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61.
Abstract— Kinetic studies of the hematoporphyrin–sensitized photooxidation of l -tryptophan and tryptamine at pH 10 in either homogeneous aqueous solutions or in aqueous dispersions of Triton X–100 and cetyltrimethylammonium bromide micelles indicate that the indole substrates are attacked via a mixed type I (electron transfer from triplet dye)/type II (1 O2 -involving) mechanism. Both reactive intermediates, generated by micelle-solubilized hematoporphyrin, can diffuse to attack substrate molecules located in either the bulk aqueous phase or a different micelle. In particular, incorporation of the substrate into a micelle has only minor effects on its reactivity toward1 O2 , although the 1 O2 —indole interaction appears to be more efficient in cationic micelles owing to a favourable orientation of the target with respect to the attacking species. On the other hand, the electron transfer from triplet porphyrin to a micellized substrate is virtually non-operative when the latter is located in an anionic micelle, whereas in neutral or cationic micelles, the efficiency of the process is again controlled by the substrate orientation. Studies of tryptamine photooxidation sensitized by meso-tetra-(4-sulfonato-phenyl) porphine in the presence of sodium dodecylsulphate micelles lend further support to the abovementioned conclusions. 相似文献
62.
We describe a simple method for the determination of phenobarbital, diphenylhydantoin, carbamazepine, and primidone in serum, by use of gas-liquid chromatography with temperature programming. The methylated derivatives of these anticonvulsants were well resolved, as was 5-(p-methylphenyl)-5-phenylhydantoin, the internal standard. In this procedure we used the jet tube for separation of the drug from the serum. The proposed procedure requires only 100 μl of serum and can be done in less than 30 min. The lower limit of detection for each of the drugs is 0.5 mg/liter. Analytical recoveries of drug from serum were excellent and peak height and concentration were linearly related up to twice the toxic concentration for serum. 相似文献
63.
S. Colonna S. De Rossi M. Faticanti I. Pettiti P. Porta 《Journal of molecular catalysis. A, Chemical》2002,180(1-2):161-168
ZrO2-supported La, Co oxide catalysts with different La, Co loading (2, 6, 8, 12 and 16 wt.% as LaCoO3) were prepared by impregnation of tetragonal ZrO2 with equimolar amounts of La and Co citrate precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. Catalytic CO oxidation was performed at 298–800 K. XRD revealed the presence of tetragonal zirconia with traces of the monoclinic phase. LaCoO3 perovskite was also detected for loading higher than 6%. XAS experiments suggested that at high loading LaCoO3 and Co3O4 were formed, while at low loading, La, Co oxide species interacting with support, and hard to be structurally defined, prevailed. The catalysis study evidenced that the catalytic activity was due to segregated and highly dispersed cobalt oxide species. 相似文献
64.
We report here our findings on a new and highly efficient strategy for the synthesis of β-amino acids involving the addition of bis(O-silyl) ketene acetals on Mannich type iminium electrophiles. 相似文献
65.
Grald Lelais Dieter Seebach Bernhard Jaun RaveendraI. Mathad Oliver Flgel Francesco Rossi Marino Campo Arno Wortmann 《Helvetica chimica acta》2006,89(3):361-403
The correlation between β2‐, β3‐, and β2,3‐amino acid‐residue configuration and stability of helix and hairpin‐turn secondary structures of peptides consisting of homologated proteinogenic amino acids is analyzed (Figs. 1–3). To test the power of Zn2+ ions in fortifying and/or enforcing secondary structures of β‐peptides, a β‐decapeptide, 1 , four β‐octapeptides, 2 – 5 , and a β‐hexadecapeptide, 10 , have been devised and synthesized. The design was such that the peptides would a) fold to a 14‐helix ( 1 and 3 ) or a hairpin turn ( 2 and 4 ), or form neither of these two secondary structures (i.e., 5 ), and b) carry the side chains of cysteine and histidine in positions, which will allow Zn2+ ions to use their extraordinary affinity for RS? and the imidazole N‐atoms for stabilizing or destabilizing the intrinsic secondary structures of the peptides. The β‐hexadecapeptide 10 was designed to a) fold to a turn, to which a 14‐helical structure is attached through a β‐dipeptide spacer, and b) contain two cysteine and two histidine side chains for Zn complexation, in order to possibly mimic a Zn‐finger motif. While CD spectra (Figs. 6–8 and 17) and ESI mass spectra (Figs. 9 and 18) are compatible with the expected effects of Zn2+ ions in all cases, it was shown by detailed NMR analyses of three of the peptides, i.e., 2, 3, 5 , in the absence and presence of ZnCl2, that i) β‐peptide 2 forms a hairpin turn in H2O, even without Zn complexation to the terminal β3hHis and β3hCys side chains (Fig. 11), ii) β‐peptide 3 , which is present as a 14‐helix in MeOH, is forced to a hairpin‐turn structure by Zn complexation in H2O (Fig. 12), and iii) β‐peptide 5 is poorly ordered in CD3OH (Fig. 13) and in H2O (Fig. 14), with far‐remote β3hCys and β3hHis residues, and has a distorted turn structure in the presence of Zn2+ ions in H2O, with proximate terminal Cys and His side chains (Fig. 15). 相似文献
66.
Miriam Rossi 《Journal of organometallic chemistry》2006,691(8):1535-1543
Using structural data from bis(bidentate)diorganotin compounds in the Cambridge Structural Database a potential pathway for trans-cis interconversion is envisaged with nondissociative Sn-donor bonds and retaining metal coordination number 6. C-Sn-C bond angles in the range 180-145° correspond to skewed trapezoid bipyramidal geometry for 6- and 5-membered O,O′ chelates; geometries that resemble the transition state of the trans-cis pathway starts forming at about C-Sn-C 134°. cis-Diorganotins explored in this work have C-Sn-C bond angles in the range 102-110°; it is the statistically favored configuration for diphenyltins. The proposed trans-cis conversion pathway is deduced from a series of geometries associated with decreasing the C-Sn-C bond angle and shows 2 weakly (secondary) bound chelating atoms lengthening their bonds until near the transition state and later strengthening; they end up cis to each other and opposite to the organic groups. Conversely, the other 2 (primary) donors shorten their bonds until the transition state is reached and later lengthen; they end up trans to each other. The entire transformation from trans to cis configuration occurs with relative rotation of 3 bonds. 相似文献
67.
Mun BS Watanabe M Rossi M Stamenkovic V Markovic NM Ross PN 《The Journal of chemical physics》2005,123(20):204717
The surface valence-band densities of states (DOS) of Pt(3)M (M=Ti,V,Cr,Fe,Co,Ni) polycrystalline alloys were investigated with ultraviolet photoemission spectroscopy. Upon annealing the ion-sputter-cleaned alloys at high temperatures, the observed valence-band DOS spectra clearly show the modified electronic structures on the surfaces suggesting the surface segregation of Pt as predicted in thermodynamic models. The measured d-band centers and widths for the annealed alloy surfaces show qualitatively the same trend as predicted by density-functional-theory calculations based on the model of a Pt "skin" on the topmost surface layer and a subsurface layer enriched in the 3d transition metal. 相似文献
68.
The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids 总被引:3,自引:0,他引:3
Silveira ET Umpierre AP Rossi LM Machado G Morais J Soares GV Baumvol IJ Teixeira SR Fichtner PF Dupont J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(15):3734-3740
The controlled decomposition of an Ru(0) organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF(6)), tetrafluoroborate (BMI.BF(4)) or trifluoromethane sulfonate (BMI.CF(3)SO(3)) ionic liquids with H(2) represents a simple and efficient method for the generation of Ru(0) nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of approximately 57 nm that contain dispersed Ru(0) nanoparticles with diameters of 2.6+/-0.4 nm. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75 degrees C). The ternary diagram (benzene/cyclohexene/BMI.PF(6)) indicated a maximum of 1 % cyclohexene concentration in BMI.PF(6), which is attained with 4 % benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI.PF(6). Selectivities of up to 39 % in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2 % in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles. 相似文献
69.
Guidoboni E de los Rios I Ienco A Marvelli L Mealli C Romerosa A Rossi R Peruzzini M 《Inorganic chemistry》2002,41(4):659-668
Reaction of [(triphos)Re(CO)(2)(OTf)] (1) [triphos = MeC(CH(2)PPh(2))(3); OTf = OSO(2)CF(3)] with P(4)S(3) and P(4)Se(3) yields pairs of coordination isomers, namely, [(triphos)Re(CO)(2)[eta(1)-P(apical)-P(4)X(3)]](+) (X = S, 2; Se, 5) and [(triphos)Re(CO)(2)[eta(1)-P(basal)-P(4)X(3)]](+) (X = S, 3; Se, 6). The latter represent the first examples of the eta(1)-P(basal) coordination achieved by the P(4)X(3) molecular cage. Further reaction of 2/3 and 5/6 mixtures with 1 affords the dinuclear species [[(triphos)Re(CO)(2)](2)[mu,eta(1:1)-P(apical,)P(basal)-P(4)X(3)]](2+) (X = S, 4; Se, 7) in which the unprecedented M-eta(1)-P(basal)/eta(1)-P(apical)-M' bridging coordination of the P(4)X(3) molecule is accomplished. A theoretical analysis of the bonding properties of the two coordination isomers is also presented. The directionality of apical vs basal phosphorus lone pairs is also discussed in terms of MO arguments. 相似文献
70.
Rafael Fogel Rafaela Rodrigues Garcia Rebeca da Silva Oliveira Denise Neves Menchero Palacio Luciana da Silva Madeira Nei Pereira Jr. 《Applied biochemistry and biotechnology》2005,122(1-3):741-752
The dilute-acid hydrolysis of sugarcane bagasse was optimized using a statistical experimental design resulting in hydrolysates
containing 57.25 g/L of xylose, which were fermented with a high inoculum concentration (10 g/L of the yeast Candida guilliermondii IM/UFRJ 50088). The addition of urea reduced the time of conversion (t
C) to 75 h (without nitrogen source addition t
C>127 h), and, consequently, improving the rates of xylitol bioproduction. Fermentator experiments, using the optimized conditions,
resulted in enhanced conversion rates, reducing t
C to 30 h. The stability of the yeast in the hydrolysate was also verified in a 480-h cultivation. 相似文献