bluuebunny/crossref_metadata_2025_split huggingface dataset:
Absolute rate Constants for the Addition of Malonyl Radicals to Alkenes in Solution
Authors: Weber Matthias , Fischer Hanns 
Published: December 2004

AbstractAbsolute rate constants and their Arrhenius parameters are obtained for the addition of a cyclic malonyl radical derived from Meldrum's acid to 20 mono‐ or 1,1‐disubstituted alkenes in (±)‐propylene oxide and for the addition of the open‐chain di(tert‐butyl)malonyl radical to six mono‐ or 1,1‐disubstituted alkenes in 1,1,2‐trichloro‐1,2,2‐trifluoroethane by time‐resolved electron spin resonance spectroscopy. At room temperature, the radicals add at the unsubstituted C‐atoms with rate constants ranging from 1.1. 105 M−1S−1 (acrolein) to 2.4. 106 M−1S−1 (1,1‐diphenylethene). The frequency factors are in the narrow ranges of log (A/M−1S−1) = 8.7 ± 0.1 for the cyclic and log (A/M−1S−1) = 8.2 ± 0.2 for the open‐chain malonyl species, whereas the activation energies vary from 12.9 kJ/mol (1,1‐diphenylethene) to 21.7 kJ/mol (acrylonitrile). They correlate with the alkene ionization potentials and, more weakly, with the reaction enthalpy. No correlation was found between the activation energies and the alkene electron affinities. Hence, the results confirm and quantify the electrophilic nature of malonyl radicals in addition reactions.
Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds part VI Derivatives with Bidentate Ligands, Crystal Structures of tetrahedro‐ Decacarbonyl{μ‐[1,1‐bis(methylthio‐χS)ethane]}tetrairidium ([Ir4(CO)10‐(μ2‐(MeS)2CHMe)]), tetrahedro‐Tri‐μ‐carbonylheptacarbonyl{μ‐{ethylidenebis‐ [diphenylphosphine]‐χP:χP'}}tetrairidium ([Ir4(CO)10(μ2‐(Ph2P)2CHMe)]), and tetrahedro‐Tri‐μ‐carbonylheptacarbonyl{μ‐{propane‐1,3‐diylbis[diphenylphosphine]‐χP:χP'}}tetrairidium ([Ir4(CO)10(μ2‐Ph2P(CH2)3PPh2)])
Authors: Lumini Tito , Laurenczy Gábor , Roulet Raymond , Tassan Augusto , Ros Renzo , Schenk Kurt , Gervasio Giuliana 
Published: December 2004

AbstractThe disubstituted clusters[Ir4(CO)10(μ2‐LL)] with one edge‐bridging ligand have a ground‐state geometry with all COs terminal (LL = (MeS)2CHMe, cluster 1) or with three edge‐bridging COs (LL = (Ph2P)2CHMe, cluste 2; LL =Ph2P(CH2)3PPh2, cluster 3) in the solid state and in solution. A comparative 13C‐NMR study of 1–3 shows that their respective ground‐state geometries are merely relative minima of energy in the same kinetic profile of successive fluxional processes consisting of a merry‐go‐round of six COs about a unique trangular face and the rotation of terminal COs about one Ir‐atom. The factors affecting the activation energy of the merry‐go‐round result from the relative bites of the bidentate ligands in the ground‐state geometry, as shown by a comparison of the molecular structures of 2 and 3.
Kinetic Study of the Reaction of Pyridoxal 5′‐Phosphate with Hydrazino Compounds of Pharmacological Activity
Authors: Echevarría‐Gorostidi Gerardo R. , Basagoitia Andrea , Pizarro Eliana , Goldsmid Ruth , Blanco José G. Santos , Blanco Francisco García 
Published: December 2004

AbstractThe kinetics of the reaction between pyridoxal 5′‐phosphate (PLP) with carbidopa, hydralazine, and isoniazid, in aqueous solution at variable pH and constant ionic strength of 0.1M was studied spectrophotometrically. The rate constants of formation and hydrolysis of the resulting Schiff base, and its stability were determined in a wide range of pH. A comparison is made of the formation rate constants with those of PLP with hydrazine. The reactivity shows the sequence isoniazid > hydrazine > carbidopa > hydralazine in the whole range of pH studied. The Schiff bases studied are more stable than those formed by PLP and hexylamine and as stable as those described for the reactions of PLP with poly(L‐lysine) or copolypeptides containing L‐lysine.
Distamycin‐NA: A DNA analog with an aromatic heterocyclic polyamide backbone. Part 2. Solid‐phase synthesis of distamycin‐NAs containing the nucleobase uracil: Unexpected solvent participation in the coupling step
Authors: Sauter Guido , Leumann Christian 
Published: December 2004

AbstractThe synthesis of the Fmoc‐protected amino acid 2 is presented. First attempts of amide‐bond formation to the homodimer 4 in solution showed only poor coupling yields indicative for the low reactivity of the amino and carboxy groups in the building blocks 1 and 2, respectively (Scheme 1). Best coupling yields were found using dicyclohexylcarbodiimide (DCC) without any additive. The oligomerization of building block 2 adopting the Fmoc ((9H‐fluoren‐9‐ylmethoxy)carbonyl) solid‐phase synthesis yielded a mixture of N‐terminal‐modified distamycin‐NA derivatives. By combined HPLC and MALDI‐TOF‐MS analysis, the N‐terminal functional groups could be identified as acetamide and N,N‐dimethylformamidine functions, arising from coupling of the N‐terminus of the growing chain with residual AcOH or DCC‐activated solvent DMF. An improved preparation of building block 2 and coupling protocol led to the prevention of the N‐terminal acetylation. However, ‘amidination’ could not be circumvented. A thus isolated tetramer of 2, containing a lysine unit at the C‐terminus and a N,N‐dimethylformamidine‐modified N‐terminus, not unexpectedly, showed no complementary base pairing to DNA and RNA, as determined by standard UV‐melting‐curve analysis.
β2‐ and β3‐Peptides with Proteinaceous Side Chains: Synthesis and solution structures of constitutional isomers, a novel helical secondary structure and the influence of solvation and hydrophobic interactions on folding
Authors: Seebach Dieter , Abele Stefan , Gademann Karl , Guichard Gilles , Hintermann Tobias , Jaun Bernhard , Matthews Jennifer L. , Schreiber Jürg V. , Oberer Lukas , Hommel Ulrich , Widmer Hans 
Published: December 2004

AbstractEnantiomerically pure β‐amino‐acid derivatives with the side chains of Ala, Val, and Leu in the 2‐ or 3‐position (β2‐ and β3‐amino acids, resp.), as well as with substituents in both the 2‐ and 3‐positions (β2,3‐amino acids, of like‐configuration) have been prepared (compounds 8–17) and incorporated (by stepwise synthesis and fragment coupling, intermediates 24–34) into β‐hexa‐, β‐hepta‐, and β‐dodecapeptides (1–17). The new and some of the previously prepared β‐peptides (35–39) showed NH/ND exchange rates (in MeOH at room temperature) with τ1/2 values of up to 60 days, unrivalled by short chain α‐peptides. All β‐peptides 1–7 were designed to be able to attain the previously described 31‐helical structure (Figs. 1 and 2). CD Measurements (Fig. 4), indicating a new secondary structure of certain β‐peptides constructed of β2‐ and β3‐amino acids, were confirmed by detailed NMR solution‐structure analyses: a β2‐heptapeptide (2c) and a β2,3‐hexapeptide (7c) have the 31‐helical structure (Figs. 6 and 7), while to a β2/β3‐hexapeptide (4) with alternating substitution pattern H‐(β2‐Xaa‐β3‐Xaa)3‐OH a novel, unusual helical structure (in (D5)pyridine, Fig. 8; and in CD3OH, Figs. 9 and 10) was assigned, with a central ten‐membered and two terminal twelve‐membered H‐bonded rings, and with CO and NH bonds pointing alternatively up and down along the axis of the helix (Fig. 11). Thus, for the first time, two types of β‐peptide turns have been identified in solution. Hydrophobic interactions of and hindrance to solvent accessibility by the aliphatic side chains are discussed as possible factors influencing the relative stability of the two types of helices.
Second‐Generation Trifluoromethyl‐Substituted Chiral Dendrimers Containing Triply Branched Building Blocks: CF3 as sensitive NMR probe for ‘remote’ diastereotopicity
Authors: Greiveldinger Guy , Seebach Dieter 
Published: December 2004

Abstract(S)‐4,4,4‐Trifluoro‐3‐hydroxybutanoic acid was used as a starting material for the synthesis of dendritic branches (16–21, 30–32) which were attached to a chiral triol (6 or ent‐6), derived from (R)‐3‐hydroxybutanoic acid, to give CF3‐substituted dendrimers (33–36, 38–40) and dendritic compounds (37) of 1st and 2nd generation. The key steps in these syntheses are diastereoselective aldol additions of dioxanone enolates (building blocks and intermediates 6–15, 22, 23, and 25–28) and Williamson etherifications of benzylic‐branch bromides with triols (intermediates and products 16–21, 28, and 30–40). The surfaces of the dendrimers are covered with MeO (33–35, 38–40) or allyloxy groups (36 and 37). The new dendrimers are characterized by NMR and mass spectroscopy. 19F‐NMR Signals of the CF3 groups reveal constitutional heterotopicities caused by substituents which are separated from the F‐nuclei by up to 15 bonds (through a 1,1′‐biphenyl‐4,4′‐diyl spacer!) (Fig. 6).
α‐Methylidene‐γ‐butyrolactones: Synthesis and evaluation of quinolin‐2(1H)‐one derivatives
Authors: Wang Tai‐Chi , Chen Yeh‐Long , Tzeng Cherng‐Chyi , Liou Shorong‐Shii , Tzeng Weng‐Feng , Chang Ya‐Ling , Teng Che‐Ming 
Published: December 2004

AbstractAs a continuation of our previous studies on the synthesis and antiplatelet activity of quinolin‐2(1H)‐ones with an α‐methylidene‐γ‐butyrolactone substituted at O(8), the O(6)‐ and N(1)‐substituted isomers were synthesized and evaluated for antiplatelet activity against thrombin (Thr)‐, arachidonic acid (AA)‐, collagen (Col)‐, and platelet‐activating‐factor (PAF)‐induced aggregation in washed rabbit platelets. These compounds were synthesized from 6‐hydroxyquinolin‐2(1H)‐one via alkylation and Reformatsky‐type condensation (Schemes 1 and 2). All of them were found to inhibit the platelet aggregation perfectly which was induced by AA and Col. 6‐Substituted isomers 5b–g exhibited very strong inhibitory activities against AA‐ and PAF‐ induced aggregation and are approximately ten times more potent than their 8‐substituted counterparts. However, the 1‐substituted (11a and 11b) and the 1,6‐disubstituted (6) counterparts were relatively inactive. Their effects on the Ca2+‐dependent vasoconstriction induced by high K+, and the phasic and tonic vasoconstrictions induced by norepinephrine (NE) in rat aorta were also evaluated. Except 5g, all of them were found to have significant inhibitory activity on the NE‐induced phasic and tonic vasoconstrictions. Compounds 6 and 11b also exhibited strong inhibitory activity on high−K+ medium, Ca2+‐induced vasoconstriction.
Acridine‐Labeled Primers as Tools for the Study of Nonenzymatic RNA Oligomerization
Authors: Kurz Markus , Göbel Karin , Hartel Christian , Göbel Michael W. 
Published: December 2004

AbstractShort, dye‐labeled oligonucleotides have been used as primers in template‐controlled polymerization reactions of RNA. The synthesis of appropriate acridine derivatives and their attachments to nucleic acids is described. In the nonenzymatic oligomerization of 2‐methyl‐1H‐imidazole‐activated guanosine 5′‐monophosphate, two observations deserve special notice: (1) reaction rates are almost unchanged by variations of the Na+concentration; (2) the conformational type of the primer‐template duplex (Avs.B) has considerable influence on the rates and yields of RNA oligomerization. When the incorporation of cytidine was studied in the presence of 1MNa+or K+, the process was almost inhibited by quadruplex formation of the oligo‐dG template. However, if these cations were omitted, an efficient primer extension could be observed using template concentrations as high as 100 μM. The chances for nonenzymatic self‐replication of RNA thus might be distinctly better than previously assumed.
Synthesis and Spectroscopic Characterization of 4‐Chlorophenyl Isocyanate (= 1‐chloro‐4‐isocyanatobenzene) Adducts with Amino Acids as Potential Dosimeters for the Biomonitoring of Isocyanate Exposure
Authors: Möller Marianne , Henschler Dietrich , Sabbioni Gabriele 
Published: December 2004

AbstractBlood‐protein adducts are used as dosimeter for modifications of macromolecules in the target organs where the disease develops. The functional groups of cysteine, tyrosine, serine, lysine, tryptophan, histidine and N‐terminal amino acids are potential reaction sites for isocyanates. Especially the N‐terminal amino acids, valine and aspartic acid of hemoglobin and albumin, respectively, are reactive towards electrophilic xenobiotics. To develop methods for the quantification of such blood‐protein adducts, we treated 4‐chlorophenyl isocyanate (1) with the tripeptide L‐valyl‐glycyl‐glycine (2a) and with single amino acids yielding N‐[(4‐chlorophenyl)carbamoyl]valyl‐glycyl‐glycine (3a), N‐[(4‐chlorophenyl)carbamoyl]valine (3b), N‐[(4‐chlorophenyl)carbamoyl]aspartic acid (3c), N‐(4‐chlorophenyl)carbamoyl glutamic acid (3d), N‐acetyl‐S‐[(4‐chlorophenyl)carbamoyl]cysteine (3e), and N‐acetyl‐O‐[(4‐chlorophenyl)carbamoyl]serine (3f), Nα‐acetyl‐Nε‐[(4‐chlorophenyl)carbamoyl]lysine (3g). For several chemicals, it was shown that blood‐protein adducts are good dosimeters of exposure and dosimeters for the target dose. The hydrolysis of the N‐terminal adducts of isocyanates release hydantoins which can be separated from the rest of the protein and analyzed using GC/MS or HPLC. This was achieved with 3a. The released hydantoin could be analyzed using GC/MS. We propose to analyze the N‐terminal adducts of isocyanates with blood protein to distinguish between arenamine and arylisocyanate exposure.
Asymmetric synthesis of pentono‐ and 6‐deoxyhexono‐δ‐lactams via hetero‐Diels‐Alder reactions with nitroso dienophile
Authors: Defoin Albert , Sarazin Hervé , Sifferlen Thierry , Strehler Christiane , Streith Jacques 
Published: December 2004

AbstractAsymmetric Diels‐Alder reaction of the pentadienoic and hexadienoic acids 2a,b with the chiral chloronitroso derivative 3 gave the primary adducts 4a,b with good‐to‐excellent enantioselectivity. Subsequent as‐ or trans‐dihydroxylation and hydrogenolytic cleavage of the NO bond led to the 5‐amino‐5‐deoxypentono‐δ‐lactams 13a, 14, 15a, and 16 in the D‐ribose, L‐arabinose, D‐xylose, and L‐lyxose series, respectively, and to the 5‐amino‐5,6‐dideoxyhexono‐δ‐lactams 13b and 15b in the D‐allose and D‐glucose series, respectively.
New Enzyme Models of Chloroperoxidase: Improved stability and catalytic efficiency of iron porphyrinates containing a thiolato ligand
Authors: Wagenknecht Hans‐Achim , Claude Cécile , Woggon Wolf‐Dietrich 
Published: December 2004

AbstractThe heme‐thiolate protein chloroperoxidase (CPO) catalyzes the chlorination of activated CH bonds. A reaction mechanism is proposed for this enzymatic transformation (Scheme 1), and a new iron(III) porphyrinate complex 13 is synthesized containing pentafluorophenyl groups at two meso‐positions and a thiophenolato ligand coordinating to the Fe‐atom (Schemes 2 and 3). Due to the presence of the electron‐withdrawing substituents, the catalyst 13 is appreciably resistant to oxidants (HOCl) and chlorinates, e.g., monochlorodimedone (5), with turnover numbers up to 1530. The redox potential of 13, E0 = ‐ 134 mV, and the Soret band (λmax 448 nm) of the CO adduct of the reduced state of 13 are close to the corresponding values of the enzyme CPO.
Stress‐generated potentials in bone: Effects of collagen modifications
Authors: Pollack S. R. , Korostoff E. , Sternberg M. E. , Koh J. 
Published: December 2004

AbstractThe effect of collagen modification on the magnitude of stress‐generated potentials has been studied. A whole bone model has been developed for determining the effects of collagen crosslinking on the electrical signals. Increased crosslinking results in larger electrical signals, and such increases result whether in vivo or in vitro changes in the collagen are made.
Characterization of self‐curing acrylic bone cements
Authors: Kusy R. P. 
Published: December 2004

AbstractFour self‐curing acrylic bone cements were surveyed by infrared, solubility, viscometry, quantitative metallography, microscopy, and physical testing techniques: CMW, Palacos R, Sulfix‐6, and Surgical Simplex P. Results show that these bone cements were primarily composed of poly(methyl methacrylate) and that no cross‐linking was evident. Solubility analysis confirmed this latter observation, as the bone cements dissolved completely except for a small insoluble fraction, which was identified as the radiopaque filler. For each bone cement, the viscosity‐average molecular weights of both the powdered phase and the cured two‐phase product remained unchanged, varying overall from 1 to 5 × 105. Using standard quantitative metallography, porosity ranged from 1 to 8% and the dispersed powder phase decreased 11–46%. Microscopy revealed the nature of the porosity, radiopaque fillers, the powder size and shape, and the fracture morphology. From tensile and fracture toughness tests, five physical properties were determined at ambient conditions and at 37°C after conditioning in distilled water at 37°C for 10 months: the modulus of elasticity, the ultimate tensile strength, the elongation at break, the fracture energy, and the mean inherent flaw size. At ambient conditions, the ultimate tensile strength decreased 33–55% when compared with commercial unmodified poly(methyl methacrylate), Plexiglas G. While the fracture energy remained rather invariant, the mean inherent flaw size increased fivefold over the commercial acrylic tested. This marked increase in the mean inherent flaw size could lower the fatigue resistance of a material, since more and/or larger fracture initiation sites are available. When tested at 37°C after protracted conditioning, the deleterious trends observed at ambient temperature continued. To some degree, porosity, particle‐matrix interfaces, residual stresses, low molecular weight products, inorganic and/or other organic additions, and water contributed to the inherent flaw size at the expense of the working stress. Several modifications are suggested by which the importance of these factors might be minimized.
Compatibility and long‐term stability of glass–ceramic implants
Authors: Blenckè B. A. , Brömer H. , Deutscher K. K. 
Published: December 2004

AbstractThe biocompatible and bioactive glass–ceramic material Ceravital, which has been developed, can be adapted to the physiological conditions of the organism by apropriately monitoring its chemical composition. The material exhibits a remarkable long‐term stability in in vitro solubility tests and when used as implants in animal experiments.
L‐Asparaginase bound to collagen membranes: Effect of glutaraldehyde crosslinking on stabilization of catalytic activity
Authors: Jefferies S. R. , LaPresto P. , Bernath F. R. , Joyeuse R. 
Published: December 2004

AbstractReconstituted bovine collagen has been used extensively in our laboratory as a carrier for immobilized E. coli L‐asparaginase. The activity and catalytic stability of these collagen–asparaginase membranes can be altered substantially by conditions used in membrane crosslinking with glutaraldehyde. As the concentration of glutaraldehyde used in tanning is increased, the initial specific activity of collagen–asparaginase membranes decreased asymptotically to a limiting value. Similar results occurred when membranes were subjected to increasing time periods of tanning at a constant glutaraldehyde concentration. These observations point to a time‐concentration relationship for glutaraldehyde tanning and its effect on the specific activity of collagen–asparaginase membranes. Specific activities of membranes tanned at glutaraldehyde concentrations of 5% or higher appear to be very stable over long periods of alternate storage and assay. This result, however, is not observed with membranes tanned at glutaraldehyde concentrations lower than 5% for short periods of time (approximately 30 sec to 1 min). It is not clear whether the instability of membranes tanned at lower concentrations of glutaraldehyde or shorter intervals of tanning is due to enzyme elution from the membrane or denaturation of the bound enzyme.
The effect of surface finish on the anodic polarization of a conventional spherical amalgam
Authors: Boyer D. B. , Chan K. C. 
Published: December 2004

AbstractThe corrosion of carved, burnished, polished, and burnished–polished samples of a conventional spherical amalgam was studied by anodic polarization in Ringer's solution. The effect of the variation of the mercury content from 40 to 60 wt% on the corrosion of burnished or polished samples was also investigated. It was found that burnished and burnished–polished samples corrode the least and carved samples the most. The current‐density peak associated with γ2 was of greater magnitude in polished specimens than in burnished specimens throughout the range of mercury concentrations investigated. It is hypothesized that lower porosity and more ready passivation of the surface due to a high amount of γ2 are the reasons that burnished samples corroded less than polished samples in this anodic polarization test.
Enzyme immobilization on heparin
Authors: Torchilin V. P. , Il'Ina E. V. , Streltsova Z. A. , Smirnov V. N. , Chazov E. I. 
Published: December 2004

AbstractWe describe the preparation and some of the properties of heparin‐bound α‐chymotrypsin that were obtained via activation of heparin with water‐soluble carbodiimide. Immobilized enzyme has unchanged kinetic characteristics toward low‐molecular‐weight and macromolecular substrates. The heparin‐bound enzymes could have a wide range of medical applications.
Mathematical models for the release of drugs from matrix tablets
Authors: Cobby John 
Published: December 2004

AbstractA previously derived cubic equation describing the release of drugs from matrix tablets is compared with a more complex equation recently presented. Using data obtained from cylindrical hydrocortisone matrix tablets, it is shown that the simpler cubic equation affords an equally acceptable description of drug release. Because the cubic equation contains a release‐rate constant Kr that is independent of tablet shape, a congruency between this equation and other historical models of diffusion may be demonstrated. It is suggested, given the complexity of drug disposition in vivo, that simplicity in modeling is a desirable goal.
Biodegradation behavior of various calcium phosphate materials in bone tissue
Authors: Klein C. P. A. T. , Driessen A. A. , de Groot K. , van den Hooff A. 
Published: December 2004

AbstractIn order to study the biodegradation behavior of calcium phosphate materials, cylinders of standard size were implanted in the tibiae of rabbits. Material parameters were stoichiometry (hydroxyapatite with a Ca/P ratio of 1.67 versus tricalcium phosphate with a Ca/P ratio of 1.50), crystallographic structure (apatite versus β‐whitlockite), microporosity, and macroporosity. The extent of biodegradation was evaluated by radiography, light and fluorescence microscopy, microradiography, and porosity measurements. All calcium phosphate materials were biocompatible in bone tissue. Hydroxyapatite ceramics had a higher osteogenic potential than β‐whitlockite materials. Depending on their porosities, sintered tricalciumphosphate (β‐whitlockite) materials were more or less biodegradable, in contrast to sintered hydroxyapatite materials, which showed no detectable resorption over a period of 9 months of implantation.