T Vaara

University of Helsinki, Helsinki, Uusimaa, Finland

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Publications (29)121.59 Total impact

  • T. Vaara
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    ABSTRACT: This is the first comparative study on the nature and occurrence of the outermost surface structures in chroococcacean cyanobacteria. It shows that all the structures are similar to those of other bacteria and not specific to only cyanobacteria. Twenty-two chroococcacean strains (1 Microcystis, 7 Synechocystis, 12 Synechococcus, and 2 Gloeothece) were included in the study. Two Synechocystis strains had a glycocalyx layer. The two Gloeothece strains had a characteristic sheath, and the space between the sheath and the cells contained loosely packed material. On the basis of their preferential staining with ruthenium red, both the sheath and the loosely packed material most probably consist of acidic polysaccharides. Microcystis firma, three Synechocystis strains, and one Synechococcus strain were surrounded by a regularly structured surface layer consisting of hexagonally arranged subunits. Microcystis firma, all seven Synechocystis, and three Synechococcus strains possessed pili. The pili of some strains occurred as bundles. None of the strains produced spinae. Additionally, some structures described in previous studies are reinterpreted according to the findings of the present study.
    No preview · Article · Feb 2011 · Canadian Journal of Microbiology
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    ABSTRACT: The local adsorption structure of the coadsorption phase of K and CO on Co(0001) has been determined at 160 K using dynamical low-energy electron diffraction. The K atoms in the (2×2) overlayer adopt the on-top sites as in the (2×2)-K structure, but the bond length to the nearest Co atom increases by 0.2 Å to 3.14±0.05 Å. The CO molecules are shifted from the on-top sites of the (sqrt[3]×sqrt[3])R30°-CO structure to the fcc and hcp three-fold hollow sites. The axis of the CO molecule is perpendicular to the surface. The adsorption induce buckling in the top Co layer, pushing the Co atom beneath the K by 0.27±0.03 Å towards the bulk. The optimum length for the C-O bond is 1.22±0.1 Å and that for the C-Co bond 2.0±0.1 Å. The obtained structure is compared to other known CO+K coadsorption structures on transition metals, as well as to the (sqrt[3]×sqrt[3])R30°-CO and (2×2)-K structures on Co(0001).
    No preview · Article · Mar 2001 · Physical Review B
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    ABSTRACT: Commercially available activated carbon supports, and carbon supported rhodium catalysts were characterised by BET, XPS, TPD, SEM, TEM and chemisorption measurements to elucidate the effect of the pore structure and chemical nature of the carbons on the Rh/C catalysts. During impregnation, both of the parameters had a significant effect on the Rh/C catalysts. First, the meso- and macropores were important for the mass transfer of the metal precursor within the support particle; the larger the pores the better the distribution of rhodium within the support particle. Second, the chemical composition of the carbon surface determined the amount of interaction of the rhodium species and the carbon surface; the pH influenced the attraction of the species, and the oxygen containing surface groups acted as adsorption sites for rhodium. During reduction, the thermal decomposition of the oxygen containing surface groups was essential for dispersion. The thermally stable (CO evolving, weakly acidic, neutral or basic) surface groups remained intact, whereas the thermally unstable (CO2 evolving, acidic) surface groups decomposed inducing agglomeration of rhodium. Thus, it is not only the amount of oxygen containing adsorption sites that affected dispersion but also their nature and stability. Evidently, the degree of agglomeration depends strongly both on the type of carbon and on the reduction conditions. Accordingly, TEM provides a good measure for the particle size since it also accounts for the hydrogen induced agglomeration, whereas hydrogen chemisorption only affords a less informative average value.
    Preview · Article · Jan 2001 · Applied Catalysis A General
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    ABSTRACT: The structures of clean Co(0001) and the p(2×2) overlayer of potassium on Co(0001) were determined at 160 K using dynamical low-energy electron diffraction (LEED). The clean Co(0001) surface shows an oscillatory relaxation in the first two layers, with a 2.1% contraction of the first layer spacing and a 1.3% expansion of the second layer spacing compared to the bulk value. The potassium atoms in the p(2×2) overlayer adopt the top site and induce a buckling of 0.16 Å in the top Co layer such that the Co atom beneath the K is displaced towards the bulk. The effects of the top site and the buckling of the top Co layer on the K/Co(0001) phase diagram are discussed.
    No preview · Article · Apr 1999 · Surface Science
  • T Vaara · J Vaari · J Lahtinen
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    ABSTRACT: The structures of potassium overlayers on Co(0001) surface at several coverages and temperatures have been studied using work function measurements, XPS, TDS and LEED. A tentative structural phase diagram was constructed. In the coverage range from 0.17 ML to 0.31 ML a (2×2) structure was seen. Splitting of the (2×2) LEED beams was observed above 0.31 ML suggesting formation of rotated domains in the potassium layer. The rotated structure occured in an abrupt manner at the onset of the second potassium layer formation. The effects of adsorbate–adsorbate and adsorbate–substrate interactions to the structural and electronic properties of the K/Co(0001) system are discussed.
    No preview · Article · Jan 1998 · Surface Science
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    ABSTRACT: The activity and characteristics of the nitrate derived CoSiO2 were determined in relation to reductive pretreatment. With increasing reduction temperature (300–450°C), the extent of reduction increased. This increase was not, however, accompanied with increased chemisorption capacity. Namely, the hydrogen chemisorption was highest at a reduction temperature of approx. 300°C. Also, the carbon monoxide adsorption was almost constant at 200–400°C, and decreased drastically thereafter. Thus, the results suggest that cobalt agglomerated at temperatures above 400°C. The activity in toluene hydrogenation correlated well with the hydrogen uptake for reduction temperatures of above 300°C. Also, the CO hydrogenation activity of the catalyst decreased with increasing temperature of reduction (400–450°C) due to the decrease in the number of active sites.
    Full-text · Article · Aug 1997 · Applied Catalysis A General
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    ABSTRACT: The effect of pretreatment atmosphere, hydrogen or carbon monoxide, on the activity and selectivity of the Rh4(CO)12/SiO2 and Co4(CO)12/SiO2 catalysts in CO hydrogenation was determined. The catalysts pretreated under hydrogen exhibited a higher degree of decomposition of the original cluster, higher hydrogen uptake and lower carbon content than those decomposed under CO. Therefore, some of the active sites of the catalysts were encapsulated by carbon, and in the case of Co4(CO)12 poorly reducible cobalt silicates were probably also formed. The activity of the hydrogen-treated Rh4(CO)12/SiO2 was fairly similar to that of the CO-treated catalyst, but significantly fewer oxygenates were formed. Thus, the carbon monoxide treatment appeared to facilitate the formation of oxygenates. Probably the original structure of rhodium carbonyl was better retained, and thereby some of the rhodium particles remained more dispersed under CO than under hydrogen. In the case of Co4(CO)12/SiO2 catalysts, however, no benefit was observed in conjunction with CO pretreatment either in activity or selectivity. Most likely the CO treatment resulted in the formation of unreactive carbonaceous species which permanently blocked most of the active sites.
    No preview · Article · Jun 1997 · Journal of Molecular Catalysis A Chemical
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    ABSTRACT: The effect of the precursor on the characteristics of the Co/SiO2 catalysts prepared from Co(NO3)2, Co2(CO)8 and Co4(CO)12 was determined. The near-surface reduction was clearly lower for the Co2(CO)8 derived catalyst than for the Co4(CO)12 based one. The hydrogen chemisorption, CO desorption, XRD and XPS measurements indicated that the dispersion of the metallic species decreased in the precursor order Co2(CO)8 > Co4(CO)12 ≫ Co(NO3)2. The TPD studies showed that CO was more strongly adsorbed on catalysts derived from carbonyls than on those based on nitrate, resulting in greater initial activity in CO hydrogenation. In pulsed CO hydrogenation reactions, the selectivity of the Co2(CO)8 derived catalyst differed from the other two. Thus, despite the well documented transformation of the Co2(CO)8 to Co4(CO)12 on the silica support, the supported catalysts derived from these two precursors exhibited distinct characteristics and reactivity.
    No preview · Article · Dec 1996 · Applied Catalysis A General
  • J. Vaari · J Lahtinen · T Vaara · P Hautojärvi
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    ABSTRACT: We have studied the coadsorption of carbon monoxide and potassium on Co(0001) using XPS, TDS, LEED, XPD, and ISS. Potassium-induced dissociation of CO was not detected. Two ordered coadsorption structures were observed, namely (2 × 2)-(CO + K) and (2 × 2)-(2CO + K). The XPD results show that the CO molecules are adsorbed perpendicular to the surface. According to the ISS measurements the K atoms form the outermost layer of the coadsorption structure.
    No preview · Article · Feb 1996 · Surface Science
  • T. Vaara · J. Lahtinen · P. Hautojärvi
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    ABSTRACT: Nitric oxide adsorption and coadsorption with carbon monoxide have been studied using XPS and TDS on clean and magnesium covered polycrystalline cobalt. At small NO exposures, the adsorption on clean cobalt at room temperature was dissociative but at exposures over 3 L molecular adsorption was also observed. Submonolayer coverage of Mg had a negligible effect on the adsorption of NO, but a full monolayer coverage decreased both the molecular and the dissociative adsorption of NO. In the NOCO coadsorption the NO molecules block the adsorption of CO and induce removal of the preadsorbed CO molecules on the clean and Mg covered cobalt. The presence of Mg stabilizes the CO molecules against the attack by NO molecules.
    No preview · Article · Jun 1995 · Applied Surface Science
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    ABSTRACT: Co/SiO2, Mg-Co/SiO2 and Co-Mg/SiO2 catalysts were prepared from acetate, nitrate or carbonyl precursors. The catalysts were characterized by XRD, XPS, SIMS and TGA. The steady-state activity and product distribution of the catalysts were evaluated in synthesis gas reactions at 0.5 MPa and 235-290C using 3 : 1 : 3 molar ratio of Ar : CO : H2. The activity in CO hydrogenation decreased in the precursor order Co2(CO)8>Co(NO3)2> Co(CH3COO)2, and the probability of chain growth decreased in the precursor order Co(NO3)2>Co2(CO)8>Co(CH3COO)2. Alcohol yields were highest with Co2(CO)8, and lowest with Co(NO3)2, Magnesium promotion influenced the catalyst activity and decreased the CO2 formation, but the promotion effects were less profound than those of the precursor. Surface studies on partially magnesium covered cobalt foil model catalysts suggested that magnesium promotes CO dissociation and chain growth, neither of which were, however, observed in the supported catalysts.
    No preview · Article · Feb 1995 · Topics in Catalysis
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    M Vaara · T Vaara
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    ABSTRACT: The cationic amphipathic insect peptide cecropin B was almost as active on wild-type enteric bacteria as it was on their lipopolysaccharide and lipid A mutants that have very defective outer membrane. The polymyxin-resistant strains, which elaborate altered, less anionic lipopolysaccharide, were completely susceptible to cecropin B. No synergism was found between cecropin B and hydrophobic antibiotics. Throughout the study, the activity of cecropin B resembled that of quaternary detergents.
    Full-text · Article · Nov 1994 · Antimicrobial Agents and Chemotherapy
  • J. Vaari · T VAARA · J LAHTINEN · P HAUTOJARVI
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    ABSTRACT: The effect of Mg on CO adsorption on polycrystalline Co foil has been studied using XPS and TDS. Mg was found to increase the adsorption capacity of the surface, promote the dissociation, and affect the sticking coefficient. The Mg overlayer is partially oxidized due to CO dissociation. Heating of the system above room temperature results in further Mg-promoted dissociation of CO. With increasing Mg coverage, the TDS peak at 387 K corresponding to CO desorption from the clean surface is gradually replaced by a C + O recombination feature around 900 K. The recombination is triggered by the decomposition of the Mg suboxide, observed as Mg desorption simultaneously with CO desorption.
    No preview · Article · Nov 1994 · Applied Surface Science
  • Timo Vaara · Martti Vaara
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    ABSTRACT: This chapter focuses on Cyanobacterial Fimbriae. Fimbriae are stiff, tubular filaments of uniform thickness that rise from the bacterial cell surface. They are usually 1-2 μm in length and 5-7 nm in diameter and comprise hundreds of identical subunits, fimbrillins, helically arranged along the filament. Fimbrillins are type specific, and apparent mass range from 8000 to 20,000 has been determined. In one respect, however, all fimbrillins are similar: they are particularly rich in hydrophobic amino acids, which results in increased hydrophobicity of fimbriate cells. Fimbriae are best visualized by using negative staining. This technique involves drying a small quantity of the sample on a grid for electron microscopy and staining it with an electron-dense material such as phosphotungstic acid. Regions of the specimen that the stain cannot infiltrate remain electron-transparent and are seen with high resolution against the electron-dense background. Phosphotungstic acid is recommended for routine staining of cyanobacterial fimbriae.
    No preview · Article · Dec 1988 · Methods in enzymology
  • P Viljanen · H Käyhty · M Vaara · T Vaara
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    ABSTRACT: Polymyxin B nonapeptide was able to sensitize Escherichia coli strains and strains of Salmonella typhimurium, Klebsiella spp., Enterobacter cloacae, Pseudomonas aeruginosa, and Haemophilus influenzae to the bactericidal action of fresh normal human serum. The degree of sensitization varied significantly within the strains. Strains of Proteus mirabilis, Neisseria gonorrhoeae, and N. meningitidis remained resistant.
    No preview · Article · Feb 1986 · Canadian Journal of Microbiology
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    ABSTRACT: The outer-membrane-disorganizing peptide (polymyxin B nonapeptide; PMBN) was able to sensitize even “antibiotic supersensitive” enterobacterial mutants to hydrophobic antibiotics. This resulted in an extreme sensitivity. The mutants included the “deep rough” lipopolysaccharide mutants, as well as the acrA mutant of Escherichia coli and the “class A, B, and C mutants of Salmonella typhimurium. Sensitization factors of approx. 30 or more were found for most antibiotics. Even minimum inhibitory concentrations as low as approx. 0.5 ng/ml (rifampicin), 1.5 ng/ml (erythromycin), 2 ng/ml (fusidic acid), 6 ng/ml (novobiocin), and 30 ng/ml (clindamycin) were achieved in the presence of 30 μg/ml of PMBN. The finding indicates that the mechanisms which mediate the increase in hydrophobic diffusion are different but synergistic in the mutants and in the PMBN-grown cells.
    No preview · Article · Mar 1985 · FEMS Microbiology Letters
  • M Vaara · P Viljanen · T Vaara · P H Mäkelä
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    ABSTRACT: The small cationic outer membrane-disorganizing peptide PMBN sensitized four smooth, encapsulated strains of Escherichia coli (serotypes 02:K1, 04:K12, 018:K1, and 018:K5) to the lethal action of serum. The concentrations of PMBN required were low (0.3 to 1.0 microgram/ml). One E. coli strain (IH 11030; 075:K5) remained virtually resistant to serum and also to anti-075 hyperimmune serum plus complement (C) even in the presence of PMBN. This strain was nevertheless sensitive to the outer membrane permeability-increasing action of PMBN. In the bactericidal system, PMBN could be replaced by high concentrations of lysine20 or protamine but not lysine4. The PMBN-dependent bactericidal activity of GPS was abolished by heating or zymosan treatment that inactivate its C but not by lack of the action of the classical pathway of the C in C4-deficient GPS. PMBN formed a bactericidal system also with normal rabbit, rat, and human serum but not with mouse serum. The bactericidal system against E. coli 018:K1 and its derivative EH 817 (018:K1-) was found to require a factor that can be removed from normal sera by absorption with a rough E. coli strain. This factor could be replaced by specific anti-018 antibodies. The bactericidal activity of fetal calf serum plus PMBN against E. coli 018:K1 was enhanced by normal rabbit or anti-E. coli 018 hyperimmune serum. We suggest that PMBN unshields the deep structures and the hydrophobic membrane milieu of the outer membrane and facilitates the insertion of the membrane attack complex of the C into this milieu.
    No preview · Article · Jun 1984 · The Journal of Immunology
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    ABSTRACT: Synechocystis CB3, isolated from the Gulf of Finland, was covered by innumerable flexible pili (fimbriae) with an approximate diameter of 6 nm. The Synechocystis CB3 pili had a tendency to attach side by side thus forming characteristic bundles consisting of several dozens of individual pilus filaments. The Synechocystis CB3 pili were isolated and purified using deoxycholate and urea, and found to be very similar to other bacterial pili in their subunit Mr (21 kDa) and amino acid composition (46% hydrophobic amino acids). The amino acid analysis revealed also small amounts of glucosamine in the pilus preparation.
    No preview · Article · Mar 1984 · FEMS Microbiology Letters
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    ABSTRACT: The isolated, outermost cell wall layer from Synechocystis sp. strain CLII is described using electron microscopy and Fourier reconstruction to study the three-dimensional structure of the proteins within the layer to a resolution of ca. 3 nm. This surface layer forms regular hexagonal arrays (a = b = 15.2 nm). The two-dimensional space group is p6. The monomer proteins form hexamers arranged around a central hollow cylinder. The linkers between the hexamers are of the delta type and are located approximately in the central section between the top and bottom of the protein layer.
    Full-text · Article · Jan 1984 · Journal of Bacteriology
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    M Vaara · T Vaara
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    ABSTRACT: Polymyxin B nonapeptide, a polymyxin B derivative which lacks the fatty acyl part and the bactericidal activity of polymyxin, was shown to sensitize smooth encapsulated Escherichia coli (O18:K1) and smooth Salmonella typhimurium to hydrophobic antibiotics (novobiocin, fusidic acid, erythromycin, clindamycin, nafcillin, and cloxacillin). The polymyxin B nonapeptide-treated bacteria were as sensitive to these antibiotics as are deep rough mutants. A lysine polymer with 20 lysine residues (lysine 20) had a largely similar effect. Larger lysine polymers and the protamine salmine were bactericidal but, at sublethal concentrations, sensitized the strains to the antibiotics mentioned above, whereas lysine4, streptomycin, cytochrome c, lysozyme, and the polyamines cadaverine, spermidine, and spermine had neither bactericidal nor sensitizing activity.
    Full-text · Article · Aug 1983 · Antimicrobial Agents and Chemotherapy

Publication Stats

1k Citations
121.59 Total Impact Points

Institutions

  • 1979-2011
    • University of Helsinki
      • • Department of Microbiology
      • • Laboratory of Physical Chemistry
      • • Department of Bacteriology and Immunology
      Helsinki, Uusimaa, Finland
  • 1985-1986
    • National Public Health Institute
      Helsinki, Uusimaa, Finland
  • 1984
    • Stockholm University
      Tukholma, Stockholm, Sweden