42) ?(3), Z = 6) and has the same coordination geometry. The solution thermodynamics of the Al(3+), Ga(3+), and Fe(3+) complexes have been studied by spectrophotometric titration. The stability constants (log K) are 23.6(1), 29.2(3), and 31.04(3), respectively, for the DFFTH(2) complexes and 22.0(1), 27.8(2), and 29.09(3), respectively, for the NDFFTH(2) complexes. Cyclic voltammograms of both iron complexes have been recorded in water at a carbon disk working electrode and in DMF at a graphite working electrode. The reduction waves measured in DMF indicate no reversibility whereas in water a quasi-reversible reduction is observed. The reduction potentials (E(1/2)'s) in water are -166 mV for [Fe(DFFT)(2)](-) and -97 mV for [Fe(NDFFT)(2)](-) versus NHE. These potentials are well in the range for biological reductants, which makes possible an in vivo reduction mechanism for the iron removal from the siderophore.     

Synthesis and properties of crosslinked polyvinylformamide and polyvinylamine hydrogels in conjunction with silica particles

Polyvinylamine hydrogels with silica particles encapsulated (PVAm/silica) were produced by a two‐step synthesis. In the first step, polyvinylformamide/silica (PVFA/silica) hybrids were synthesized from vinylformamide (VFA) and 1,3‐divinylimidazolidin‐2‐one (1,3‐bisvinylethyleneurea, BVU), as the crosslinker, by radical copolymerization in silica/water suspensions using different compositions of VFA/BVU. The target product PVAm/silica was obtained by acidic hydrolysis of the PVFA/silica hydrogels in a second step. The chemical structures of both hydrogels, PVFA/silica and PVAm/silica, respectively, were revealed by solid‐state ¹³C(¹H) cross‐polarity/magic‐angle spinning NMR spectroscopy. Both hydrogels swelled significantly in water. The swelling capacity of the two systems was characterized by the correlation length ξ (or hydrodynamic blob size) of the network meshes with small‐angle neutron scattering experiments. ξ is significantly larger for PVAm/silica than for PVFA/silica, which corresponds to the observed higher swelling capacity of this polyelectrolyte material. Furthermore, the swelling behavior of the hybrid hydrogels was quantitatively described in terms of free swell capacity, centrifuge‐retention capacity, adsorption against pressure, and free swell rate as compared with values of the corresponding copolymer hydrogels. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3144–3152, 2002     

The pH dependence of phenolphthalein: A 13c NMR study

The pH dependence of the ¹³C NMR spectra of phenolphthalein has been measured and interpreted. The spectrum of the neutral compound 1 is compared with the spectra of the dianion 4, the trianion5 and the carbocation7. At the first time spectroscopic evidence for the carbinol 6 is given.     

PHYTOCHROME EVOLUTION: A PHYLOGENETIC TREE WITH THE FIRST COMPLETE SEQUENCE OF PHYTOCHROME FROM A CRYPTOGAMIC PLANT: (Selaginella martensii SPRING)

We have sequenced cDNA and genomic clones coding for phytochrome of the fern Selaginella. On the amino acid level, this phytochrome shares sequence homologies with phytochromes of higher plants which range between 62 (phytochrome B of Arabidopsis) and 55 (56)% [phytochrome C of Arabidopsis (Avena)]. Introns in the Selaginella gene are short and occupy positions known from phytochrome sequences of higher plants. A rooted phylogenetic tree based on mutation distances puts Selaginella phytochrome closest to the hypothetical ancestor. A similar tree arises if the tree is constructed with partial sequences (about 200 amino acids) around the chromophore attachment site. An extension of this tree by sequences of other cryptogamic plants (Mougeotia, Ceratodon, Psilotum) shows all these sequences including those of the phytochromes B and C of Arabidopsis on a branch, well separated from the branch formed by phytochromes known to accumulate in etiolated plants. The rooted phytochrome phylogenetic tree, however, is difficult to reconcile with the fossil record.     

Mechanisms of reduced human vascular cell migration after photodynamic therapy

Restenosis results from intimal hyperplasia and constrictive remodeling following cardiovascular interventions. Photodynamic therapy (PDT) has been shown to inhibit intimal hyperplasia in vivo by preventing neointimal repopulation of the treated vessel. This study was undertaken in an attempt to further dissect the mechanisms by which PDT acts on secreted and extracellular matrix proteins to inhibit migration of cultured human vascular cells. PDT of three-dimensional collagen gels inhibited invasive human smooth muscle cell (SMC) migration, whereas cell-derived matrix metalloproteinase production remained unaltered. Additionally, PDT generated cross-links in the collagen gels, a result substantiated in an ex vivo model whereby PDT rendered the treated vessels resistant to pepsin digestion and inhibited invasive migration of SMC and fibroblasts. These data support the premise that by inducing matrix protein cross-links, rendering the vessel resistant to degradation, in vivo PDT inhibits repopulation of the vessel and therefore intimal hyperplasia.     

Improving the catalytic activity of Candida antarctica lipase B by circular permutation

Lipases (EC 3.1.1.3) play an important role in asymmetric biocatalysis. Tailoring these enzymes to novel, unnatural substrates is one of the primary challenges of protein engineering. We have used circular permutation, the intramolecular relocation of a protein's N- and C-termini, to explore the effects of altered active site accessibility and protein backbone flexibility on the catalytic performance of lipase B from Candida antarctica (CALB). Our combinatorial approach identified 63 unique functional protein permutants of CALB, and kinetic analysis of selected candidates indicated that a majority of enzyme variants either retained or surpassed wild-type CALB activity on a series of standard substrates. Beyond the potential benefits of these tailor-made lipases as new catalysts for unnatural substrates, our study validates circular permutation as a promising general method for lipase engineering.     

HPTLC analysis of organotin compounds in preservative solutions and preservative-treated wood

A high-performance thin-layer chromatographic (HPTLC) method is described for the determination of tributyltin compounds (bis(tri-n-butyltin) oxide, TBTO, and tri-n-butyltin naphthenate, TBTN) and their degradation products (dibutyltin and monobutyltin compounds). The organotin compounds are extracted from wood with ethanol containing 0.5% (v/v) of hydrochloric acid and the separation of the defferent kinds of organotin compounds is achieved by thin-layer chromatography. The sample spots are measured using a scanning densitometer after decomposing the organotin compounds to inorganic tin by ultraviolet irradiation and visualization of the spots with pyrocatechol violet. Applications of the method to detection and quantification of organotin compounds in preservative solutions, in recently impregnated wood, and in wood samples from five-year-old window frames are described.     

Anomalous behavior of atomic hydrogen interacting with gold clusters

The change in the electronic structure of Au(n)- clusters induced by the exchange of an Au atom by hydrogen is studied using photoelectron spectroscopy. Au anion clusters react with one hydrogen atom but not with molecular hydrogen. The spectra of Au(n)- and Au(n-1)H- clusters show almost identical features for n > 2 suggesting that hydrogen behaves as a protonated species by contributing one electron to the valence pool of the Au(n)- cluster. This behavior is in sharp contrast to that of the commonly understood electronic structure of hydrogen in metals; namely, it attracts an electron from the conduction band of the metal and remains in an "anionic" form or forms covalent bonding. We discuss the influence of the unique electronic structure of H on the unusual catalytic behavior of Au clusters.